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Circadian Lighting –  

 An Engineer’s Perspective

by  | Nov 14, 2019 | Health & Architecture | Published: 19/11/14

Whether you call it “circadian lighting,” “biologically effective lighting” or some other name, the principle is the same: the color and intensity of light can be used to regulate the timing of our biological clocks, or “circadian rhythms.” For architects and lighting designers, this is an opportunity to provide healthy and comfortable environments for building occupants.

Light Transmittance Through Greenhouse Glazing

by  | Nov 14, 2019 | Horticulture | Published: 18/10/01

Look at a greenhouse manufacturer’s product specifications and you will see that the light transmittance of single-pane clear glass is typically 88 to 91 percent. Compared to double-wall polycarbonate with a transmittance of 80 percent, it would seem that glass is the better choice. However, if you measure the photosynthetically active radiation (PAR) at the leaf canopy within the greenhouse, it is often only 40 to 60 percent of that measured outside the greenhouse. Why is this?

Far-Red Lighting and the Phytochromes

Most LED grow lights feature blue and red LEDs whose peak wavelengths – approximately 450 nm for blue and 660 nm for red ­– have been chosen to coincide with the spectral absorption peaks of chlorophyll A and B molecules. In doing so, they optimize the conversion of electrical energy into plant photosynthesis.

Some manufacturers, however, are now offering grow lights with “far-red” LEDs that feature peak wavelengths of approximately 735 nm. Unfortunately, they offer little if any information on why these LEDs are useful.

In order to make an informed choice when purchasing these grow lights, it is necessary to understand some of the science behind far-red radiation and how plants perceive and respond to it.

The Science of Near-Infrared Lighting

There is a common-sense argument being presented in the popular media that since humans evolved under sunlight, our bodies must surely make use of all the solar energy available to us. Given that more than 50 percent of this energy is due to near-infrared radiation, we are clearly risking our health and well-being by using LED lighting that emits no near-infrared radiation whatsoever.

Melanopic Green

Numerous medical studies have shown that exposure to blue light at night suppresses the production of melatonin by the pineal gland in our brains and so disrupts our circadian rhythms. As a result, we may have difficulty sleeping. It is therefore only common sense that we should specify warm white (3000 K) light sources wherever possible, especially for street lighting.

Solid Angles

Do you suffer from math anxiety? A surprising number of us do (e.g., Wigfield 1988). I would tell you the exact numbers, but you would need to understand statistical analysis …

Fortunately, we can mostly muddle through our lives without having to deal with statistics, vector calculus, differential geometry, algebraic topology and all that. As an electrical engineer in the 1980s for example, I never needed anything more than a four-function calculator to do my work designing billion-dollar transportation systems. 

Understanding Mesopic Photometry

If you are involved with outdoor area or roadway lighting design, you will undoubtedly encounter such terms as mesopic multipliers, scotopic lumens, and S/P ratios, and you will sooner or later need to consider mesopic photometry in your design efforts. For example, you may encounter a government specification that states, “Luminaires must have a minimum S/P ratio of x.” What does this mean and, equally important, where can you find this information?

Thoughts on Color Rendering

UPDATE 14/10/06 – LightingEurope, the “Voice of the Lighting Industry,” has just published their LightingEurope Position Paper on Color Quality. To summarize:

  1. LightingEurope supports to continue the use of the existing Color Fidelity metric CRI including eight reference colors.
  2. LightingEurope supports to keep legal minimum requirements on CRI on the current level as defined in the EU Eco-design Regulation.

The Kruithof Curve

UPDATE 16/04/09 – This metastudy:

Fotois, S. 2106. “A Revised Kruithof Graph Based on Empirical Data,” Leukos. (Published online 08 April 2016, DOI 10.1080/15502724.2016.1159137.) critically examined 29 studies in which the Kruithof curve was investigated. The author concluded that “… these [studies] do not support Kruithof. For pleasant conditions, these data suggest only avoiding low illuminances and do not favor any CCT.”

Sports Lighting Regulations

This blog article has a somewhat frustrating history. About a year ago, I was asked to volunteer my time to write a primer of light and color as it relates to sports lighting regulations. I was told the name of the organization I was volunteering my time for, but I did not pay much attention — it seemed like a good cause.

I should have perhaps paid more attention before agreeing to volunteer — the Green Sports Alliance is not the poorest of socially responsible organizations.

Upon completing the primer, I was told that it was far too technical for its intended audience. Hopefully, you as my readers will disagree.

Solar Illumination

Lighting design is based in part on the reasonable assumption that photometric units have precise definitions. The candela, for example, has a precise mathematical definition:

“The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.”

and from this all other photometric units are derived, including luminous flux, luminous intensity, luminance, illuminance, and so forth.

You might expect then that the same applies for daylighting design … but you would be wrong.

Seeing Ultraviolet

UPDATE 15/11/08 – The following text briefly notes that some people can see near-ultraviolet radiation following cataract surgery due to the UV transmittance of their artificial intraocular lens. An example of this is reported in considerable detail here.

What does it mean to “see?” The word is ubiquitous in the English language, with dozens of different meanings. However, according to the Oxford English Dictionary, the most common definition is to “perceive with the eyes.” It is so common, in fact, that it ranks as one of the thousand most frequently used words in English.

“I see,” said the blind man, “what you mean.”

Rethinking the Photometric Data File Format

If you perform lighting design calculations today, you can thank the efforts of the IES Computer Committee (IESCC) some thirty years ago. Its members recognized an industry need, and so developed and published IES LM-63-86, IES Recommended Standard File Format for Electronic Transfer of Photometric Data. With the growing popularity of the IBM Personal Computer for business applications, it was an idea whose time had come.

The need was clear: Lighting Technologies (Boulder, CO) had released its Lumen Micro lighting design and analysis software product in 1982, and luminaire manufacturers needed to provide photometric data for their products. For them, IES LM-63 was a god-send in that it established an industry-standard file format7.

Phytochrome and PSS

Horticultural lighting is currently one of the fastest-expanding markets in commercial lighting, with projected revenues of several billion dollars in less than a decade. From the perspective of a professional lighting designer, the market opportunities are enticing. Whether it is lighting for greenhouses or vertical farms and plant factories, the basic principles of lighting design remain the same.

Photometry and Photosynthesis

UPDATE 15/04/13 — This article was first published on December 10, 2014. A revised version was published as “LED Lighting for Horticulture” in the Mar/Apr 2015 issue of LED Professional Review ( This update includes information from the published article.

UPDATE 15/11/05 — due to several Excel spreadsheet errors, the lux-to-PPFD conversion factors presented in Table 2 were miscalculated. These errors have been corrected.

UPDATE 16/01/12 — the description of the Emerson effect has been corrected.

UPDATE 16/02/10 — Added discussion of calculating lumen-to-PPFD conversion factors for overcast skies, as well as expanded notes and references on green and ultraviolet LEDs.

Mobile Light Pollution

At first glance, this appears to be an innocuous question:

How much light pollution is attributable to automotive headlights?

It is also a good question in that if we are to address light pollution, we need to know what causes it. For this, we first need to look at the U.S. Department of Energy publication, 2010 U.S. Lighting Market Characterization (DOE 2010).

Section 4.2.4, Outdoor Results, tabulates the estimated number of outdoor lamps and their wattages nationwide by application in Tables 4.27 and 4.28 (Fig. 1), while Table C.2, System Efficacy Assumptions, tabulates the lamp efficacies (Table 1).

Mesopic Photometry and Statistics

One of the joys of statistics is that you can never be proven wrong …

In a previous All Things Lighting article titled “Understanding Mesopic Photometry” (October 8th, 2013), I wrote:

Some publications on mesopic lighting have indicated that the S/P ratio of a lamp can be estimated from its correlated color temperature (CCT), but this is incorrect …

I continued on with an example that compared the spectral power distributions and scotopic-to-photopic (S/P) ratios of a phosphor-coated white light LED:

Light Pollution and Uplight Ratings

“Oh, East is East, and West is West, and never the twain shall meet.”

When Rudyard Kipling wrote this line in his poem The Ballad of East and West (Kipling 1892), he was referring to cultural misunderstandings between the British and their colonial subjects in India (where “twain” means two). As a proverb, however, it has worked equally well for the lighting industry and the astronomical research community for the past four decades.

The meeting concerns light pollution, wherein roadway and area lighting contribute to the diffuse sky glow that limits our ability to observe the stars at night. The International Dark-Sky Association (IDA) has campaigned since 1988 to limit the use of outdoor lighting, and to employ luminaires that are designed to limit undesirable spill light. Unfortunately, the equivalent of cultural misunderstandings have until recently worked against this effort.


LICASO and DAYSIM are daylighting analysis software programs that perform climate-based annual daylight simulations, including the calculation of annual daylight metrics. This white paper compares their performance in terms of accuracy and calculation times. Average illuminance and spatial daylight autonomy values agree to within 8 percent, while LICASO is approximately 250 times faster than DAYSIM for a benchmark model.

Kruithof Revisited

To summarize the article, “The Kruithof curve itself was thoroughly debunked a quarter-century ago with three exhaustive studies involving up to 400 participants (as opposed to two people in Kruithof’s study, including himself).” These studies were Bodman (1967), Boyce and Cuttle (1990), and Davis et al. (1990).

In other words, the weight of scientific evidence is firmly against use of the Kruithof curve as a guide to modern lighting design practices. Case closed, yes?

In the Blood

We most often think of lighting design in terms of lumens, color temperature, and CRI, but there are occasional situations where a deeper analysis is required. One such situation is as close as your doctor’s office: the examination room.

An examination room is typically windowless and illuminated only by linear fluorescent lamps. In examining the patient’s skin for anything from bruises to lesions, the doctor relies on experience to assess skin color. Anything that influences this perceived color should be a concern, for it could potentially lead to a misdiagnosis.

In Search of Luminance

The IES Lighting Handbook, Tenth Edition (IES 2010), describes luminance as “perhaps the most important quantity in lighting design and illuminating engineering.” This is an accurate but curious description, as the editors neglected to include an entry for Section 5.7.3, Luminance, in the handbook’s index.

The section itself is a mere five paragraphs long, informing the curious reader that luminance is the “local surface density of light emitting power in a particular direction,” defined mathematically as:

Horticultural Lighting Metrics

UPDATE 17/08/26 – This article was first published on August 25th, 2017 in Urban Ag News.

It was all so easy until recently. Plants require light in order to grow, and so we provided them with daylight and/or electric lighting. Given the singular choice of high-pressure sodium (HPS) lamps, we only needed to be concerned about measuring Photosynthetically Active Radiation (PAR) and Daily Light Integrals (DLI).

The introduction of light-emitting diodes (LEDs) and solid-state lighting (SSL) has changed everything. With the ability to independently control the light source spectrum from ultraviolet through visible light to far-red, researchers and growers are discovering that plant species and even cultivars respond differently to different spectral power distributions. From these discoveries are coming ìlight recipesî for optimal plant growth and health.

Giving Light

Giving light … this phrase symbolizes a new philosophy of lighting design, a philosophy in the sense of how we think about the lighting design process. Much like the modernist movement in architectural design a century ago, it offers a reconciliation of lighting design practices with today’s rapid technological advancements and societal changes.

The innovations we are seeing in lighting hardware today are fascinating, but we are as always in danger of seeing these innovations in terms of existing technology. It is much like the first automobiles, which looked just like what they were called — horseless carriages. In some cases, these early and primitive vehicles came complete with buggy whip holders. As useless as they were, these accessories symbolized the inability of designers to fully adopt the new technology of internal combustion (and yes, electric) engines. The horse may have been absent, but it was still basically a 19th-century carriage.

Filtered LEDs and Light Pollution

The problem is astronomical — the blue light emitted by LED roadway luminaires has been shown to contribute to light pollution, especially when cool white LEDs are used. Blue light is preferentially scattered by air molecules, and so the higher the correlated color temperature (CCT), the greater the light pollution problem becomes. It is for this reason that the International Dark Sky Association requires a maximum CCT of 3000K for its Fixture Seal of Approval outdoor lighting certification program.

Sometimes, however, even warm white LED street lighting is not enough. For cities that are in close proximity to astronomical observatories, such as Flagstaff, AZ and the nearby US Naval Observatory Flagstaff Station, any amount of blue light is bad news.

Entraining Circadian Rhythms

There is a fascinating research paper called “Colour as a Signal for Entraining the Mammalian Circadian Clock” that has just been published in the open access journal PLOS Biology (Walmsley et al. 2015). While it is an exceedingly technical paper, the basic premise is this: the change in yellow-blue color ratio during twilight may be more effective in entraining circadian rhythms (at least in mice) than changes in daylight intensity.

Why is this important to professional lighting designers? Well, the answer involves the current interest in circadian-based (or biologically-effective) lighting. Quoting a recent LD+A article called “The Case for Circadian Correct Lighting” (Roos 2015), lighting designers are advised to:

Daylight Factors

Nine out of ten daylight simulation programs agree … and therein lies a story worth retelling.

Daylight in History

The story begins in the sixth century with the publication of Corpus Juris Civilis (“Body of Civil Law”) by order of the eastern Roman emperor Justinian I [Scott 1932]. Written in four volumes, it included the Digest, being extracts from the writings of earlier Roman jurists. Book VIII, Title 2, “Concerning Servitudes of Urban Estates,” includes this legal distinction between daylight and views:

Light is the power of seeing the sky, and a difference exists between light and view; for a view of lower places may be had, but light cannot be obtained from a place which is lower.

Controlling Multicolor LED Luminaires

Color Temperature and Outdoor Lighting

UPDATE: Sports field lighting analysis added 15/10/12.

[An edited version of this article was published as “STREET LIGHTS: Light pollution depends on the light source CCT” in the October 2015 issue of LEDs Magazine.]

Most of you will be familiar with the International Dark-Sky Association (IDA), which was founded in 1988 to call attention to the problems of light pollution. It reminds us that light pollution threatens professional and amateur astronomy, disrupts nocturnal ecosystems, affects circadian rhythms of both humans and animals, and wastes over two billion dollars of electrical energy per year in the United States alone.

Climate-Based Daylight Modeling

daylight, n. The light of day.

Apart from having a wonderfully circular definition in most English-language dictionaries, daylight really is just another form of illumination. As such, most people would expect lighting designers to be able to simulate daylight with the same ease that we simulate electric lighting … but ah, I see you blushing.

We have for the past one hundred and fifty years relied on daylight factors to predict the distribution of daylight in architectural spaces. The daylight factor metric is exceedingly simple to calculate, but it is not very useful in understanding how daylight illuminates an interior space. All it really tells us (and our clients) is whether there will be sufficient daylight to read a newspaper indoors on an overcast day. We have, in other words, good reason to blush.

Botanical Light Pollution

Blue-rich light from LED streetlights, we are told, is the nemesis of professional and amateur astronomers. Blue light is preferentially scattered by the atmosphere, resulting in potentially unacceptable levels of light pollution for astronomical observations. Unfortunately, LED streetlights emit more blue light on a per-lumen basis than the high-pressure sodium streetlights they are rapidly replacing.

Botanists and horticulturalists, however, may choose to differ. For them, it is red light from streetlights that is the problem. Depending on the species and various environmental factors, even low levels of light trespass from roadway and outdoor area luminaires can have harmful effects on both wild and domesticated plants. LED streetlights likewise emit more red light on a per-lumen basis than high-pressure sodium streetlights.

Blue Light Hazards and Television

Blue Light Hazard… Or Not?

As a professional lighting designer, you will likely have read about the “blue light hazard” associated with white light-emitting diodes. You will have seen warnings like this (Willmorth 2014a):

“… long term exposure to blue light at 441nm caused lesions on the retinas of rhesus monkeys,”

and recommendations like this from the same author:

“Use the lowest CCT LED color with the highest CRI available to suit the lighting application – including avoidance of high CCT (> 5000K), low CRI (<80) sources altogether, and eliminate use of blue-light rich products, such as those generating >5500K at <65CRI.”

and even this (Kitchel 2000):