Light A WELLography™
First Edition
International Well Building Institute

Table of Contents


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International WELL Building Institute PBC authorizes personal use of this Light WELLography™, which includes the ability by the user to download and print a single copy of the Light WELLography™ for the user’s own education and reference. In exchange for this authorization, the user agrees:

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The International WELL Building Institute also acknowledges the important work of Melcher Media in bringing this document to market in its current state.

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The International WELL Building Institute (IWBI) and Delos Living LLC (Delos) acknowledge the work of the following IWBI and Delos technical staff that developed and created the WELLographies: Oriah Abera; Niklas Garrn; Trevor Granger; Soyoung Hwang; Michelle Martin; Vienna McLeod; Anja Mikic; Renu Nadkarni; Brendan O’Grady; Chris Ramos; Eric Saunders; Sara Scheineson; Nathan Stodola; Regina Vaicekonyte; Sarah Welton; Kylie Wheelock; Emily Winer.

The IWBI also is grateful for the input and insight provided by the following Subject Matter Experts:

Air: Terry Gordon, PhD; Eric Liberda, PhD; Tim McAuley, PhD; Ellen Tohn, MCP

Water: Eric Liberda, PhD; Tim McAuley, PhD; Margret Whittaker, PhD, MPH, CBiol, FSB, ERB, DABT, ToxServices LLC

Nourishment: Sharon Akabas, PhD; Alice H. Lichtenstein, DSc; Barbara Moore, PhD

Light: Chad Groshart, LEED AP BD+C; Samer Hattar, PhD; Steven Lockley, PhD, Consultant, Delos Living LLC and Member, Well Living Lab Scientific Advisory Board, Neuroscientist, Brigham and Women’s Hospital and Associate Professor of Medicine, Harvard Medical School

Fitness: Dr. Karen Lee, MD, MHSc, FRCPC, President & CEO, Dr. Karen Lee Health + Built Environment + Social Determinants Consulting; Jordan Metzl, MD

Thermal Comfort: Alan Hedge, PhD, CPE, CErgHF; David Lehrer, MArch; Caroline Karmann, PhD, MArch

Acoustics: Arline L. Bronzaft, PhD, Professor Emerita of The City University of New York; Charles Salter, PE

Materials: Clayton Cowl, MD; Matteo Kausch, PhD, Cradle to Cradle Products Innovation Institute; Megan Schwarzman, MD, MPH; Margret Whittaker, PhD, MPH, CBiol, FSB, ERB, DABT, ToxServices LLC

Mind: Anjali Bhagra, MBBS; Lisa Cohen, PhD; Keith Roach, MD; John Salamone, PhD; Nelida Quintero, PhD


None of the parties involved in the funding or creation of the WELL Building Standard™ and the WELLographies™, including Delos Living LLC, its affiliates, subsidiaries, members, employees, or contractors, assume any liability or responsibility to the user or any third parties for the accuracy, completeness, or use of or reliance on any information contained in the WELL Building Standard™ and the WELLographies™, or for any injuries, losses, or damages (including, without limitation, equitable relief) arising from such use or reliance.

Although the information contained in the WELL Building Standard and the WELLographies is believed to be reliable and accurate, all materials set forth within are provided without warranties of any kind, either express or implied, including but not limited to warranties of the accuracy or completeness of information or the suitability of the information for any particular purpose.

The WELL Building Standard™ and the WELLographies™ are intended to educate and assist building and real estate professionals in their efforts to create healthier work and living spaces, and nothing in the WELL Building Standard and the WELLographies should be considered, or used as a substitute for, medical advice, diagnosis or treatment.

As a condition of use, the user covenants not to sue and agrees to waive and release Delos Living LLC, its affiliates, subsidiaries, members, employees, or contractors from any and all claims, demands, and causes of action for any injuries, losses, or damages (including, without limitation, equitable relief) that the user may now or hereafter have a right to assert against such parties as a result of the use of, or reliance on, the WELL Building Standard™ and the WELLographies™.

Welcome to WELL

The buildings where we live, work, learn and relax have a profound effect on our well-being: how we feel, what we eat and how we sleep at night. By examining our surroundings and our habits, and making key optimizations and changes, we have the power to cultivate spaces that promote wellness, and support efforts to live healthier, active, mindful lives – a right for every human.

The WELL Building StandardTM (WELL) envisions this reality and opens this critical dialogue. It provides a roadmap and a comprehensive set of strategies for achieving building and communities that advance human health.

WELL consists of a comprehensive set of features across seven concepts (Air, Water, Light, Nourishment, Fitness, Comfort and Mind). Together, these components address the various individual needs of the people inside buildings, while also setting forth a common foundation for measuring wellness in buildings as a whole. The standard was developed by integrating scientific research and literature on environmental health, behavioral factors, health outcomes and demographic risk factors that affect health; with leading practices in building design and management. WELL also references existing standards and best practice guidelines set by governmental and professional organizations, where available, in order to clarify and harmonize existing thresholds and requirements.

The result is the premier building standard for advancing human health and wellness – and a blueprint for creating better buildings that can enhance productivity, health and happiness for people everywhere.

How to Use This WELLography™

WELLographiesTM present research relevant to health and well-being in buildings and communities. The sources included span health, wellness, and scientific and professional literature specific to the seven concepts within WELL, and other core focus areas. WELLographies are meant to complement the WELL Building Standard TM (available at and provide architects, building managers, engineers, and interior designers, among others, with health- and science-focused background to support and guide their efforts to advance the healthy buildings movement.

WELLographies have three primary goals:

  1. Provide background information for key topics relevant to understanding human health as it relates to the built environment.

  2. Synthesize and present the science that underpins the WELL Building Standard.

  3. Outline specific, evidence-based strategies that building professionals can apply to create spaces that promote health and well-being in buildings and communities.

There are nine WELLographies:

  1. Air
  2. Water
  3. Nourishment
  4. Light
  5. Fitness
  6. Thermal Comfort
  7. Acoustics
  8. Materials
  9. Mind

The Light WELLographyTM has the following sections:

Light and the Built Environment, which broadly describes how Light relates to the human experience in buildings.

Properties of Light, describes important technical components, including any terms that will be discussed throughout the WELLography.

Light and the Human Body, which provides an explanation of the biological mechanisms relating to Light, describing how the body functions under normal, healthy conditions.

Elements of Light, which describes environmental conditions or behaviors that are linked to health, focus, or occupant comfort, and that are subject to interventions in building design or policy. Each element includes coverage of associated health effects as well as solutions, interventions that can be implemented to impact the element. Some solutions may address several different elements.

Explanations of Solutions, which provides a definition and/or more detail for each solution.


Light profoundly influences our well-being . The way we design our lighting environments not only impacts our ability to perform visual tasks, it also affects our comfort, moods, and biological processes.

The timing, quality and frequency of our exposure to light have significant impacts on our bodies. Untimely exposure to light also severely affects a wide range of physiological functions including cognition and sleep quality.

Light quality influences human vision as we form images and perceive color through the light in our environment. Our perception of a scene’s crispness and beauty relies on the light that surrounds it. In mammals, the eyes detect light and send this information to the brain, triggering a number of events. The brain responds as light stimulates photoreceptor cells on the retina, instigating processes and functions that fall under three categories:

  1. image formation and color perception;
  2. calibration of the body’s biological clock and circadian rhythms;
  3. direct effects on alertness, mood and cognition.

Light also triggers subconscious effects that are not associated with image-forming vision but are essential for health and wellbeing. 1 One such “non-imaging forming” function includes synchronizing our internal biological circadian rhythms to match the external solar day. When our circadian rhythms are misaligned with the solar day it can cause problems that are similar to the symptoms of jet lag. 1 This is because a range of body processes—related to sleep, body temperature, metabolism, hormone regulation, immune function and many more—all appear to be regulated to some extent by the circadian clock. 2 3 4

Light also has a series of acute non-image-forming effects that impact our day-to-day mood, alertness, cognitive ability and overall health and wellbeing. 5 6 7 Light can suppress the pineal hormone melatonin, regulate pupil constriction, have direct effects on alertness and cognition, and have antidepressant effects. 5 6 7

Light is ubiquitous and the applications for lighting are widespread. Anywhere we want people to be alert, safe and productive might benefit from improved light.

Light and the Built Environment

Since all visible light—not just sunlight—can affect physiological processes, we have the opportunity to create indoor lighting environments that are optimal to human health and wellbeing.

Just as the pupil provides the gateway to light entering the eye, buildings provide the gateway to the amount and quality of light that reaches us. Thus, buildings can have an enormous impact on our light exposure to both daylight and electric sources.

Buildings must therefore tune the brightness and spectrum of indoor electric lighting to meet people’s needs as appropriate throughout the day, in addition to promoting access to daylight. In order to understand what light is considered “appropriate” and how to set it as such, it is first necessary to understand the properties of light and how it is measured.

Properties of Light

Light is a specific type of electromagnetic radiation: a form of energy that is transmitted by photons. Photons, the basic constituent of all electromagnetic interactions, are particles that exhibit wave-like behavior as they travel through space.

A photon’s energy content is directly related to its frequency: a photon from high-frequency radiation types (e.g., X-rays) carries more energy than a photon of lower frequency types (e.g., microwaves). This whole range of lower to higher frequency/energy makes up the electromagnetic spectrum.

Figure 1: Electromagnetic Radiation Of Different Frequencies.
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Only a small portion of the electromagnetic spectrum is visible to the human eye: wavelengths between roughly 380 nanometers (nm) and 750 nm. This is what is typically referred to as (visible) light (Figure 1). Humans can detect electromagnetic radiation outside of the visible range in other ways, such as through the sensation of heat created by infrared radiation.

Measuring Light

There are objective measures of the amount of power emitted by a light source, but our perception of certain distinctions is contingent on the eye’s capacity to detect them. The way the human eye measures light and then perceives visual targets has to do with our cells’ sensitivity to specific wavelengths of light, and how that sensitivity varies depending on which cell type perceives the light. See Light and the Human Body for more on this topic.

It is owing to these considerations that there are different metrics for quantifying the amount of light in terms of energy emitted from a source, versus the amount of light in terms of what appears bright to the eyes. Take, for example, the difference between radiant flux and luminous flux, which highlights this point. Radiant flux, also known as radiant power, is a measure of the total electromagnetic power emitted by a light source, measured in watts. However, this measure of light does not take into consideration the limitations of human vision.8 Luminous flux on the other hand does, and describes the total ‘light’ output of a source, weighted to the visual sensitivity of the human eye. Luminous flux is measured in lumens, where one watt of light at 555 nm is equal to 683 lumens.9 See Light and the Human Body for more on the significance of 555 nm.

Luminous intensity builds on luminous flux by taking into consideration the way light is emitted from a source: it measures the power emitted by a light source in a particular direction. Luminous intensity is measured in candela (cd).

If a light source of one lumen emits all of its light through an angular area of one steradian, then the luminous intensity is one lumen/steradian, or one candela. A steradian is the angle made at the center of a sphere, which has an area of the square of the sphere’s radius. The equivalent in a circle is a radian, where one radian is defined as the angle made at the center of a circle by an arc whose length is equal to the radius of the circle (Figure 2).

Figure 2: Radians and Steradians are Unitless Angular Measurements.
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The surface area of a sphere is found by A=(4)(3.14)(r²), meaning that there is a total angular area of 12.56 steradians (4x3.14=12.56). Therefore, a light source that produces 12.57 lumens equally in all directions produces a luminous intensity of one candela.

Taking it one step further, luminance describes the luminous intensity across a given unit of area, measured in of one candela per square meter (cd/m²). This is the primary measure of the sensation of brightness, as luminance refers to the amount of light coming to the eye from a surface or point.10 Luminous flux, luminous intensity and luminance all do not take into account the distance from the light source to the target surface. Illuminance does, detailing the amount of light reaching a surface, such as a floor or table, and is measured in lux, or in foot-candles in non-metric systems (1 lux = 1 lumen/m², and 1 fc = 1 lumen/ft²).8 Illuminance then measures light on a surface, which is then reflected to the eye, and so is not directly observed.

Figure 3: Metrics for Light.8 10
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Figure 4 displays the illuminance levels of common lighting environments. The range of lux in the figure also highlights the eye’s ability to adapt to different intensities and amounts of light for visual perception.

Figure 4: Approximate Illuminance Levels for Various Environments.11
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Color and Color Temperature

The color temperature of light is significant both for visual and non-visual reasons. For one, the color temperature of a light affects the color of the objects it illuminates and can greatly impact the quality of a visual scene. Beyond that, the color temperature of a light—which corresponds to its wavelength—differentially affects the body, because different cells in the eye are sensitive to different wavelengths of light.

Figure 5 depicts all visible hues in all possible saturations in a map known as the International Commission on Illumination (CIE) color space. The sides of the “arch” are the monochromatic prismatic colors (wavelengths in nm shown), while those along the straight line at the bottom connecting red and blue are extra-spectral hues. All colors inside the shape are made by the combination of two or more monochromatic wavelengths, though the most saturated colors are prismatic colors from a single wavelength.

Figure 5: The CIE 1931 Color Space With Blackbody Locus.12
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Most light that humans encounter on a daily basis, though perceived as a single color, is composed of a combination of frequencies/wavelengths, or emissions spikes, which makes up what is known as its particular spectral power distribution (SPD).

Polychromatic light is the combination of multiple frequencies and can remain tinted or appear white in certain combinations. Lighting used in buildings, except for architectural highlighting, is usually close to the central near-white point (around about 5,000 K) of the CIE color space. This light illuminates colored objects similar to the way they would under natural illumination.

To understand what would constitute “ideal” artificial light for seeing color, it is useful to refer to the construct of a blackbody, which is an ideal object that absorbs all radiation directed at it, and that, to stay in thermal equilibrium, emits electromagnetic radiation in a way proportional to its temperature. Planck’s blackbody radiation function calculates the SPD for various blackbodies of different temperatures and describes how, as temperature increases, peak radiation output occurs at shorter wavelengths, affecting the color at which we perceive the temperature.

The color temperature of light is measured in kelvin (K) and can be ascertained with reference to the peak of the curve of SPD within the visible range, which describes the “whiteness” of a given blackbody with a specific temperature (Figure 6).

Figure 6: Blackbody Radiation as a Function of Temperature.
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The curved line on the CIE color space in Figure 5 connects different temperatures to specific colors based on Planck’s blackbody radiation function, creating what is known as the blackbody locus. Lamps that do not produce light by glowing (e.g., light-emitting diodes (LEDs) and fluorescent lamps), create the illusion of a blackbody’s “whiteness” by combining several different narrow-band emissions and glowing phosphors (substances that are luminescent).

The SPD from LED and fluorescent lamps does not follow Planck’s law (i.e., Planck’s blackbody radiation function), so they technically do not have a color temperature. Instead, their output is described by correlated color temperature (CCT), which is the point on the blackbody locus that is closest to their output, measured in kelvin. By selecting the emission characteristics of the lamp, it can be tuned to the desired correlated color temperature.

The International Organization for Standardization (ISO) and the CIE provide joint publications offering definitions and information for standard colorimetric observers and different types of lamps. For example, the industry standard illuminant for daylight is known as D65 and has a CCT around 6,500 K. Typical office illumination refers to F2 lamps—also known as cool white fluorescent (CFW) lamps—which have a CCT around 4,200 K.13

Color Rendering and Quality

There are further considerations relevant to creating aesthetically pleasing and comfortable visual environments that have to do with both the kind of light hitting surfaces of the environment, and the types of surfaces in question. CCT simply describes the color produced by a lamp on a white surface. However, depending on the SPD of the lamps used, colors may not appear as expected when the source of illumination is not a glowing blackbody The relative response of the eye’s cone cells determines the colors we perceive, based on these cells’ sensitivity to specific wavelengths of light (discussed further in Light and the Human Body).

For all light, the light reflected by an object is the product of both the spectrum hitting it and the object’s spectral absorbance. For example, chlorophyll in plants reflects green light and absorbs the rest, so under sunlight, plants appear green. When illuminated by blue, red or magenta light however, the chlorophyll simply absorbs all of the radiation and the plants appear black. What this means is that when some specific spectral distribution hits a target, the target has its own properties that allow it to absorb specific frequencies of that spectrum, and that which is not absorbed is reflected back out. When the majority of light is absorbed, leaving virtually no light to reflect back out, the object approaches blackness.

The color rendering index (CRI) is a scale from 0-100 (higher scores being better) that indicates how closely a given light portrays a series of colors, in comparison to how the colors would be portrayed under a standardized ideal light of the same warmth or coolness (i.e., color temperature). It is in effect a measure of a light source’s ability with a given temperature to behave like a blackbody radiator of that temperature, which is defined as a CRI of 100.14 Candles, incandescent lamps and the sun are very close to theoretical blackbodies, so light from these sources has a CRI of nearly 100. CRI is not a definitive measure of lighting quality. For example, candlelight, with its near-100 CRI, contains very little green and blue light and therefore will not illuminate these colors well. Furthermore, a low-quality fluorescent light, despite a lower CRI, will actually render objects more closely to sunlight than a candle with a near-perfect CRI. For more on the specific properties of common lamps, see Explanations of Solutions.

The CRI of a lamp is often expressed as an average known as Ra.15 To determine Ra, typically eight standard CIE color samples (known as R1—R8) are illuminated by the light source in question and then by the reference light source. The difference in the chromaticity (color appearance) of each sample between the two light sources is found using a standard calculation and then averaged over the number of samples taken to yield Ra.15

These differences can have major impacts on the visual aspects of the space and lighting comfort. Color quality often suffers in the drive for energy-efficient lights, and programs such as the Environmental Protection Agency’s (EPA) Energy Star set minimum The CRI requirements in addition to maximum energy consumption caps for light sources that bear its label. 16 This feature sets an acceptable value for Ra, as well as a separate minimum for R9 (saturated red), which can help to ensure that deeply saturated colors are likewise well illuminated. The value for R9 can often be obtained from lighting manufacturers or calculated from spectral measurements.16

Electric Lighting

There are several lamp types that act as a light source via different mechanisms, each with their own pros and cons. Comparing incandescents, fluorescents, and LEDs, incandescents have the shortest life while LEDs tend to have the longest. As of January 1, 2014, incandescent light bulbs, known for their warm colored light, are no longer manufactured in the United States (U.S.) as they do not meet federal energy-efficiency standards, however lamps in stock may continue to sell in stores until already existing supplies drop. The phasing out of incandescent lamps has been matched with the increasing popularity of compact fluorescent lamps (CFLs) and LEDs—both of which are more energy-efficient. LEDs can be particularly relevant for circadian considerations in 24-hour environments, due to their color tunability for dynamic lighting systems.

Figure 7: Correlated Color Temperature of Some Common Electric Light Sources.17
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Incandescent lamps shine by heating a tungsten filament. Although incandescent lamps produce a continuous spectrum of wavelengths, giving them a CRI of 100, their light output is skewed towards the red end of the spectrum, giving them a low color temperature. This result comes from a need to balance efficiency and the type of light emitted with operating life: although higher filament temperature produces higher color temperature and greater efficiency, it also increases the rate at which the metal melts, leading to blackening of the bulb and a weaker filament.18 19

Tungsten melts at 3,695 K, and a filament at 3,000 K in air would oxidize.20 21 To decrease degradation of the filament, the lamps are filled with inert argon. Halogen lamps, an advanced type of incandescent lamp, also use bromine or iodine (halogen-series elements) to create a reaction that continuously re-deposits tungsten which has vaporized back on the filament, extending the lamp life and enabling slightly higher operating temperatures.20 21

Decreasing the voltage supplied to an incandescent lamp simply reduces the operating temperature, thus diminishing light output and color temperature. In addition to this inherent dimmability, such filament lamps are extremely inexpensive due to their complete lack of electronics. However, the filament will evaporate on the order of about 1,000 hours.


A fluorescent lamp consists of a phosphor-coated tube with an electrode at each end filled with low-pressure mercury vapor and a buffer gas such as argon.18 22 Electricity arcing through the mercury vapor causes its electrons to jump to a higher energy state; when they revert, ultraviolet light is emitted, which is then converted into visible light by the fluorescent coating on the interior of the glass. The coating is often made of a combination of phosphor crystals, the selection of which determines the spectrum of light produced by the lamp. Newer fluorescent lamps use three types of phosphors, resulting in a CRI as high as 90 compared with 60 for older lamps, and output between 75 and 90 lumens per watt, compared with between seven and 22 for incandescent lamps.18 22

A 60 watt Energy Star certified compact fluorescent lightbulb (CFL) costing $6 USD will have a payback of a year or less, and save between $30-80 USD in electricity costs over its lifetime.23 The distinct emissions by the phosphors are one of downsides of fluorescent light sources. Although the light’s color can be altered as desired, the spectrum has gaps at frequencies missing an associated phosphor, resulting in a low CRI. Modern fluorescent lamps use a larger collection of phosphors, and therefore render colors better than earlier generations. The phosphors slowly degrade, and fluorescent lamps can be rated from anywhere between 7,500 to 24,000 hours, though some lamps have even longer life rating depending on various factors such as ballast configuration and the operating environment.19

Light Emitting Diode (LED) Lamps

LED lamps are relatively new lamps, rapidly growing in popularity. The way these lamps produce light yield a more efficient light source compared to incandescent or fluorescent lamps, and other characteristics of the lamp also allow the LED to be a more versatile lamp, which can render benefits both for visual and wellness-related purposes.

Because LEDs promise greater efficiency, reliability, and a longer lifespan than other types of lighting, they have been more frequently adopted for use in outdoor applications where these characteristics are particularly important.

The U.S. Department of Energy estimates that 10.1% of the installed base of outdoor lighting now consists of LEDs, compared with 2.8% of the indoor.24

Minimum performance standards for commercial LED products used in a variety of outdoor applications have been set by the DesignLights Consortium, a nonprofit partnership between energy efficiency groups and utilities that works to promote high-quality commercial LED products. All outdoor lamps are required to have a lifespan of 50,000 hours at L70, a 5-year warranty, and a minimum efficiency of 65, 70, and 75 lumens per watt for low (up to 5,000 lumen output), medium (10,000), and high (over 10,000) output applications.25

In an LED, a supplied electrical current causes electrons to flow over a p-n junction (the positive and negative) semiconductor.19 The two sides of the p-n junction are separated by a band gap, dependent on the materials used to make the semiconductor. As electrons cross between the sides, it emits a photon of light, and the band gap determines the photon’s frequency and can result in visible wavelengths.19 Each p-n junction therefore produces only monochromatic light. Multiple types of p-n junctions can be combined to produce white light, or LED lamps can employ phosphors similar to those used in fluorescent lamps.

LEDs often last 50,000 hours and have efficiencies often between 50 lumens and 85 lumens per watt.26 In 2012, Cree developed LEDs that provide more than 200 lumens per watt.27 Because of the small size of the light emitting components, LEDs can provide dynamic spectral adjustment for tunable lighting.

Light and the Human Body

In learning how light affects the body it is imperative to understand both how our eyes perceive light, and how different parts of the brain respond to light. Such responses then trigger downstream effects throughout the body.

Defining Light

Electromagnetic rays affect the human body in different ways depending on the wavelength. “Infrared” radiation (electromagnetic radiation on the order of 1,000 nm) is felt as heat on the skin. “Ultraviolet” radiation (electromagnetic radiation between 200 nm to 400 nm) allows the skin to produce vitamin D, though excess exposure can be harmful. Lower wavelength radiation carries even more energy and exposure is considered carcinogenic. 28

Rays between 380 nm and 750 nm can be sensed by receptors in the eyes and this spectrum makes up what is known as “light”. 8 The Light WELLographyTM employs this definition of light, referring specifically to a visible range of electromagnetic radiation, between 380 nm and 750 nm, and the effect that light has on the brain through the eyes, which then involves other physiological reactions downstream in the body.

Photoreceptor Cells in the Eye

When light enters the eye, two different types of photoreceptor cells contribute to image-forming vision: rod cells and cone cells. These cells express photopigment, which is granular material in cells that selectively absorb specific wavelengths of light. Different pigments are sensitive to different wavelengths of light.29 All rod cells express the photopigment rhodopsin, whereas cones express three distinct cone opsin pigments that absorb short-wavelength, medium-wavelength, and long-wavelength light rays.

A third type of photoreceptor cell with its own accompanying pigment, melanopsin, has functions which predominantly involve non-image forming responses, including synchronizing and resetting the circadian clock, regulating sustained pupil responses, and alerting the brain.30 31 These cells are located in the ganglion cell layer of the retina, a different area than where the rods and cones are located, and are directly light sensitive even when the cells are studied after being taken out of the eye.32 33 These third types of photoreceptor cells are known as intrinsically photosensitive retinal ganglion cells (ipRGCs). Studies on various mammals reveal that there are several subtypes of ipRGCs (M1—M5) with diverse functions.34 35 Of particular note for their predominant involvement in non-image forming visual functions are the M1 ipRGCs.36 37

The ipRGCs, while making up only a small subset of RGCs, are spread across the retina in a network to provide a broad irradiance detection function, regardless of whether an individual is looking directly at the light and without the need for the high density that visual acuity requires.38 32 33 39 The majority of the cells project to the site of the master circadian clock, the suprachiasmatic nuclei (SCN) and other brain areas mediating non-image-forming functions.40 41 42 Furthermore, ipRGCs have a diverse range of functions, including influences on sleep quality and affective states such as mood in humans.39 41 36 43 Overall, rods, cones, and ipRGCs work in a complementary fashion to respond to light of all intensities.41

In order to understand the relationship between light and the human brain, it’s important to understand the functions of classical rod and cone photoreceptor cells as well as the novel ipRGCs. By reviewing the structures and neural pathways associated with these cells, we can begin to understand light’s impact on human health and wellbeing, and how to harness light to improve indoor conditions in ways that can support proper biological functioning.

Responding to Light

The retina is a thin layer in the back of the eye containing millions of nerve cells that are highly sensitive to light, called photoreceptors. The photoreceptors in our eyes are rods, cones, and ipRGCs. The dendrites of ipRGCs cover the entirety of the retina to make up a “photoreceptive net”.44 45 The light-sensitive nerve cells located on the retina are primarily sensitive to two properties of light: wavelength and intensity.

When light travels through the eyes, the cornea and lens refract (i.e., bend) the light so that the rays converge on a point on the retina to bring the images to focus for vision.46 29 The lens has a natural yellow tint, which affects the spectrum of light that passes through by preferentially blocking light at lower (bluer) wavelengths. This characteristic of lens transmission is particularly important as people age, and their lenses yellow further. Lens density also increases with age.47 Taken together, this has implications on what kind of light is ideal for an observer based on their age.

Figure 8: Diagram of the eye and its Photoreceptors.48
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Figure 8 shows how rods and cones are in the outer nuclear layer, while RGCs (including ipRGCs) are located in their own ganglion cell layer, the inner nuclear layer.48 The axons of RGCs comprise the optic nerve. The optic nerve from each eye converges and crosses at a point called the optic chiasm. From that point on, different neural paths are activated depending on the type of information (in this case, the “information” is light).46

Neural paths are activated when photoreceptor cells absorb a sufficient amount of light. The cells are sensitive to light because they express a specific photopigment that is activated by absorbing a single photon of light. This is called a cell’s unitary response, also known as the “single-photon response”.49 50 A physiological reaction to light that triggers downstream activity depends on whether the sum of each photopigment’s contributions passes a certain threshold.

A photopigment’s unitary response is considered “large” when a single photopigment responds on a large scale.49 50 In other words, relatively few of that type of photoreceptor need to be activated in order to sufficiently trigger some response. Conversely, a “small” unitary response means that comparably more numbers of that type of photoreceptor needs to be activated to pass the required threshold for initiating a series of events or functions in the body.49 50

Classical Photoreceptors

Distinct cells are associated with specific pathways and bodily functions. As such, different types of light instigate reactions from different types of cells.51

Rod cells are cylindrical over their whole length, while cone cells thin to a point at one end. Both rods and cones are mostly concentrated at the center of the retina (though only cones are present at the fovea, which is a tiny point in the middle of the retina), and while their distribution tapers off from that point, rods are predominantly responsible for providing adequate peripheral vision.46

Rod cells have peak efficiency at 498 nm and are more sensitive to light than cone cells, and therefore are able to facilitate vision in dim lighting conditions.52 9 53 “Dim” conditions refer to when luminance is less than 0.001 cd/m² (approximately 1/100 lux on a surface), and in such conditions, rod cells provide the primary visual signal to mediate what is defined as scotopic vision. Rods, however, saturate above 3 cd/m² (approximately 10 lux), wherein cones take over to mediate what is defined as photopic vision. When cells saturate, that means the cells no longer respond to variations in light intensity. All photoreceptors have a saturation point, after which an increase in light intensity does not correspond to an increase in stimulation of the cell. Intermediary lighting levels that engage both the rods and cones contribute to what is known as mesopic vision, relevant in lighting levels between 0.001 cd/m² to 3 cd/m².52 9 53 Vision at night is mediated by the scotopic visual system, while most daytime vision is mediated through the photopic system.

The reason for the difference in sensitivity to light between rods and cones may have to do with their unitary responses to light.50 54 Rod cells have a large unitary response, meaning relatively few number of rods need be activated to accommodate image perception. Cone cells on the other hand have a small unitary response, meaning we require more of them to be activated in order to accommodate visual perception. This is true despite the fact that both rods and cones express a similar number of pigment molecules per cell (i.e., the density of rods and cones is similar). Taken together, this means that cone cells are particularly important during daylight and for most indoor environments, as they are less sensitive to light and will not saturate as rods do at higher light levels. 5054

Figure 9: Light Intensity and Sensitivity of Different Photoreceptors.
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Color Perception

While the colors we see have to do with the quality of light as well as the target object itself, which absorbs and reflects that light (see Properties of Light – Color and Color Temperature), the way humans see color also depends on the way our brains perceive color based on information it receives in the form of light. Different photopigments selectively absorb specific wavelengths of light, which our brains interpret as distinct colors. There are three sub-types of cones, each of which is most sensitive to a distinct wavelength of light: S (short wavelength sensitive) cones have peak sensitivity to 419 nm light, M (medium wavelength) cones are most sensitive at 531 nm, and L (long wavelength) cones reach peak sensitivity at 558 nm. 55 9 Figure 10 shows sensitivity curves for each cone as described above, where the color of each line represents the monochromatic wavelength at each peak, based on studies mapping photoreceptor sensitivity to different wavelengths of light. 55 56 In vivo, lens filtering shifts spectral sensitivity towards longer wavelengths, because the lens has its own color—yellow—which is relatively on the longer-wavelength end of the visible spectrum. 57 This is simply to say that if the cells were studied taken outside of the eye, they would should peak sensitivity to a slightly shorter wavelength of light, but the yellow filter of the lens makes it such that the sensitivity shifts to longer-wavelengths.

Figure 10: Peak Sensitivity Wavelengths For The Three Types Of Cones.55
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There is only one type of rod, and rods cannot distinguish colors.52 This is because different types of cells (with different pigments) are sensitive to different wavelengths of light—as in the case of the different types of cone cells —and forming a comprehensive image requires combining input from other cells. Dim light vision is predominantly (possibly solely) rod-mediated. This type of vision is not color vision, as provided by cones at brighter, photopic, light intensities. Therefore, the mechanisms driving dim light vision may be less integrative than those driving bright light, cone-mediated color vision. 52

The three types of cones work in conjunction to facilitate the perception of color.58 In general, theories of color vision contend that the relative activation of the three types of cones by different wavelengths of light result in the perception of different colors.58

There are many combinations of wavelengths which result in the same ratio, so different spectra of light can appear the same color. However, these seemingly identical alternates, known as metamers, do not reflect from illuminated surfaces in the same way. Metamers are lights that are physically different (i.e., they have different SPDs/are composed of a different combination of wavelengths), but that we perceive as identical because the wavelength ratios are similar.

Figure 11: Metamers - Identical Light Colors With Similar Ratios Of Different Spectral Compositions.
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Figure 11 shows how metamers work. The light on the bottom is a monochromatic yellow light at 580 nm. On the top-left, there is red light at 650 nm and green light at 510 nm on the top-right. The wavelengths of the red and green light have a combined average of 580 nm, resulting in the same color yellow. Although one would perceive these two lights as identical colors, there would be discernible differences in the way the lights illuminated the colors of the room. This is owing to the fact that since there would be a difference in the spectral composition of the two lights, objects would absorb the lights differently and therefore reflect light back differently as well. Thus, metamers highlight the problem with considering CCT alone to describe spectral content: two lights of the same CCT may have very different spectra and therefore different blue content, meaning they will also have different circadian efficacy. This is because the photoreceptor cells that mediate circadian responses are activated most strongly by 480 nm light—blue light. These cells, the ipRGCs, are most sensitive to blue light, which is why lights with higher blue content have greater circadian efficacy. This is explained further in the Intrinsically Photosensitive Retinal Ganglion Cells (ipRGCs) section.

All this aside, our sense of sight is not founded on a perfect empirical instrument, but rather a mechanism that has evolved to gather information in a way that prioritizes detecting differences (contrast). For example, the first and third circles in Figure 12 are of identical color, despite the one with the yellow background appearing blue and the one surrounded by blue appearing yellow. This is another example of how physical characteristics of light alone does not adequately explain our perception of that light.

Figure 12: The first and third circles are of identical colors.
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Brightness is the luminance of a visual target as perceived by our brains, and this perception varies depending on how long we are exposed to the light, and the contrast that surrounds the visual target. The M and L cones contribute to the sensation of brightness at typical daylight and indoor lighting levels, and have a combined peak response at 555 nm (see the convergence point for the M and L curves in Figure 10). In illuminating measurements, the electromagnetic power is weighted to the eye’s sensitivity to the combined M and L cones. This M-L curve with peak sensitivity at 555 nm is what makes up the aforementioned photopic visual system.59 60 In other words, the metrics described in Light and the Built Environment are all weighted to/based on the response of the M and L cones, meaning they define bright light as 555 nm light.

In daylight, the typical human eye perceives light of 555 nm as greenish-yellow, and this is the wavelength of light to which the eyes are most sensitive, meaning it is interpreted as maximum “brightness”. This being the case, something like monochromatic 480 nm light must emit much more power compared to monochromatic 555 nm light in order to induce a similar impression of brightness. This is because our perception of brightness is mediated by the M and L cones, which together show peak sensitivity to 555 nm light. This also highlights the point that the eyes are sensitive to both the spectral composition (wavelength) and power (intensity) of light. Furthermore, while our perception of brightness is often connected to the cones, rods also play a role.

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Due to the patterns of distribution of cones and rods on the retina, the proportional contribution of rods versus cones to brightness depends on the angle from which we observe light, referring to the scotopic/photopic ratio (S/P ratio).62 The S/P ratio is a measure of the scotopic luminance over the photopic luminance, wherein a value of one represents light levels that only engage the cones, and a value of zero represents light levels that only engage the rods.62

As a final consideration, perceptions of brightness also have to do with the contrast between the visual target’s luminance and the background’s luminance.63 Contrast is measured via the process of adaptation, which considers the difference between luminance (the physical measure of light output), and brightness (the perception of luminance, which is contrast-dependent and weighted to the sensitivity of M+L cones).10 What this means for designing comfortable lighting environments is that a strategy which limits luminance contrast can yield a more visually comfortable space and reduce opportunities for glare. Lighting organizations provide luminance ratio recommendations (e.g., luminance at the brightest to darkest points within a room), discussed further throughout the Solutions sections of this WELLography™.

Non-Image-Forming Responses to Light

While previous sections have focused on image-forming visual processes related to light, there are other, non-image forming responses to light which have to do directly with cognition and alertness, as well as downstream physiological reactions in the body that have to do with circadian rhythms.

In 1995, a seminal study published in the New England Journal of Medicine showed that some totally visually blind people, testing negative on standard neuro-ophthalmology tests, still retained circadian photoentrainment and melatonin suppression responses to light. 64

At the time, it had been largely assumed that the non-image-forming responses to light were mediated by rods and cones, but this study and other data in animal models of visual impairment and color blind individuals strongly suggested that there must be an additional photoreceptor system.8 65 It has since been confirmed in additional studies that a small proportion of totally visually blind individuals can retain non-image-forming responses, particularly to blue light, if their eye disease is limited to the outer retina, which is the rod and cone layer.66 67 68 The discovery of melanopsin, a novel photopigment, elucidated the mechanism underlying these effects.

Studies in totally blind individuals illustrate the functional and anatomical separation of the image-forming and non-image-forming photoreceptor systems. This separation provides the basis for the development of sophisticated lighting that can stimulate, or avoid stimulating, the visual and non-visual responses to light differentially. This is because these studies illustrate that different kinds of light—specifically blue light—have effects on humans that have to do with circadian rhythms, while other kinds of light might serve a purely image-forming purpose, aiding in visual processes without having non-image forming effects.

Intrinsically Photosensitive Retinal Ganglion Cells

In 1998, researchers discovered a novel pigment called melanopsin in the dermal melanophores of frogs, which are photosensitive much like retinal photoreceptors. 69 A study published in 2000 identified melanopsin in the human eye and noted that the unique localization of the pigment in the ganglion cell layer of the retina, rather than the outer retina as for rods and cones, indicated that it was not involved in image-formation, but instead seemed more likely as a candidate for a circadian photopigment. 30 By 2001, action spectra in humans provided supporting evidence that a non-rod and non-cone photoreceptor system existed which mediated circadian rhythms 70 57 and shortly thereafter, researchers studying mice and rats identified key characteristics of a group of ipRGCs that contained the novel pigment melanopsin and were responsible for circadian synchronization.71 57 32 33

As noted previously, traditional photoreceptors’ sensitivity peaks at 498 nm and 555 nm for rods and the combined M+L cones, respectively. The ipRGCs on the other hand are most sensitive at about 480 nm—to short-wavelength blue light. 32 66

Figure 13: The Melanopsin And Opsins Of M+L Cones’ Sensitivity Curves.
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There are some important differences in how ipRGCs absorb light compared to other cells. First, ipRGCs have the largest unitary response of the three types of photoreceptors, but are also the least sensitive to light. 54 72 This may seem contradictory, but studies suggest how this may be the case. The reason behind the insensitivity may have to do with pigment density per cell: ipRGCs express melanopsin to yield a density of about 3 molecules per micrometers2, whereas rods and cones express their respective pigments for a density about 25,000 molecules per micrometers².32 50 36 54

This sparsity results in a low probability of photon capture in ipRGCs compared to rods and cones, but again, the large unitary response means that even absorbing a single photon greatly increases the chance of meeting a threshold level of activation. Further marking the unitary response of ipRGCs as unique is a slow kinetic response, even when compared to typical RGCs: the unitary response of ipRGCs lasts nearly 10 seconds, which is 20 times longer than that of rods, and 100 times longer than that of cones. 50 This seems to makes sense, as melanopsin, a more ancient photopigment that evolved before the rods and cones used for vision, is essentially trying to measure daylight; to tell the brain whether it is day versus night, or summer versus winter. As one study notes: “This prolonged kinetics improves sensitivity by conferring high temporal summation, while also smoothing the response to fluctuating light levels.” 54 This means that ipRGCs fundamentally operate over long time scales, relative to the rods and cones. 54

Circadian Oscillations

Mammals function on an approximately 24-hour cycle, even in continuous dark, following what are referred to as circadian rhythms (circa meaning around in Latin, dies meaning day).73 The circadian system controls many aspects of our biochemistry, endocrinology, physiology, metabolism and behavior, including at least an estimated 10% of the human genome.74 75

The circadian system’s function is to ensure that our biology is appropriately synchronized with the external world, and that our internal processes are accordingly properly synchronized.76

The circadian clock achieves this by having self-sustained internal rhythms that can anticipate environment time in order to be ready when necessary for tomorrow, rather than just responding passively to changes in the environment (the ‘early bird’ has to be awake before the worm).

Humans are diurnal—day-active—and so the circadian clock promotes wakefulness during the day and sleep at night. 77 Temperature is higher during the day, peaking in the late afternoon, and falling during the night. 78 Alertness, mood and performance show daily variation too: upon waking, we experience ‘sleep inertia’ as the brain gradually wakes up, which can take up to 4 hours to fully dissipate. 79 Our alertness then stays quite high and relatively steady until the middle of the evening, when the brain begins to promote sleep. 78 The opening of the ‘sleep gate’ coincides with the onset of melatonin production, considered the biochemical signal of darkness. 78 Melatonin does not induce sleep but rather tells the brain that it is night. It is therefore often referred to as the “darkness hormone”, and in humans and other diurnal (day-active) animals, night means it is time to sleep. 51

As day-active animals, we metabolize food better during the day compared to the night. Eating at night will cause higher glucose, insulin and fat levels in the blood compared to when eating the same meal during the day. 78 Some people experience a mid-afternoon or post-lunch “dip” or lethargy (which is not actually related to food), which is worse if we are sleep-deprived or have disrupted our circadian rhythms. 78 There are also rhythms in heart function, lung function, liver function, immune markers and many other processes that are tuned to our temporal niche—to be awake during the day and asleep at night. 78

The site of the central circadian clock is the suprachiasmatic nucleus (SCN), which is located in the anterior hypothalamus of the brain. The clock runs naturally with a rhythm close to, but not exactly 24 hours (about 24.2 h on average but ranging from about 23.6-25 h), and so the clock has to be reset each day to 24 hours, i.e., it must undergo circadian photoentrainment. 80 This calibration occurs via the ipRGCs, which send information to the SCN via a dedicated neural tract, and then the SCN in turn synchronizes the various local clocks in peripheral tissues and organs. In this fashion, mammals’ circadian system is organized into a “hierarchy of oscillators” that starts with the SCN, considered the “conductor” of the orchestra, that then keeps the other players to time. 81

It is important to note that blue-light sensitive melanopsin are not the only photoreceptors that influence circadian entrainment. The eyes’ rod and cone cells play a lesser but still significant role in circadian entrainment, particularly at low light levels or at the start of light exposure. Their influence declines at higher light levels and with increasing light duration, when melanopsin and therefore blue light sensitivity dominates. The ipRGCs are the only cells that provide input to the SCN for photoentrainment; however, the process of melanopsin phototransduction—wherein photons are converted to electrical signals—incorporates information from rods and cones, travelling to the SCN via the ipRGCs. 82 83 This means that nearly all light, particularly if it’s otherwise dark or has just been dark, can initiate circadian responses. This may be good in the mornings when we desire alertness to begin the day, in which case we would want specifically high blue-content light, but also has implications on light in the evenings and underscores the importance of minimizing light exposure of all kinds at night, but particularly blue-content light.

Key Hormones

Light influences us in many ways, and we can review its effects by studying fluctuations in hormone levels throughout the body in response to light. Without external time cues that entrain the clock, the circadian cycle would run on its own internal time, and gradually desynchronize from the daily changes of day and night. A total lack of light input to the clock, such as that which occurs in many totally blind patients, leads to its failure to synchronize to the 24-hour day and causes non-24-hour sleep-wake disorder, a serious clinical problem. 84

Exogenous signals that synchronize or entrain the biological clock to its surroundings are known as zeitgebers. 85 Light is the most important, but there are other weaker “non-photic” time cues such as exercise or food. Their weakness is illustrated by the fact that most totally blind people cannot synchronize to the 24-hour day even in the presence of very strong social and other non-photic zeitgebers. 85

One of the main ways that the SCN helps to signal light information to the brain is by being the key regulator of the pineal hormone melatonin, which signals to the body that it is a time of darkness. 76 Melatonin rhythm is regulated by the SCN, so it will cycle with an approximate 24-hour period even in the absence of light or other time cues. As with other circadian rhythms, entrainment of the SCN leads to entrainment of the melatonin rhythm and therefore the 24-hour light-dark cycle is needed to maintain appropriate entrainment. 76 In other words, while the cycle will still persist as a nearly 24-hour cycle without light or other cues, the entrainment will not be retained without cues, and it is entrainment of the SCN that tells the body how to align its internal time with the external world, making sure the body’s biological “noon” corresponds to the noon of the solar day.

In addition to its cycle being entrained by light, melatonin is also acutely suppressed by light. 86 87 Exposure to light after dusk, an unnatural event in the natural world, causes the brain to induce daytime physiology, as the presence of light at this time would seem to indicate that the circadian system has not done its job, and that there is a mismatch between what the circadian clock anticipated and the apparent time of day. Post-dusk light therefore triggers the circadian clock to reset, suppresses melatonin, elevates heart rate and temperature, and has a direct stimulating effect on the brain related to alertness and performance, all of which are physiological responses associated with daytime in a diurnal (as opposed to nocturnal) animal such as humans. 86 87 Of note, light exposure after dusk in a mouse or rat will enhance sleepiness and decrease arousal, as melatonin and darkness usually correspond with the active part of the day in such nocturnal animals, whereas light is associated with sleep. 7

In the absence of light or in minimal light, the melatonin rhythm is often used as a proxy for SCN activity since direct SCN measurement is not possible. 88 The rapid rise in melatonin production at the start of the biological night apparent in dim light or darkness is often used as the phase marker, and is known as Dim Light Melatonin Onset (DLMO). 89 In less well-controlled field conditions, the more robust rhythm of 6-sulfatoxymelatonin (aMT6s), the major urinary metabolite of melatonin used as an indicator of melatonin activity, is preferable. 90

Cortisol is another important hormone whose daily 24-hour rhythm is controlled by the circadian clock. In contrast to melatonin, cortisol levels naturally rise in the morning, but are slightly elevated with increased morning light. 91 92 93 Overall, cortisol levels begin to rise near the midpoint of sleeping and peak near the time of waking. Cortisol engages the body’s metabolic processes to waking operation. 73 It is also the “fight-or-flight hormone”, which the body releases in response to threats. As such, an occasional spike in cortisol may be necessary and good, but long-term elevated cortisol levels stress the body’s systems. Urinary cortisol measures can be used to assess the underlying circadian rhythm without the influence of the temporary “spikes” due to stress, along with aMT6s. 94 90 66 Figure 14 shows how the circadian rhythms of melatonin and cortisol in the blood stream relate to alertness and time of day.

Figure 14: Cortisol And Melatonin Cycles Over Two Days.76 93
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Properties of Light and the Circadian System

Light has several properties that determine its ability to influence circadian rhythms or to induce acute non-visual responses. These include light intensity, duration, timing, pattern, light history and spectrum (wavelength). These factors largely have been studied separately for their unique contributions to non-image-forming mechanisms induced by light, but the way they interact with each other is also beginning to be examined more closely. This is necessary given that in real-world conditions, most of these factors contribute simultaneously to a net response to light.95


One way to assess effects on the circadian system is to observe the phase shift that occurs. Phase refers to “relative” time; relative to the current rhythm or cycle.87 In humans, phase is usually defined from a consistent point in the cycle (such as the onset, peak, or minimum). A phase relationship describes the alignment (or lack of) between two phase markers (e.g., the melatonin onset and the external light-dark cycle, or the relative timing of the melatonin onset and sleep).87

Light can either phase advance or delay the circadian pacemaker, depending on the timing of the light exposure.

Assuming normal conditions, a phase delay occurs when light exposure happens late in the day or during the early night (about 1800-600 h). Light instigates a phase advance if the light exposure occurs late in the night or early in the day (about 600-1800 h). This means that timing affects the direction of a phase shift. Timing also affects the magnitude of the resultant shift, with the circadian system being most sensitive to light during the night. The relationship between these variables (stimulus timing, effect direction, and effect magnitude) can be described by a Phase Response Curve (PRC). 96 Some animal species appear insensitive to light during the middle of the subjective day, yielding a “dead zone” in the PRC. Humans are continuously sensitive to light with a low sensitivity occurring in the middle of the subjective day. Specifically, this means the circadian pacemaker is continuously sensitive to light, underscoring again how important it is to ensure proper periods of darkness when the body needs darkness. Note that circadian resetting responses occur during the day when melatonin is not being produced and cannot be suppressed, and therefore circadian resetting and melatonin suppression are not interchangeable when considering how the properties of light affect non-image-forming responses (see Direct Effects of Light).98


A phase shift’s magnitude is largely affected by the intensity of the light stimulus that triggered the shift. There is a non-linear relationship between light intensity and the magnitude of phase shifting, wherein exposure to relatively dim indoor lighting (about 100 lux, 4,100 K) induces 50% of the effect stimulated by exposure to illuminance 10- to 100- times greater (with timing and duration kept constant—at night for 6.5 hours in one study). 87 98 99 100 Overall, the circadian system is minimally responsive to increases in illuminance levels above about 1,000 lux, and saturates at about 550 lux, at least at night.87 98 99 100 Prior light exposure may alter this threshold slightly but in general, the human circadian pacemaker is sensitive to relatively low light levels commonly produced by electrical indoor illumination. 8798 99 100 This means it is particularly important to have bright days, and dark nights—a sufficiently bright day means that the body is less sensitive to light at night, and therefore the potential for negative impacts from light exposure in the evening may be attenuated. However, if we experience poorly or insufficiently lit environments during the day, then we are all the more sensitive to any light exposure at night, which can shift our circadian rhythms and impact sleep onset and quality effect. 101 102 103

Duration and Pattern

There is also a non-linear relationship between the duration of light exposure and melatonin suppression, or the magnitude of phase shifting.104 Short exposures to light induce a greater effect per minute than would be expected if the relationship between these variables could be defined linearly. For example, 12 minutes of light exposure (9,000 lux, 4,100 K fluorescent) can produce a phase shift similar to that experienced after an hour of exposure, despite the exposure’s duration being five times shorter. 104

Similarly, the effects induced by continuous light exposure compared to intermittent exposure follow a non-linear relationship as well. One study found that six bright light pulses (about 10,000 lux) lasting for 15 minutes each but delivered once an hour over the span of 6.5 hours at night of otherwise very dim light (<1 lux) induced 75% of the effect compared to continuous exposure to bright light at night (resulting in a 2.3-hour delay compared to a 3-hour delay, respectively). 105 This meant that while the intermittent pulses of light only represented 23% of the total duration of time, this exposure was able to cause about 75% of the phase resetting response observed in the continuous exposure group. 105 Such studies suggest that the circadian system integrates light information over time and substantial benefits from light during the day can be realized even without long durations of continuous light exposure.

Light History

A light stimulus’ effect on the circadian system is dependent on the intensity of light as well as the relative change in intensity. Prior immediate light exposure modestly desensitizes responses induced by subsequent light stimuli. 101 102 103 The inverse is true as well: prior exposure to dim light or darkness my lead to modestly heightened sensitivity to light later on. 106 107 The circadian system therefore adapts to a given light based on individual light history.

These responses are not huge, changing sensitivity by about 15%, but they could be important in optimizing lighting to enhance or minimize its effect. 101 102 103 At a cellular level, ipRGCs undergo similar adaptation, and this process may be both wavelength- and intensity-sensitive. 108 The time course of this adaptation response is not yet fully understood and further research is necessary, but initial studies suggest that the circadian system integrates light information over hours, days, and potentially longer. 101 102 108 103


As previously described, the spectral sensitivity of the circadian system to light, detected and transduced by melanopsin, is blue-shifted relative to photopic and scotopic systems. Multiple action spectra for melatonin suppression and pupil constriction responses in humans and other primates show a peak in the blue visible range (about 480 nm). 70 57 109 66 Further, comparisons of 460 nm and 555 nm monochromatic, equal photon density light at night show that 460 nm light is two times as effective at inducing phase delay shifts, melatonin suppression and alerting the brain in both sighted and blind individuals compared to 555 nm light. 110 111 66 82 112 Similarly, early morning exposure to polychromatic blue light has also been shown to be at least as effective at stimulating phase advances compared to exposure to cool white light of 185-fold higher photons. 113

Direct Effects of Light

In addition to resetting the circadian clock, light has a number of acute “non-visual” effects, some of which are important when considering light’s practical applications.

Light is an acute stimulant that directly alerts the brain during the day and at night, without going through circadian rhythms.86 This has been shown in a number of ways including higher subjective alertness ratings, improvements in a range of cognitive performance tests, changes in EEG brain activity patterns that indicate a more alert state, and activation of brain areas that mediate alertness, such as the thalamus and brainstem, in functional magnetic resonance imaging (fMRI) studies. The alerting effect is dependent on the intensity of light and the wavelength, with similar sensitivities to those described for circadian resetting and melatonin suppression.114 When controlled for photons, short-wavelength light will improve multiple markers of alertness more than longer wavelength light, for example.115

Light has potential to be used to improve both daytime or nighttime alertness, productivity, and safety in any settings where light is used. 6

Light also has antidepressant properties and has been used to treat clinical affective disorders.116 117 118 119 It may also have use, however, in improving mood in non-clinical settings in a similar manner to the alerting effects. Some fMRI studies have shown activation of the amygdala in response to light, a brain area that helps to regulate mood.120 These responses are best triggered by blue-biased light—or more specifically, light that corresponds to 480 nm, the wavelength to which ipRGCs are most sensitive and therefore become activated. Conversely, when an alerting response is not desired, then neither is blue-biased light, and so high red-content or amber-like light, to which ipRGCs are not as sensitive, may be preferred for evenings if light is needed at all.

As explained previously, light also induces a number of other responses such as stimulating cortisol production, increasing heart rate and temperature, and controlling pupillary constriction. It can even stimulate circadian clock gene expression.121 122 These are “day-associated” responses in diurnal humans and so light exposure at night will cause activation of such daytime physiology as well.

While not yet fully understood, there is an increasing understanding of the range of effects that light induces and the pathways that mediate them. Given the only-recent discovery of the ipRGCs, it is likely that many of the “non-visual” effects of light that were previously thought to be visual responses will turn out to be mediated by the melanopsin-based system.

Elements of Light

This section outlines the elements of light and lighting environments that impact the human body based on how indoor spaces are designed and constructed. Humans react to different parameters of light in ways that affect comfort and health. The elements of light are interdependent and less meaningful in isolation.

Light-Dark Pattern

Lighting designs will vary based on different motivations or desired outcomes. In general, light exposure and darkness, at different points, are of equal importance. An underlying principle is to ensure a 24-hour pattern of both light and darkness. 95 The circadian system expects to be exposed to very stable, regularly timed light days and dark nights, just as in nature, and this should be a founding principle for good lighting design.

An optimal daytime lighting environment incorporates strategies that address visual performance as well as light’s biological effects.

1. Circadian Daytime Lighting

Many factors are intimately connected, so efforts may not be effective if certain relationships are not considered. In other words, the success of any one solution is closely dependent on how lighting is handled in other ways, and the context that the lighting design serves. As one study states, “an important note of caution here is that it is not always clear whether lighting design should aim to maximize or minimize non-visual responses.”123

For example, when designing lighting for shiftworkers, lighting that promotes high alertness and performance may be desirable in the short-term, but repeated exposure to light at night may have longer-term health risks. Whether to avoid the alerting effects of light, through filters or glasses, is also a difficult question; while avoiding light on the drive home after night shift work may help circadian adaptation to the night shift, reducing the light’s alerting effects at this time may increase the risk of less alert driving. It is important, therefore, that the consequences of lighting design are considered to their full conclusion for both visual and non-visual responses. The solutions presented here may aid in creating alerting and energizing environments, but promoting such strategies outside of a normal day-night cycle should be weighed against the importance of maintaining an overall 24-hour pattern of light-dark.

Health Effects

Most of the considerations for the health effects of daytime lighting are to ensure optimal visual function and to take advantage of light’s acute alerting effects. While optimal circadian entrainment requires stable, robust, and distinct light-dark, day-night cycles—and therefore light during the day—these effects are discussed under Health Effects—Light at Night.

Nervous System

Age-related non-visual sensitivity. The eye lens naturally yellows with aging, resulting in reduced transmission of shorter-wavelength light to the retina. This poses a concern that the aging populations are at higher risk for inadequate light exposure and ensuing melatonin suppression. However, there is evidence that the photoreceptor system may adapt to this change by slightly lengthening the peak sensitivity of the circadian photoreceptor system. 124 Cataracts, also more common in older people, block the transmission of light across the entire spectrum. These cases may warrant a recommendation to increase light exposure duration or intensity as appropriate, while taking the precautions to ensure that glare does not become problematic. Additionally these “adapted” populations may benefit from increasing exposure to light with shifted peak spectral power (e.g., away from 480 nm to 490 nm) to ensure that enough light of the right wavelength reaches the retina. 124

Alertness and cognition . Light is an acute stimulant and therefore can directly help to increase alertness and performance directly during the day. High-intensity white light improves reaction time and subjective alertness when compared to dimmer light exposure. 125 126 127 128 129 130

Studies examining the alerting effects of individual wavelengths of monochromatic light during the day provide insight into the photoreceptor mechanisms involved. Several fMRI studies have shown that short exposures to blue light (< 1 minute) can activate brain areas involved in alertness, arousal and mood more easily than violet or green light, suggesting that melanopsin-based photoreception is active in the day-time. 6 Monochromatic short-wavelength blue (460 nm) light during the day can improve reaction time and lapses of attention compared to the same photon density of 555 nm green light, although subjective ratings of sleepiness do not differ. These performance-enhancing effects were accompanied by suppression of activity in the theta/low alpha frequency band (6–9 Hz), a response indicative of higher alertness and consistent with previous white light studies during the night. 99

Further, there are a number of real-world studies showing improvements in alertness and performance following installation of blue-enriched polychromatic light sources. 131 132

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Some “dynamic lighting” programs using variations in light spectra have been tested in schools, with a number of different patterns and settings. Although the studies vary in their size, duration and lighting characteristics employed, the results are broadly consistent indicating that students perform better in standardized tests of concentration, reading speed, and comprehension when under dynamic lighting. 133 134 135 The general scheme of these ‘dynamic lighting’ systems is to provide an option to switch between a “standard lighting” mode and an “enriched lighting” or “focus” mode. In these studies, the standard mode tended to be 3,000–4,000 K at about 300–500 lux and the blue-enriched lighting varying from about 5,500–11,000 K from 300–12,000 lux. Some plans also included short exposures to lower intensity, and lower CCT lighting to promote calmness temporarily (for example after breaks or lunch).

Similar advantages of higher CCT or bright light on measures of sleepiness have also been shown in college-aged students during the daytime,136 particularly in autumn, and during evening exposures.137 Exposure to polychromatic blue-enriched white light sources later in the evening have also been shown to increase alertness and performance in laboratory settings, and these are discussed in more detail under Light at Night.

Depression, mood and fatigue. Light also has antidepressant properties that are used therapeutically in a clinical setting,116 including blue or blue-enriched light.117 118 119 In clinical populations, light therapy should only be recommended and performed under supervision of a physician.

Similarly the alerting effects of light have been used in populations suffering from daytime fatigue as a non-pharmacological approach to improve alertness and quality of life. For example, blue and blue-enriched white light has been used to improve fatigue in patients following a traumatic brain injury 138 and in patients undergoing chemotherapy.139 Blue-enriched light may therefore be useful under non-clinical conditions to improve general mood and feelings of wellbeing.

Overall, a fundamental design principle critical to circadian-friendly daytime lighting is ensuring access to daylight, and supplementing this with electric light sources.


Adequate light exposure during the day is necessary for basic visibility, and for timely and accurate visual performance. Furthermore, healthy circadian rhythms depend on adequate light exposure during the day. What is considered adequate, however, may depend on several factors, including the task at hand and the subject’s age.

1. High Electric Illumination Levels

Receiving an adequate amount of light is especially important in the morning to help with circadian alignment (see Light at Night) and in the afternoon to counteract the common mid-afternoon “slump” in alertness. This does not necessarily suggest that lighting levels must be constantly tuned throughout the course of the day, however—having blue-enriched light all day may achieve this goal.

The American National Standards Institute (ANSI) has approved IES standards documents for various environments, including the American National Standard Practice for Office Lighting (RP-1-12). This document recommends illuminance targets based on the typical tasks undertaken in a given environment, and based on the visual age of the observer. Meeting illuminance targets between about 150 lux to 1,500 lux is advised.10 However, these lighting levels are set using photopic lux, a lighting metric based on the visual photoreceptor system (i.e., cones), and thus may not represent the most appropriate solution for the circadian photoreceptor system.

2. Maintaining Equivalent Melanopic Illuminance (Lux)

A novel metric proposed by Lucas et al., 2014 presents a function for calculating equivalent melanopic lux. 123 This unit can be found by multiplying the photopic lux with a melanopic ratio, the latter of which depends on the type of light source and the color temperature of the source. Overall, achieving at least the same amount of melanopic lux as photopic lux (i.e., maintaining a melanopic ratio of one) represents an energy-efficient solution for lighting design. A melanopic ratio of one indicates high activation of the relevant parts of the spectrum for circadian considerations, and so it represents an energy efficient way to introduce light into the space for circadian impact without simply increasing overall illumination levels.

3. High CCT Lamps

Traditional US lighting practice tends to favor office lighting levels up to 4,100 K, but office lighting color temperature of 5,300 K or higher may be considered during the standard workday. 26 Many of the studies referenced in Health Effects—Circadian Daytime Lighting found alerting effects with 6,500 K lights.

4. Reflective Surfaces

Walls, ceilings and floors that are reflective help distribute light throughout a space to maintain brightness for visual performance. Luminance balance for alerting lighting can be met through reflective walls, ceilings and floors that help to distribute light across a space 140 10 and should be designed to maximize, or at least not diminish, reflectance of shorter-wavelength light. 141

5. Daylight

Windows and skylights introduce natural light into a space, balancing out the use of electric lighting. Access to daylight can also involve a consideration of controlling the amount of daylight experienced by people in buildings. In addition, ensuring that work or recreational areas are not too far from windows can be an important consideration for allowing each person to receive adequate sunlight.

Defining lighting levels by using daylight as a reference also helps to circumvent many issues concerning phase shifting, as sunlight provides exactly the quality of light for duration of time optimal for the human body. 142 143

5. Wellness Walks

Walking outside is one way people can get exposure to bright light during the day, which may help to improve alertness. Even intermittent exposures via several short walks throughout the day are helpful to achieve these ends.

2. Light at Night

Light exposure at night can be problematic. Any exposure to light after dusk can be considered unnatural compared to a natural light-dark cycle. When the eyes, and therefore the brain, detect light at night, the brain interprets it as a signal of daytime and induces “daytime” physiology. Given that humans are diurnal mammals, daytime physiology promotes being awake, alert and active and includes elevated heart rate, temperature, brain activity, alertness and performance. 78 The brain also shifts the timing of the circadian clock as light at night is interpreted as suddenly being asleep at the wrong time of day (i.e., the day) and tries to reset the clock as quickly as it can (interestingly, light induces sleep in nocturnal rodents as exposure to light at night would mean that they are active at the wrong time of day).

The majority of these “non-visual” effects are mediated by stimulation of melanopsin although the visual rod and cones photoreceptors can also contribute. Rods and cones can project to the ipRGCs 144 35 and the visual system can affect non-visual responses in humans. 82 This means that rods and cones represent a path by which the circadian system is engaged under certain circumstances, particularly under dimmer light conditions and at the start of a light exposure. 82

Health Effects

Nervous System

Alertness and sleep disruption.

Light is a stimulant.99 145 5 146 86 The direct alerting effects of light, while beneficial during the daytime, are disruptive at night. Exposure to visible light at any time of day will have an alerting effect, but the magnitude of the effect will depend on light intensity and wavelength. Higher intensity and shorter wavelength light is more alerting.99 145 5 146 86

The alerting effects of light can affect sleep in two ways. If an individual wakes during the night, for example to go to the bathroom, exposure to light will make it more difficult to go back to sleep. These alerting effects continue for at least several hours after the light is turned off. For example, blue light exposure ending at 11:30 p.m. has been shown to affect sleep initiated at 1:15 a.m.147 Evening light exposure in the hours before bedtime increases the time taken to fall asleep, reducing sleep duration, and suppresses slow-wave (deep) sleep, reducing sleep quality. These effects are greater with higher intensity and shorter wavelength light.148 129 149 150

Reducing the duration and quality of sleep compromises the recovery obtained from sleep and results in sleepiness and performance problems the next day. This reduces productivity and increases the risk of accidents and injuries.151 Longer-term, habitually short sleep has been associated with an increased risk of heart disease, diabetes, depression and some cancers.151

Circadian photoentrainment. While daytime light exposure is necessary to maintain circadian rhythm synchronization and to promote alertness, productivity, and health, the circadian system also requires periods of darkness for every 24 hours. Exposure to stable cycles of light days and dark nights are optimal for circadian synchronization and the systems it controls such as sleep and metabolism. 152 153 2 In addition to increasing alertness and making it difficult to fall asleep, evening light exposure will cause the circadian system to be entrained at an abnormal phase, as illustrated in comparisons of rhythms in individuals with and without access to electric light in the evening. In a study comparing circadian rhythms in individuals living at home, with electric light, or when camping without it, campers’ melatonin rhythms shifted earlier by several hours and the melatonin rhythms peak was correctly aligned with the middle of the biological night. 154 Collectively, these studies illustrate the profound systematic impact that electric light has on melatonin levels, circadian rhythm disruption and sleep quality, in addition to shifting our sleep to a suboptimal circadian phase each and every day.

There are many environments where ensuring robust and appropriately timed circadian entrainment may be beneficial, particularly in places where there may not be a strong light-dark cycle, such as care homes, hospitals, schools and prisons.

Studies have shown benefits of increasing daytime light exposure in the common areas of care homes on the time course of dementia, sleep and depression 155 156 including blue-enriched light. 157

“Light days and dark nights” is the principle upon which 24-hour light-dark cycles should be based. Exposure to variable light-dark cycles or light at night can disrupt circadian rhythms, sleep and have adverse health outcomes. These adverse effects are best illustrated by the most extreme example of disruption to circadian entrainment as observed in shiftworkers.

Circadian rhythm and sleep disruption—Circadian rhythm sleep disorders. Circadian rhythm sleep disorders (CRSD) are typified by sleep disturbances that have to do with a misalignment between internal circadian rhythms and the desired or appropriate time for sleep, affecting the timing or duration of sleep. There are several types of CRSDs, including Jet Lag Disorder, Shift Work Disorder, Advanced Sleep Phase Disorder and Delayed Sleep Phase Disorder. 1 158 Some of these disorders are largely environmentally induced, for example Shift-Work Disorder and Jet-Lag Disorder, in which individuals often must sleep and wake at an adverse circadian phase, whereas others are caused by misalignment between the biological drive for sleep and the 24-hour solar or social day (Delayed Sleep Phase Disorder, Advanced Sleep Phase Disorder, Non-24-hour Sleep-Wake Disorder). 1 158 159 Sleep patterns which do not show a regular 24-hour cycle are also classified as circadian rhythm sleep disorders (Irregular Sleep Wake Rhythm) and include those associated with a medical or neurological condition, such as dementia. 1 158 160

Light can help those whose sleep schedules are out of alignment with their internal circadian clock and/or the external light dark-cycle. Light therapies to shift Circadian rhythm are based on the principle of the PRC (as outlined in Light and the Human Body; light can either phase advance or phase delay the circadian system depending on the timing of exposure. Under normal conditions, light exposure in the later day/early night causes a phase delay of the pacemaker whereas light exposure in the late night/early day will phase advance the clock.87 158 Delayed Sleep Phase Syndrome can be combatted by the use of half an hour of bright lights in the morning and regularly timed bedtimes in the evening.159

Endocrine System

Hormonal rhythms. Many hormones are under circadian control including melatonin, cortisol, prolactin, growth hormone, parathyroid hormone, and thyroid stimulating hormone. 161 162 Inappropriate light exposure at night can cause circadian disruption that may alter hormonal function or disrupt local or homeostatic processes. Light’s disruptive effects on sleep may also disrupt hormones under the control of sleep, affecting endocrine activity related to pituitary and adrenal hormones. 163 161

Melatonin production. Melatonin is affected by light in two ways. First, the daily pattern of melatonin is strongly circadian. 164 100 Appropriate entrainment of the circadian clock is required to ensure appropriate entrainment of the melatonin rhythm. Note, melatonin’s rhythm is controlled entirely by the circadian clock and will persist in the absence of light, e.g. in complete darkness or in total blindness. 90 Neither a light-dark cycle, nor light, is needed to generate or maintain the melatonin’s rhythm. What light will do, however, is acutely suppress melatonin when light exposure occurs at night. 164 100

The intensity of light regulates the level of melatonin in circulation and therefore the functions mediated by the hormone. Exogenous melatonin (e.g., oral consumption) has been shown to induce drowsiness in humans, though this effect is limited to when endogenous levels are low. 165

Since melatonin suppression is strongest with short-wavelength blue light, 70 57 111 82 higher color temperature light sources are more effective at suppressing melatonin than lower color temperature (“warm white”) lights until a saturating dose is reached. 167 166 168 For example, tests with common lamps showed that 40 lux of 6,500 K fluorescent light for two hours reduced melatonin levels in subjects to 40% below the levels achieved by similar exposure of 3,000 K incandescent or 2,500 K fluorescent lamps. 129

Cortisol production. Like melatonin, cortisol is also under strong circadian regulation. It is also affected by acute and homeostatic short-term changes. Due to these similarities, the same principles apply to the generation and maintenance of the cortisol rhythm as outlined above for melatonin.

There are apparently conflicting data on the acute role of light on cortisol. Evidence shows that light will suppress cortisol during the biological night 43 but light exposure may elevate levels directly in the morning.91 These effects may not be incompatible however, and may reflect a circadian gating in the acute response related to time at which exposure occurs and expectations of cortisol levels at that time of day. Given that elevated cortisol is considered a stress response, one could also term light exposure that increased cortisol as a stressor, consistent with the general activation and mood elevation associated with acute exposure to light.

Digetsive System

Metabolic disorders. The circadian clock controls many processes and products of metabolism, including cholesterol, glucose, insulin and cortisol. 169 170 171 172 173 174 As such, disruption of circadian rhythms from inappropriate circadian entrainment will also disrupt metabolic processes, including glucose and lipid metabolism. The same meal eaten at night will cause higher post-meal glucose, insulin and fat levels in the blood compared to when the same meal is eaten in the day, and this may underpin the higher risk of diabetes, obesity and heart disease in shiftworkers. 169 170 In non-shiftworkers, exposure to more robust light-dark cycles (brighter days and darker nights) may improve metabolism of carbohydrates, likely due to better internal and external synchronization of circadian rhythms in digestive and metabolic processes. 171 Longer term disruption of metabolism may lead to elevated levels of glucose, insulin and fats and increase the risk of weight gain, insulin resistance, and diabetes. 172 173 174

Carbohydrate and lipid metabolism. There is evidence that light exposure may have a regulatory role in the metabolism of lipids and carbohydrates. 175 In one study, subject groups were exposed to 2,000 lux versus 20 lux during a meal at 5 p.m. in the evening. The 20-lux group showed improved absorption of dietary carbohydrates compared to the 2,000-lux group. 171 Further research indicates that exposure to bright light in the evening does not impact carbohydrate digestion the next morning. 176 Thus, bright light exposure during the day and dim light exposure during the evening may be beneficial for digestion of evening meals. 171 175

Cardiovascular System

Cardiac rhythms. Many cardiac rhythms are under the control of the circadian system, including heart rate, heart rate variability and blood pressure. There is a diurnal rhythm in the timing of heart attacks, which peaks in the morning, which may be in part due to circadian rhythms in cardiac function and related processes. 173 Disruption of circadian rhythms through inappropriate light-dark cycles may therefore affect cardiac function. Notably, shift workers, who regularly disrupt their light-dark cycles and circadian entrainment, have a high prevalence of cardiovascular disease. 177 Exposure to high light levels at night also elevates heart rate directly in addition to shifting the timing of the clock. Repeated exposure to light at night may therefore also impair cardiac circadian rhythms and cardiac function. 178

Immune System

Immune function. Aspects of the immune system are under circadian control, such as the daily rhythms in some cytokines and chemokines, or counts of T-cells and other lymphocytes. Disruption of circadian rhythms from inappropriate circadian entrainment will therefore disrupt immune function. 179 Poor sleep also affects immune responses and so disruption of sleep by inappropriate light exposure after dusk or during the night may also contribute to impaired immune function. 180 181

Reproductive and Endocrine System

Breast and prostate cancer. Disruption of circadian rhythms has been associated with increased risk of hormone-dependent cancers. 182 183 184 Proposed mechanisms for the link include disruption of circadian clock organization, either between the internal clock and the external environment, or disruption of internal clock relationships; disruption of sleep; and suppression of the hormone melatonin by light. 185 183 184

In non-shiftworkers, sleep duration has also been considered a proxy for dark duration (as people tend to be in light when awake) and shorter sleep duration or sleep problems have been shown to be associated with an increased risk of breast 186 and prostate cancer. 187 Similarly, there is epidemiological evidence that exposure to greater light at night is associated with increased breast cancer rates. 188

Cancer and shift work. Shiftworkers who work at night and sleep during the day are at particular risk for a degraded circadian rhythm, as their sleep-wake and light-dark cycle are in opposition to the internal circadian rhythms. 189 In theory, one can fully adjust to sleep during the day and wake at night if one stayed on night shifts permanently, without days off; in practice, this is more difficult.

Shiftwork involving circadian desynchrony was designated a “probable carcinogen” by the World Health Organization (WHO) in 2007. 190 191 192 Shiftwork alters many aspects of our physiology; as outlined above, eating at night impairs metabolic function. Disruption to sleep quality and duration is virtually guaranteed in shiftworkers when they sleep in the day in order to be awake at night. Light exposure at night is inherent to shiftwork—without light at night, there cannot be work at night, and as outlined above, exposure to light at night disrupts circadian rhythms and acutely suppresses the hormone melatonin There are therefore multiple factors affecting physiology and metabolism in shiftworkers.

While the mechanisms are not fully understood, there is strong evidence of a link between light exposure and melatonin: suppression of melatonin via constant light exposure or removal of the pineal gland increases tumor growth in a dose-dependent manner in experimental animal models, and the presence of melatonin can inhibit or slow down tumor growth. 193194

Further investigation of the relationship between light exposure at night and cancer risk is underway. It is possible that a greater understanding may lead to the development of technological solutions to optimize shiftworkers need to see at night, while reducing the risk of melatonin suppression or cancer. Although the causal link between light exposure and cancer prevalence in shiftworkers is not fully conclusive, a precautionary approach would minimize the shiftwork entirely, maximizing sleep and dark duration and minimize light exposure at night.


Light at night should be abolished if possible. If light is required, it should be as dim as possible and red-wavelength enriched.

1. Evening Lights

A policy statement published in 2012 by the American Medical Association (AMA) advocates the use of dim, red lights in the evening. 195 Lights should be as red and as dim (low intensity) as possible during the evening. 195 Ideally, a gradual dimming and reddening of the light should occur from dusk until bedtime, or earlier if bedtimes are earlier (for example in children). If this is not possible, as long a duration of dim and red-enriched light before bedtime as possible should be promoted, along with restrictions on use of electronic devices.

2. Night Lights

Night lights can aid in navigating through a space when an individual wakes up during the middle of the night. Light sources should not be directly visible to assuage the relative perception of brightness. The AMA advocates the use of dim, red lights as nightlights. 195 Lighting to aid orientation, for example pathway lighting along floors to aid walking 196 or around doorways to aid orientation upon waking 197, should also be as low as practically possible and in the yellow to red end of the spectrum. Electronic devices emitting light, such as bedside radios, should have red displays or be covered during sleep.

3. Evening Digital Detox

The use of electronic devices in the evening introduces bright, blue-enriched light to users’ eyes during a time of day when such light will have negative effects. Even brief, intermittent exposure to bright light at night can have phase-shifting and alerting effects and should be avoided, particularly when an alerting effect is not desired. 95 In general, electronic devices, including TVs, phones, tablets and computers, should not be used in the bedroom, and should be avoided when gradually dimming and reddening the light in the evening. There should be a distinct break in the use of such devices for as long as possible before bed, as a rule of thumb at least 30 minutes and ideally longer, as the alerting effects may persist for several hours after lights are turned off. 147 150

4. Blackout Shades and Eye Masks

The non-visual alerting and circadian resetting effects of light are mediated exclusively through the eyes and therefore blocking light to the eyes can prevent its effects. Blackout curtains or shades can be used to block external light sources, and eye masks used to block internal light sources. Traveling with an eye mask may help prevent negative effects of light in non-standard environments.

5. Morning Lights

Blue, blue-enriched, or polychromatic dawn-simulating lights can be used in the morning to promote alertness and circadian alignment.198

6. Shift workers

Lighting for shiftworkers should be designed to promote performance and safety during night work through higher intensity and/or higher blue-wavelength content light. This is in contrast to what is typically recommended for daytime workers specifically because shiftworkers uniquely require alertness in the evening, both for during their work hours and for any alertness required during travel after shifts.

Aesthetic and Ergonomic Lighting

Visual abilities depend on proper ocular function in receiving light input, and also on the properties of the visual targets and of the lighting environment in general. It’s important to consider the interplay between physiological capabilities and our buildings’ components and structure to learn how to optimize conditions not just for adequate visual perception, but also for creating a rich visual scene. Visibility requires a minimum threshold of luminance, which primarily engages the photopic visual system via cones. This may be achieved by incorporating proper electric lighting with adequate daylighting strategies.

Lights’ spectral composition and intensity distinctly engage photosensitive cells to guide image-forming vision, which includes the ability to perceive differences in brightness and color. These factors play a huge role in our enjoyment of a visual scene, both in terms of aesthetics and comfort upon viewing.

1. Illumination

Visibility refers to the ability to detect objects or patterns within some defined distance, as facilitated by an appropriate lighting environment. It is influenced by the relationship between four factors:199

  1. task luminance;
  2. background contrast;
  3. task size;
  4. observer age and eye function.

As these factors suggest, the luminance level required for visibility is largely context-dependent.

There is an initial relationship between improved visual performance and increased luminance, but the effect of luminance plateaus at a certain point, beyond which increasing luminance does not render increased benefits in visual ability, as measured in terms of the speed and accuracy with which an observer completes a task. 200 199 While these and other studies have focused on luminance, task luminance is generally dependent on room illumination, thus the element of interest here is illumination see Properties of Light

Health Effects

Comfort and Focus

Visual performance. There have been a number of experiments over the decades exploring the suprathreshold of visual performance, or, the threshold of luminance, which represents a sufficient stimulus for a visual response. One of the main practical goals of such experiments has been to establish whether a relationship exists between higher luminance and visual performance, and if such a relationship could be leveraged to create lighting environments that promote greater productivity. For example, in the school studies referenced above, simply seeing better might help classroom performance and shorter wavelength light facilitates better visual responses.201

While several studies support the observation that visual performance improves with higher luminance, it is clear that there is a low threshold over which the magnitude of measurable improvement plateaus. The relative visual performance (RVP) model describes this relationship, assessing visual performance against contrast and luminance.202 200

Age-related visual sensitivity. The RVP model is affected by the age group of the study participants from which the model is derived. Several studies show that there are age-related changes in vision, which require different conditions and luminance levels to accommodate adequate visual performance. These changes begin to noticeably affect vision around age 40. 199 It is still unclear exactly what factors contribute to the decline in visual ability, though many point to increasing lens density, pupillary miosis (constriction), and a mix of other optical, cortical, and retinal changes that occur over time. 203 204

Older individuals (65 and older) not only tend to have poorer visual performance compared to young counterparts in scotopic light levels, but studies have also found that spatial contrast sensitivity in photopic light levels also declines with age. 205 206 207

Older eyes also take longer to adapt to differences in light levels. While a slight adaptation phase is normal for any person, changes in the eye that occur with age render it less sensitive to changes, requiring a longer response time. 208

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1. Adequate Illumination Levels

The ANSI/IES document American National Standard Practice for Office Lighting (RP-1-12) recommends illuminance targets based on the typical tasks undertaken in a given environment and on the visual age of the observer. Meeting illuminance targets between about 150 lux to 1,500 lux is advised, depending on the task and on the average age of the person in the particular space.10

2. Task Lighting

The lighting levels achieved by a task light should be factored into the space’s overall illuminance to achieve general illuminance target levels.210

3. Lighting Uniformity

The luminance ratio between any two points within the same room should not exceed 1:10. The luminance ratio between a task and its immediately adjacent surroundings or visual display terminal (VDT) should not exceed 1:3.10

2. Color Quality

The quality of the colors we perceive depends on the interaction between three variables: the light source, the target object, and the viewer. Color quality is a function of the source’s spectral output, the object’s spectral absorbance/reflectance and the sensitivity of the eye’s photoreceptors to different wavelengths of light, which it perceives as color.

Man-made lights create whiteness through different SPDs to illuminate colored objects in a way similar to how they would appear under natural daylight. This is because sunlight is considered to be “full-spectrum” light, meaning its spectral distribution spans wavelengths fairly uniformly across the visible range of the electromagnetic spectrum. “Full-spectrum” lighting is ideal for optimizing a visual scene’s color quality because it is always able to complement the way an object absorbs and reflects specific wavelengths, resulting in specific colors. However, lamps are often not characterized by uniform and wide-ranging spectral distribution; many have spikes and dips in wavelengths such as the fluorescent source in Figure 15.

Figure 15: The Spectral Output Of A Typical Low CCT Fluorescent Lamp.211
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The spectral output of a lamp affects the environment’s color quality by the extent to which it can accommodate its targets’ spectral absorbance and reflectance. Under the fluorescent lamp shown in Figure 16, for example, an object that would normally reflect back yellow would suffer in the color quality perceived by our eyes.

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Health Effects

Comfort and Focus

Experience. Light sources with good color rendering ability facilitate greater accuracy in color vision and contribute to a person’s comfort level within a space. A reduction in CRI from 100 to 80 would not likely yield a significant difference in an individual’s experience, nor would it greatly affect color discrimination abilities. A reduction to a CRI of 40 however would result in the majority of viewers feeling uncomfortable and experiencing a negative aesthetic impact.213

There are a number of social, cultural and other variables that may affect a person’s lamp preference in terms of CCT. One review notes that in residences, people reported a preference for “medium white” color temperature over “cool white” or “warm white.” However, the definition of these types of white lamps varied by manufacturer, and the overall finding may simply be that consumers tend to prefer “medium” colors.214 There are many competing theories to explain the basis of color preference, ranging from ecological valence theories premised on affected responses to color-associated objects, color-emotions and evolutionary theory.215 216 217

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The following solutions must be given simultaneous consideration to create an optimal lighting environment for color. Focusing on just one may not constitute an adequate strategy for enhancing color quality in a space.

1. High CRI

Higher CRI values are an imperfect but useful way to ensure that the colors of a space are accurately portrayed and saturated. The Illuminating Engineering Society recommends illuminants with a CRI of 80 or greater for general office lighting.10 The EPA’s Energy Star program also has criteria set at CRI ≥ 80 and R9 > 0 for LED bulbs. 219

2. “White” Chromaticity Range

CCT values between 2,700 K and 6,500 K represent a wide range of color temperatures as they approach the “whiteness” of natural light. The spectral output of a light source should be considered alongside the CRI; for example, the CRI of candlelight is higher than many other light sources even though its color (1,700 K and very yellow-orange) is outside the range generally considered white.

3. Broad Spectral Power Distribution (SPD)

If the SPD of a light source is broad (energy emits fairly continuously across the full visible spectrum), then the color rendering ability of that source is likely good.140

3. Glare

Glare is defined as either “excessive brightness of the light-source, excessive brightness-contrasts, and excessive quantity of light”. 220 It is caused by light scattering within the eye (intraocular scattering), thereby creating a “veil” of luminance that reduces the luminance contrast as received by the retina . The effects of glare can range from slight visual discomfort (discomfort glare) to visual impairment or even injury (disability glare). 221 222

Health Effects

Nervous System

Headaches and migraines. There is some evidence of a relationship between migraine headaches and visual stress and stimuli. 223 224 People with migraine headaches report increased sensitivity to glare compared against control groups. 225 In one self-report study, 38.8% of children with migraines and 54.9% of children with tension-type headaches reported glare and brightness as triggers. 226 While the neural mechanisms by which migraines occur is unclear, there is evidence that the exacerbation of migraines by light exposure may involve the ipRGCs. 227

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Comfort and Focus

Visual Discomfort . Although glare is a facet of lighting design, the threshold for discomfort due to glare is largely subjective. Sensitivity to glare appears to peak at around 510 nm to 550 nm, resulting in visual discomfort that can be “manifested as annoyance, squinting, distraction, blinking, tearing, and light aversion”. 222


Many of these solutions work in conjunction to minimize glare in a space. 228 Glare can be one of the most distracting elements of poor lighting design, and adopting effective strategies against glare can contribute greatly to overall comfort and focus for people inside buildings.

1. Direct/Indirect Lighting

Direct/indirect lighting refers to lighting that distributes some percentage of the light towards the ceiling (uplight) and some percentage downward (downlight), which represents a strategy for avoiding the creation of glare, while also still providing sufficient ambient lighting. The ratio between the brightest and darkest area (e.g., the luminance ratio) of the ceiling should not exceed 10:1. 10

2. Shielding Angles

Lamps should be shielded in order to help prevent glare. Recommended shielding angles vary based on the luminance of the lamp in question. 26

3. Matte Furnishings

Matte furnishings are preferred whenever appropriate and possible to reduce veiling reflections.

4. Shading Devices

Blocking out or significantly reducing intense light that comes through windows can control solar glare.

5. Variable Transmission Glass

Lessening the intensity of the light that comes through windows is a way to control solar glare. Solutions that accomplish this include micro-mirrors and special types of glass that reduce light transmission.

4. Flicker

Light flicker refers to “quick, repeated changes in light intensity,” the frequency of which determines whether or not the flicker is discernible to the average person.229 Typically, individuals can see flicker up to about 50 Hertz (Hz), or 50 cycles per second. While 50 Hz may represent the cutoff for visible flicker, it is possible that the eye may respond to flicker at higher frequency, too.229 This threshold for visual flicker, known as the flicker fusion threshold, can be up to 90 Hz, depending on the conditions under which the light is viewed. The human brain may respond to frequencies higher than 100 Hz.230

Health Effects

Nervous System

Headaches and migraines. Visual stressors are associated with potentially triggering and exacerbating migraine and other headaches.223 225

Visible flicker, among other environmental lighting stimuli, has been implicated in processes related to headaches.230

Comfort and Focus

Visual Discomfort. Flickering lights are rated by many as annoying and may be a cause of visual discomfort.231


Solutions for flicker are fairly straightforward and may do a great deal to reduce annoyance, visual discomfort or headaches.

1. Electronic Ballasts

As noted by the Canadian Centre for Occupational Health and Safety, electronic ballasts can reduce flicker in fluorescent lighting. 229

2. Regular Lamp Replacement

Older fluorescent and discharge lamps tend to flicker more often. Keeping on schedule for lamp replacements is a simple strategy for limiting flicker. 229

Explanations of Solutions

High Electric Illumination Levels

Illumination levels can facilitate appropriate visual capability and alerting effects on people within buildings. Workplaces require high levels of light, in part for visual acuity, but even more importantly for circadian and hormonal effects. Given the large fraction of the waking day many people spend at work, insufficient illumination can have negative impacts on the circadian system.

Visual performance in typical, daily-life-related or work-related environments are organized into three age groups: (A) half of the people are less than 25 years of age; (B) half of the people are between 25 and 65 years of age; and (C) half of the people are greater than 65 years of age.

Since these values are specific to offices, IESNA use desktop illuminance as a reference. Standard desktops are estimated as a horizontal surface 0.75 m (2’-6”) above the floor. 10

There is a declining return on very high light levels, both in visual performance and the direct alerting effects of light.199

Workers exposed to 2,500 lux rated subjective alertness as higher than 500 lux, but physiological alertness and subjective well-being, mood, and satisfaction with the lighting was unchanged. 232

Since the variety of electric lights span many spectral outputs, their effect on non-visual responses varies as well, based on the output in the wavelengths to which the ipRGCs respond. Electric sources of “white light” can vary in circadian impact by a factor of 1.4 to 1.9. 233 129

Because of the small size of the light emitting components, LED modules can provide dynamic spectral adjustment by having multiple LEDs in the same lighting unit and controlling the brightness of individual diodes. Figure 16 shows the spectral output of two LED lamps. This makes LEDs a useful tool for circadian emulation lighting. The high efficiency can sometimes be achieved by focusing the output of the light on the parts of the spectrum to which retinal photoreceptors are most sensitive.

Figure 16: Spectral Output of Two LEDs, With a High Blue and Low Cyan Output.
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LEDs sometimes have high circadian effect, compared with an equivalently bright fluorescent or incandescent source. Figure 16 above shows the spectral output of two LEDs at different color temperatures. Both the 2,700 K and the 4,000 K lights have a strong “blue spike” of light very close to the frequencies to which the circadian system is most sensitive. This means that even warm white LEDs can have unexpectedly strong melatonin suppression, given their color temperature. For incandescent lamps, the blackbody emissions of 2,700 K peak at about 1,100 nm (in the infrared), meaning that the bulk of an incandescent lamp light is outside of the visible range. Most traditional lamps are able to produce 10–17 lumens per watt; halogens, due to their slightly higher operating temperature, are more efficient, putting out 12–22 lumens per watt.234

Maintaining Equivalent Melanopic Illuminance (Lux)

Current lighting design and measurement is founded on the extent to which the three-cone photopic system is stimulated by an amount of light measured in terms of the unit lux for describing illuminance . Due to this photopic weighing however, lux values are inadequate for describing the amount of light stimulating the circadian system—although, so long as the light source remains constant, lux can quantify relative changes in light amount. The development of circadian-appropriate units and companion light sensors is a promising new field of work. The success of these efforts may go a long way in improving our ability to design lighting environments that are biologically and ergonomically compatible with the way our eyes and bodies interact with light.

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High CCT Range for Daytime

High CCT lights can have an alerting effect through their high blue component. However, there is a problem inherent in using CCT alone to describe spectral content: two lights with the same CCT can have very different spectra and therefore different blue content, yielding different circadian efficacy.

6,000 K to 8,000 K LEDs with a “spectral composition leaning towards the short wavelength (blue) end of the visible spectrum” may be good candidates for biologically effective lighting. As a rule of thumb, 5,300 K or higher lights are cooler, closer to “daylight white”, and may be good for alerting effects. 26

Reflective Surfaces

Light reflected off walls comes from a very large surface area and is of low luminance . While it does not appear especially bright, light emitting from a large surface area can cause the total illuminance reaching the eye to be quite large. As a result, highly reflective walls can have a substantial impact on the circadian rhythm .

In general, walls in workplaces should have a Light Reflectance Value (LRV) of 70% or greater for high reflection of light and energy efficiency. 210 IES-NA RP-1-12 notes that in offices, walls should achieve at least 30 cd/m2. 10 Darker surfaces tend to absorb light, while lighter surfaces can support a feeling of brightness.

The General Services Administration’s (GSA) Facilities Standards for the Public Buildings Service (P-100) further recommends LRV values for interior spaces. 210 For ceilings, walls, and floors: 210


Achieving high-intensity light with general electrical lighting is energy-intensive, but natural light can supplement electric light during daytime hours, greatly increasing illumination without adding to power consumption. Exposing individuals to adequate daylight ensures that they not only receive broad-spectrum light, but that light exposure aligns with the solar day and contributes to circadian photoentrainment.

From a design and behavior perspective, windows and skylights are the easiest way to expose the body to sunlight’s synchronizing and anti-depressive effects.

Having sufficiently large windows to allow appreciable amounts of sunlight is one of the most straightforward and cost-effective strategies for ensuring adequate light levels during the day. A daylight factor (DF) of 2% to 5% is preferable for an office space. 143 DF is a ratio of the illuminance in a space from natural light compared with the illuminance of diffuse daylight outside.143

Strategies for optimal window placement are a well-studied engineering and architectural subject that is described in greater detail by several prominent lighting organizations. The primary challenge in good window design is to allow for maximal light transmission while minimizing heat transmission. In the northern hemisphere, south-facing windows generally accrue more heat. Additionally, while daylight is an excellent source of light for biological effects during the day, the positive benefits of this must be measured against potential issues of glare and solar heat gain.

Rooms with large windows that face the sun for part of the day reduce recovery time in hospitals from severe depression 235 and after heart attacks,236 compared to rooms with windows facing buildings or other obstructions.

Wellness Walks

Taking short walks outside throughout the day is a simple way to benefit from the alerting nature of daylight. Even brief, intermittent exposure to sunlight may have an energizing effect on individuals.

Evening Lights

In 2012, the AMA adopted recommendations regarding indoor lighting based on studies that indicated that nighttime lighting was harmful to health, and may be linked to breast cancer among other serious health issues. 195 Low-intensity, red-shifted can help to minimize unwanted phase shifting. Lights should be kept as low and as warm as comfortable for evenings.

Programmable lighting is increasingly available for residential and other settings, and software is available to change the properties of the light emitted by electronic devices. Such approaches are likely to be beneficial by automating the lighting changes and not relying on individual to remember to change the lights or do it by hand. The ambient light environment can have a powerful impact on behavior (for example, see the camping study described in Health Effects—Light at Night). There are also screens for use on electronic devices or individuals can choose to wear glasses that filter short-wavelength light entering the eye but these will require active, rather than passive, participation to gain the benefits of light changes.

In the short-term, these approaches should increase evening sleepiness, make it easier to fall asleep and improve sleep quality. Longer-term, dimmer and redder light in the evening may phase advance the circadian clock, further facilitating earlier sleep and a longer sleep duration.

Night Lights

Night lights help mitigate sensitivity when adapting to different light environments. Assuming an individual goes from complete or near-complete darkness to light, the heightened sensitivity of photoreceptors in such a scenario can be somewhat assuaged by lower-intensity and longer-wavelength light. There are however conflicting findings based on laboratory and observational studies on what minimum thresholds of light trigger phase-shifting effects. 237

Lighting conditions typified by less than 10 lux engage the scotopic system and can be used to facilitate adequate vision at night. Red or amber light is preferred for night lights to avoid the alerting effects of blue or blue-enriched light. This means sources up to, but preferably lower than, 3,000 K.

Evening Digital Detox

The body begins to produce melatonin several hours before it falls asleep. During this time, the body has a strong phase response. Exposure to light during this time can therefore substantially delay the circadian system.

People often use televisions, computers and other electronic devices until close to the time they go to bed. In 2011, the National Sleep Foundation reported that 90% of Americans surveyed (N = 1,508) used an electronic device in the bedroom an hour before sleeping. 238

These products can produce light of a wavelength that is close to the maximum circadian sensitivity. LED screens can have twice the output of 464-nm light as equivalently bright non-LED backlit screens and create a larger increase in alertness and attention. 148 The use of tablet computers for two hours at night has been shown to diminish melatonin levels, although one hour of exposure did not cause a significant change. 239 Keeping computers at least 14 inches from the face and using a lower brightness setting for the screen reduces the risk of melatonin disruption. 240 150 Stopping use of electronic devices at least an hour before bed may be a good strategy for improving sleep onset, duration and quality. Additionally, there may be some technologies that change the color temperature of the light on devices throughout the day, shifting to warmer temperatures towards the evening.

Blackout Shades and Eye Masks

Blackout shades and eye masks prevent the melatonin-inhibiting blue wavelengths of light from disrupting sleep, and overall blackout shades allow greater control over home lighting. Eye masks are helpful to block out light specifically during the night in preparation for and during sleep. For blackout shades, motorized controls can allow for timed, automatic use. It is important to avoid light leakage around the perimeter of the shades, since this can be an oft-overlooked source of light pollution at night.

Blackout shades are less useful during times when residents desire some level of natural light. When lowered halfway, they do succeed in bringing in a limited quantity of light, but this approach is less than ideal. The uncovered portion of the window will seem disproportionately bright, especially in contrast to the completely dark upper section.

Morning Lights

The alerting and phase advancing properties of exposure to morning light, particularly blue or blue-enriched morning light, or a polychromatic dawn-simulating light, can be used to support shifting the circadian clock to an earlier phase, or improving impaired morning alertness and potentially mood from sleep inertia.198

Shiftworker Lights

Shiftworkers are at a high risk of sleepiness-related accident and injury and therefore lighting should be designed to promote performance and safety during night work through the acute alerting effects of light, i.e., through higher intensity and/or higher blue-wavelength content light.

While it has been postulated that avoidance of light, or blocking blue-wavelength light, in the morning after the shift will enhance circadian adaptation to shiftwork, 241 this is not advisable if driving given the high risk of sleepiness when driving home at this time. 242 243 In practice, adaptation to shiftwork is rare given common shift patterns and preference to remain on a “day” schedule during days off and vacations.

Adequate Illumination Levels

ANSI/IES lighting requirements for visual performance in typical, daily-life-related or work-related environments are organized by three age groups: (A) half of the people are less than 25 years of age; (B) half of the people are between 25 and 65 years of age; and (C) half of the people are greater than 65 years of age. 10

These age groups are further categorized by task with recommendations for lux at the vertical and horizontal plane.10

Task Lighting

In environments typified by a wide age range or range of visual ability, task lighting may be a solution to accommodating the needs of diverse populations without increasing the space’s overall illuminance levels.

Targeted task lighting can provide the necessary light at the workspaces, while not over-illuminating hallways and the spaces between desks. Ambient light levels of 300 lux are sufficient. Adjustable direct task lighting coupled with indirect or diffuse ambient lighting allows users customization and good visual acuity while providing a more subtle and agreeable background light. 26

Lighting Uniformity

Distributing light evenly throughout a building can help to minimize the need to adapt and adjust vision to different intensities, which is particularly important for older eyes. 208 This applies to lighting levels within a room and between rooms in corridors. A minimum of 100 lux is recommended for corridors and stairways. 26

Another strategy for lighting uniformity depends on contrast ratios within a room, and RP-1-12 recommends upper limits for luminance contrast ratios. Abiding by these ratios can help reduce the adaptation time needed to adjust between different light levels. As noted previously, luminance ratios should not exceed 3:1 or 1:3 between a task and the immediate adjacent surface, and luminance ratios should not exceed 10:1 or 1:10 between any two points in a room. 10

High CRI

Higher CRI values are an imperfect but useful way to ensure that the colors of a space are accurately portrayed and saturated. As noted previously, a CRI of 80 is typically adequate for general office work. IES notes that illuminants with a CRI of 90 or greater are recommended for environments where precise color discrimination is particularly important. 10 140

The GSA also recommends CRI values:

“White” Chromaticity Range

The “whiteness” of daylight has to do with the polychromatic nature of the light, meaning that lighting sources that adequately simulate this “whiteness” incorporate some mix of several wavelengths, making them better suited to accommodate the spectral absorbance/reflectance of any given object.

White light can be loosely categorized into three groups:26

Broad Spectral Power Distribution

SPDs for light sources show the wavelength composition for particular lamps, and can be narrow and sparse or broad. The power distribution indicated by a source’s SPD is a good indicator of its color rendering ability, wherein a broad and continuous (lacking spikes or dips) distribution is likely to facilitate good color quality in a visual scene.140

Direct/Indirect Lighting

While some buildings may prefer one type of lighting style over another (e.g., relying on a direct lighting approach), designing a system that relies on both direct and indirect lighting helps off-set the shortcomings typical of one approach. For example, direct lighting leaves the ceiling dark, affecting the overall perception of brightness. On the other hand, indirect lighting can make the space feel flat, and loses some of the aesthetic appeal and accenting qualities of direct lighting.

There are many ways to integrate elements of both direct and indirect lighting into a space. In a space relying primarily on direct lighting, even pushing just 10% or 20% of the total lumen output to the ceiling with a reflectivity of at least 65% may be sufficient to facilitate a positive result. If the space relies primarily on indirect lighting, then the addition of a few controlled downlights may be helpful to create highlights or shadows in specific areas. 10

Shielding Angles

As general rules, overhead glare is most bothersome at angles above about 55˚ to 60˚ with respect to the observer’s vertical field of view, and a direct view of the lamp should be avoided. In particular, most people will likely perceive bare lamps with a luminance 12,000 cd/m² to 16,000 cd/m² and above as glaring. 140 Since the output of a point-source is concentrated in a small area, improper use will create glare. Recessing ceiling overhead lights and shielding task lights minimize this issue. High luminance levels are especially distracting on computer and other electronic screens.

Figure 17 suggests luminance levels for various minimum shielding angles based on European recommendations. 26

Figure 17: Suggested Shielding Angles For Recessed Lights To Avoid Glare.26
expand this figure

Matte Furnishings

While reflective walls, ceilings and floors are good for purposes of illumination, matte finishes and furnishings can reduce glare in a space.26

Shading Devices

Solar shades provide a good solution for glare while preserving the view out the window. These shades are made of semitransparent weaves that block 80% to 99% of incoming light of all wavelengths (the remaining percentage is the openness). They not only reduce glare, but also block thermal radiation, reducing a building’s cooling and heating requirements.

Other shading devices can include regular shades, blinds, awnings and overhangs. Any combination of such tools can help with periodic blocking of glare-inducing sunlight through windows. 140

Variable Transmission Glass

Variable transmission glass is an alternative to solar shades. These window panes can dynamically change their opacity through the use of electrochromic materials, suspended particles, liquid crystals or micro-blinds. While most of these technologies cannot achieve 100% light blockage, they can be used to limit solar glare and heat gain through windows.

In the morning and evening, the sun is low in the sky on the east or west sides of a building. In the winter at midday, the sun will be low in the southern sky for much of the northern hemisphere (and northern sky in the southern). This will cause significant glare through windows on these sides, making shades or adjustable transmittance glass helpful.

Concentrated high levels of light can cause visual discomfort at desks by creating glare. Direct glare results from bright light sources that are in a person’s direct field of vision. Indirect glare is caused by lights reflecting off of work surfaces.

To manage the effects of direct and indirect solar glare , workstations should be oriented such that workers face a direction parallel to the plane of the nearest window.244

Designers may also want to consider placing low transmittance glass on windows facing the east, west, and south, letting less sunlight through, while north-facing windows can have higher transmittance. Likewise, lower transmission windows (30% to 40% transmission) may be preferable when they are level with viewers, while windows located higher up can be good for daylighting and can allow more light to come through (70% to 80% transmission).

Electronic Ballasts

Ballasts control the voltage and frequency of the voltage supplied to fluorescent lights. While magnetic ballasts cannot change the frequency, electronic ballasts can convert the supplied power to a higher frequency, between 20,000 Hz and 60,000 Hz.229

Regular Lamp Replacement

Older lamps tend to flicker more often. Replacing lamps on a regularly scheduled basis is an easy way to avoid flicker due to old bubs. 229 However, with LEDs and long-life fluorescents, this issue can be avoided.

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In one study, subjects were asked to judge which light between two options appeared brighter, with luminance ranging from 30 cd/m2 to 67 cd/m2.61 In a comparison between the luminance level of a lamp at 52 cd/m² and the same lamp at 40 cd/m², the former was judged brighter 115 out of 120 times, as expected. However, upon comparing metamers—lamps that appeared similar but were different and had different SPDs—10 out of 12 subjects judged the light source that was 33% photopically dimmer as seeming brighter. This photopically dimmer light emitted a greenish-blue light with a peak at 505 nm—peak sensitivity for the scotopic system. When the same ratio was tested again but at higher luminance levels, nine out of 12 subjects again judged the light source that was 33% photopically dimmer as brighter. Brightness equality between the two metamers occurred at 49 cd/m² for a warm white fluorescent and 30 cd/m² for the scotopically-weighted illuminant.61 These findings suggest that rods do indeed appear to contribute to perceptions of vision.

In one cross-over study set in an office, 17,000 K blue-enriched white light fluorescent lamps were compared to conventional white 4,000 K fluorescent lamps across two floors of a building. Participants experienced both lighting conditions for one month each. Subjective reports showed improvements in alertness, mood, evening fatigue and sleep quality, alertness during the day, focus, concentration, eye discomfort and headache, corresponding to the high CCT lamp in the office setting.132

Another parallel-designed study compared a blue-enriched 17,000 K lamp with a 2,900 K lamp across two floors of an office, randomly assigning subjects to experience one condition for 14 weeks.131 Consistent with the findings of the previous study, participants under the 17,000 K condition reported improvements in concentration, fatigue, alertness, vitality and mental health. Neither study reported any short-term negative side effects.131

In a field study that measured the luminance levels 51 workplaces and assessed legibility performance measured in terms of speed and accuracy using a number of tasks, the researchers found that for one task, the longer duration task, older workers took 6% less time with additional lighting, while the comparison younger group did not see an improvement in performance with additional lighting.209

As described in Light and the Built Environment, correlated color temperature (CCT) describes the overall color output of a lamp by comparing it to a blackbody of a specific temperature. Color rendering index (CRI ) considers the color output of the lamp at its temperature, and compares it against an ideal version of a lamp of that temperature based on a blackbody radiation function. Given that CCT describes only the color of the light itself (and not how it reflects off objects), and that CRI is always relative in nature, there are limitations in the way these measures describe color quality if considered in isolation. While there continues to be on-going research on the development of other metrics, CCT and CRI represent current industry standards for estimating the color accuracy rendered by a lamp.212

One study compared various CCTs of lights kept constant at around 500 lux. Preferences for lights ranged between 3,500 K to 5,500 K when they were judged in terms of how pleasant they found the visual scene, and also how bright and colorful they found the visual scene. Overall, the study concludes that there did not appear to be a single, definitive “preferred spectrum for office lighting”.218

One study compared 21 individuals with migraines and 19 individuals with tension-type headaches and 21 healthy control subjects to evaluate discomfort thresholds to various wavelengths of light.224 At medium wavelength light, people with migraines and tension-type headaches had similar thresholds for discomfort, and controls had a higher discomfort threshold (meaning they were not as sensitive to the light). At low and high wavelength light, individuals with migraines had the lowest threshold for discomfort compared to both the tension-headache and control groups. When unfiltered white light was applied, the migraine group had the lowest discomfort threshold, followed by the tension-type headache group, and then the control group. These findings suggest that there is some relationship between glare and headaches.224

A study on visual environmental stimuli assessed the impact of glare, flicker, pattern and color on migraines. Study participants were 1,044 women who completed a questionnaire, and whose responses were compared against 121 female controls. Participants frequently complained of glare as a visual stressor.225

Novel metrics are being developed for the measurement of light weighted to the circadian system, which is primarily mediated by the ipRGCs but also receives input from rods and cones.123 Such metrics facilitate an ability to consider the effects of light on the circadian system, as opposed to focusing lighting decisions solely on considerations to accommodate visibility.123

Continued research will help to pave the way for consensus on a single unit of light measurement. This effort is contingent upon the successful development of a suitable spectral weighting function for non-visual responses spanning circadian, neurobehavioral and neuroendocrine factors which achieved industry adoption.

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