Phytochrome and PSS

Think Beyond Pink

Ian Ashdown, P. Eng., FIES

Senior Scientist, Lighting Analysts Inc. / SunTracker Technologies Ltd.

[Please send all comments to]

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.

FIG. 1 – Horticultural lighting in greenhouses. (Source: Colorado State University)

FIG. 1 – Horticultural lighting in greenhouses. (Source: Colorado State University)

There are, however, design metrics that will be unfamiliar to most lighting designers. One of these – the subject of this article – has the rather unwieldy name of phytochrome photostationary state (PSS). While rarely discussed outside of horticultural research publications, it represents an important concept for horticulturists, and particularly floriculturists.

To understand this metric, it is necessary to review some aspects of plant biology.


The development of plants, from seed to flowering, is very much dependent on the electromagnetic radiation they are exposed to. This developmental process, called photomorphogenesis, is completely separate from the process of photosynthesis. It relies on various photopigments, including phytochromes, cryptochromes, phototropins, and UVR8, to sense and respond to radiation ranging from ultraviolet to near-infrared.

Our interest is in the photopigment family of phytochromes, which are mostly sensitive to red and far-red visible radiation. They mediate the germination of seeds (photoblasty), the growth of stems and leaves toward visible light (etiolation), the time of flowering based on the length of day and night (photoperiodism), the synthesis of chlorophyll for photosynthesis, and more (e.g., Smith 2000). While there are six known members of the phytochrome family, it is convenient to refer to them generically and collectively as “phytochrome.”

Phytochrome exists in two states, or isoforms. In its ground state (identified as Pr), phytochrome strongly absorbs red light, and so appears turquoise-blue in concentrated solution in vitro (Figure 2). When it absorbs a red photon, however, it changes its physical shape to form its physiologically active state Pfr. In doing so, its peak spectral absorptance shifts towards the far-red, with a concentrated solution of phytochrome appearing more greenish in color.

When phytochrome is in its Pfr state, it may absorb a far-red photon and change once again into its Pr  state. This bistable behavior makes phytochrome an ideal biochemical switch, with the Pfr isoform serving as the “signaling” state to the plant. As one example of this biological function, red light typically penetrates several centimeters into loose soil (e.g., Borthwick et al. 1952, Botto et al. 1996). As the sun rises higher each day in the spring, an increasing amount of red light reaches the seeds, until a sufficient concentration of phytochrome switches from its Pr isoform to its Pfr isoform. This signals the cellular mechanisms of the seed that it is time to sprout. If, on the other hand, the seed is buried too deeply, it will never sprout and will eventually die.

FIG. 2 – Phytochrome spectral absorptance. (Source: Sager et al. 1988)

FIG. 2 – Phytochrome spectral absorptance. (Source: Sager et al. 1988)


The existence of phytochrome was first suspected nearly a century ago, when Garner and Allard (1920) studied the effects of day length on flowering plants. They observed that tobacco plants could be made to flower in summer by reducing the hours of daylight with artificial darkening, and that they could also be made to remain in a vegetative state during the winter by providing supplemental electric light. They called this effect photoperiodism.

Some plant species flower only when exposed to short periods of light (such as poinsettias – Islam et al. 2014), and so are called short-day plants, while others flower only after to exposure to long periods of light (such as spinach and radishes), and are called long-day plants. In some day-neutral plants (such as tomatoes), flowering is not regulated by photoperiod.

The reason for these reactions in both short-day and long-day plants is the response of phytochrome to red and far-red light. With short-day plants, exposure to a brief period of light during the night inhibits flowering, while the same exposure with long-day plants promotes flowering. Floriculturists can therefore use supplemental electric lighting to delay or advance the flowering of plants to meet market needs.

The traditional techniques for photoperiodic control include (Boyle 1992):

  • Increasing the day length

While high-pressure sodium (HPS) lamps have traditionally been used for supplemental greenhouse lighting, incandescent lamps have the advantage of being rich in red and far-red radiation. Compact and linear fluorescent lamps have also been used, but their relative lack of red and far-red radiation makes them ineffective for phytochrome response manipulation.

  • Night interruption

The phytochrome response to red and far-red radiation does not require continual exposure. Consequently, the flowering period can be influenced with only a few hours of electric lighting during the night. This has the advantage of being more energy-efficient.

  • Cyclic (intermittent) lighting

If incandescent rather than HPS lamps are used, it may be sufficient to pulse the lighting with a short duty factor, such as one minute every half hour. (The optimal duty factor will depend on the irradiance at the plant canopy.)

  • Shortening the day length

The plants are covered with an opaque material to reduce the irradiance, preferably to the equivalent of less than 20 lux of visible light. Typical materials are black sateen cloth, woven polyolefin sheeting, and black polyethylene films.

The problem with these techniques is that they are mostly trial-and-error with different plant species and greenhouse operation conditions. The goal is to manipulate the plant growth and development through the phytochrome response, but there is no practical means of quantitatively predicting the impact of these techniques.

Solid State Lighting

The introduction of solid state lighting to the horticultural industry has been nothing short of revolutionary. In addition to the energy savings afforded by the use of blue and red LEDs whose spectral power distributions (SPDs) are optimal for photosynthesis (Figure 1), the recent commercial availability of high-flux red and far-red LEDs from manufacturers such as Lumileds, Osram, and Cree means that horticulturists and floriculturists now have the ability to precisely tune the light source SPDs for optimal photoperiod control on a per-species basis.

While the LED manufacturers’ product names vary, the products of interest have peak spectral outputs at 660 nm and 730 nm, corresponding to the peak spectral absorptances of phytochrome isoforms Pr and Pfr, respectively (Figure 2). The key here is that the ratio of red to far-red light can be easily set or varied on a daily basis as required for photoperiodic control. Along with blue LEDs, this ability to precisely control the light source SPD leads to the promise of plant “light recipes,” where the SPD and other environmental factors can be chosen on a per-species basis, and possibly varied over the life cycle of the plant growth and development.

The problem, of course, is that in order to control something, you need to measure it. For professional lighting designers, you also need the ability to specify it.

PSS Metric

The phytochrome photostationary state (PSS) metric was introduced some two decades ago (Sager et al. 1998). It has been mostly of academic interest with HPS and incandescent lighting, but the introduction of LEDs for horticultural lighting has suddenly brought this metric to the forefront as a useful design tool.

The metric is conceptually simple: it is the ratio of the concentration of the Pr isoform of phytochrome to the total concentration of both isoforms:

phytochrome-and-pss-eqn-1    (1)

under constant irradiation by a light source. (The maximum value is less than unity because of the spectral overlap between the two isoforms.)

By itself, this seems of little to no interest to lighting designers – how do you measure the relative concentrations of the phytochrome isoforms in a plant? (It took nearly forty years from the time of Garner and Allard (1920) just to isolate phytochrome in the laboratory – Butler et al. 1959.)

What Sager and his fellow researchers did was to note that each phytochrome molecule could be conceptually modeled as an opaque sphere that fully absorbs any incident radiation. If you measure the spectral absorptance of the molecule in solution and know the concentration, you can calculate the equivalent photochemical cross-section of the molecule for each wavelength. (As an aside, LED manufacturers use exactly the same approach when modeling the optical characteristics of LED phosphors embedded in an epoxy or silicone matrix.)

With this, Sager et al. measured the spectral absorptance of the Pr and Pfr isoforms (reproduced in Figure 2) and expressed the results as phytochrome photochemical cross-sections, measured in square meters per mole (i.e., 6.022 x 1023 molecules), represented as sr and sfr, respectively. Equation (1) then becomes:

phytochrome-and-pss-eqn-2    (2)

where N(l) is the measured spectral photon flux for wavelength l over the range of 300 nm to 800nm.

For photometric test laboratories characterizing horticultural luminaires, all that needs to be done is to measure the luminaire’s relative spectral power distribution. Calculating the PSS metric using the photochemical cross-section data from Sager et al. (1988) in accordance with Equation 2 is then a simple spreadsheet exercise.

For professional lighting designers, it is even simpler: the PSS metric is a direct indication of the ability of the horticultural luminaire to manipulate the phytochrome isoforms. While this will also depend, of course, on the absolute irradiance at the plant canopy, the PSS metric reduces the spectral power distributions of the red and far-red LEDs to a single number that can be specified.

LED Color Binning

We are not done yet! Professional lighting designers are all too familiar with the issue of precision in lighting design metrics. For example, lamp and luminaire manufacturers typically report the CIE General Colour Rendering Index (CRI) using two digits, such as CRI = 92. If you refer to the history of the CRI metric’s development, however, you will learn that the intended precision of this metric is five units (van Trigt, 1999). In other words, the difference between CRI = 90 and CRI = 92 is visually imperceptible and so meaningless in terms of practical application.

The same question must be asked of the PSS metric. Is, for example, PSS = 0.39 quantitatively different from PSS = 0.38? Perhaps surprisingly, this is not a question for horticultural researchers. Rather, it is a question for lighting researchers and the lighting industry.

The underlying problem is a familiar one: LED color binning. Taking Lumiled’s LUXEON SunPlus 20 series of horticultural LEDs as typical examples, we have:

Table 1 – LUXEON SunPlus 20 peak wavelength binning. (Source: Lumileds 2016)

Product Name Peak Wavelength (nm)
Minimum Maximum
Deep Red 655 670
Far Red 720 750

The Lumileds product datasheet provides typical spectral power distributions for these two products with typical peak wavelengths, which can be digitized and shifted to represent the minimum, typical, and maximum peak wavelength SPDs, as shown in Figures 3 and 4. These figures also display the red Pr and far-red Pfr phytochrome spectral photochemical cross-sections (i.e., their spectral absorptances), with the SPDs normalized to the peak cross-sections for display purposes only.

FIG. 3 – Lumileds SunPlus 20 Deep Red

FIG. 3 – Lumileds SunPlus 20 Deep Red

FIG. 4 – Lumileds SunPlus 20 Far Red

FIG. 4 – Lumileds SunPlus 20 Far Red

From this information, and assuming that the peak spectral photon intensities of the red and far-red LEDs are the same, the phytochrome photostationary state (PSS) metric values can be calculated as follows:

Table 2 – Example PSS values range (2-nm resolution)

  Minimum Typical Maximum
PSS 0.3563 0.3797 0.4036

In other words, the PSS value may vary by approximately ±6 percent for a given luminaire manufacturer’s product. This is useful information for lighting designers when specifying or qualifying horticultural luminaire products, similar to the meaningful precision of the CRI metric.

There is one further question to address. Sager et al. (1988) reported the photochemical cross-section values sr and sfr at 2-nanometer intervals. This is useful from an academic perspective, but perhaps not so much from that of a photometric testing laboratory. Unless the laboratory performs the LED spectral power distribution measurements in-house, it is likely that the SPDs will be available in 5 nm increments only. While this data can be interpolated at 2-nm intervals for the purposes of calculating the PSS metric in accordance with Equation 2, will the difference in calculated results be significant?

To answer this question, the sr and sfr values published in Sager et al. (1988) were interpolated at 5-nm intervals using a cubic spline approximation, and Table 2 was recalculated using 5-nm resolution for the LED spectral power distributions:

Table 3 – Example PSS values range (5-nm resolution)

  Minimum Typical Maximum
PSS 0.3639 0.3873 0.4033

In this situation, the PSS value may vary by approximately ±5 percent for a given luminaire manufacturer’s product. More significantly, the PSS value for 5-nm resolution was only two percent higher than the PSS value with 2-nm resolution.

These results will of course vary for different typical PSS values, but likely not significantly.

As rules of thumb, therefore:

  • Differences in PSS values of less than ±5 percent are likely not significant.
  • It likely does not matter whether the PSS values are calculated using 2-nm or 5-nm resolution.

The qualifier “likely” recognizes that, while the PSS metric is some two decades old, greenhouse operators have yet to make use of it as a production tool. Future experience may indicate that these rules of thumb are too lax. In the meantime, however, the above analysis provides some guidance for both lighting designers and horticulturists.


Horticultural lighting presents interesting opportunities for professional lighting designers. It is a rapidly developing field where the use of blue and red LEDs for optimal photosynthesis is only the beginning. Solid state lighting has energized horticultural research into plant responses to light sources with different spectral power distributions, and there will surely be discoveries that improve our understanding of both photosynthesis and photomorphogenesis, as well as improvements in horticultural lighting design.

In the meantime, the phytochrome photostationary state (PSS) metric is an example of existing knowledge that will likely prove useful in designing and specifying horticultural lighting systems.


Borthwick, H.A., S. B. Hendricks, M. W. Parker, E. H. Toole, and V. K. Toole. 1952. “A Reversible Photoreaction Controlling Seed Germination,” Proceedings of the National Academy of Science 38:662–666.

Botto, J. F., R. A. Sánchez, G. C. Whitelam, and J. J. Casal. 1996. “Phytochrome A Mediates the Promotion of Seed Germination by Very Low Fluences of Light and Canopy Shade Light in Arabidopsis,” Plant Physiology 110:439-444.

Butler, W. L., K. H. Norris, H. W. Siegelman, and S. B. Hendricks. 1959. “Detection, Assay, and Preliminary Purification of the Pigment Controlling Photoresponsive Development of Plants,” Proc. National Academy of Sciences 45:1703-1708.

Boyle, T. 1992. “Photoperiod Control Systems for Greenhouse Crops,” Floral Notes 4(6):2-4. (See also Photoperiod Control Systems for Greenhouse Crops.)

Garner, W. W., and H. A. Allard. 1920. “Effect of the Relative Length of Day and Night and Other Factors of the Environment on Growth and Reproduction in Plants,” Journal of Agricultural Research 18:553-606.

Islam, M. A., D. Tarkowská, J. L. Clarke, D.-R. Blystad, H. R. Gislerød, S. Torre, and J. E. Olsen. 2014. “Impact of End-of-day Red and Far-red Light on Plant Morphology and Hormone Physiology of Poinsettia,” Scientia Horticulturae 174:77-86.

Lumileds. 2016. DS171 LUXEON SunPlus Series for Horticulture, Lumileds Product Datasheet 20160908.

Sager, J. C., W. O. Smith, J. L. Edwards, and K. L. Cyr. 1988. “Photosynthetic Efficiency and Phytochrome Equilibria Determination Using Spectral Data,” Trans. ASAE 31(5):1882-1889.

Smith, H. 2000. “Phytochromes and Light Signal Perception by Plants – An Emerging Synthesis,” Nature 407:585-591.

van Trigt, C. 1999. “Color Rendering, A Reassessment.” Color Research & Application 24(3):197-206.


Topics of Interest

It’s All About You

Ian Ashdown, P. Eng., FIES

Chief Scientist, Lighting Analysts Inc.

[ Please send all comments to ]

What interests you? This is a question that all journalists must face at some point: if I write an article on some esoteric topic, will anyone care? Will you care?

Thankfully, in this age of Big Data, I can at least do a post-mortem analysis of your interests. What really interests you as lighting designers is … well, it’s interesting.

The data collection aspect of this analysis was easy: I simply counted your visits (and nothing more) to the individual All Things Lighting blog articles, then plotted the results:

Topics of Interest - FIG. 1

FIG. 1 – All Things Lighting article page visits.

Understanding your interests is more challenging. For example, what is your ongoing fascination with Daylight Factors? It is a complete mystery to me, as daylight factors were arguably the worst idea that has ever been proposed for daylight design when they were introduced 150 years ago. Their usefulness as a daylight design tool is even less today.

Your interest in Photometry and Photosynthesis is equally fascinating. This is basically about lighting design for horticultural applications. The article itself came from an enquiry about lighting design for … umm, medicinal plants … in Colorado, but the popularity of the topic is still surprising. (On the other hand, a projected market of ten billion dollars a year might explain some of the interest – that’s a lot of horticultural lighting business.)

Nighttime light pollution? This topic comprises Color Temperature and Outdoor Lighting, Light Pollution and Uplight Ratings, and the decidedly offbeat Mobile Light Pollution and Botanical Light Pollution articles. What is perhaps the most surprising about this is that the Light Pollution and Uplight Ratings article is arguably more germane to outdoor lighting design than Color Temperature and Outdoor Lighting, and yet look at the difference in popularity. Very strange, but it is heartening to see that you are taking an interest in this social responsibility topic.

Your interest in mesopic photometry, including both Understanding Mesopic Photometry and Mesopic Photometry and Statistics, is equally puzzling. Mesopic roadway lighting was a hot topic five years ago, but without official recognition as a design criterion by the IES Roadway Lighting Committee and other roadway lighting standards organizations, it seems like a somewhat esoteric topic today.

Now, The Kruithof Curve and Kruithof Revisited … if only there were some way to delete this topic from the collective consciousness of the lighting design community! If daylight factors are useless, the Kruithof Curve is in a category of its own. Still, it continues to attract your attention.

What is equally surprising is the apparent lack of interest in human centric lighting (Entraining Circadian Rhythms and Seeing Ultraviolet), intelligent lighting, visible light communications, and IoT (Giving Light), and the purported dangers of high-CCT light (Blue Light Hazard … or Not?). Not even Web search terms have managed to rescue these articles from relative obscurity.

The remaining articles are, as they say, background noise. They were very interesting to research and write about, but clearly not leading Web search terms.

Not to worry, however – even knowing your interests provides me with ideas for future blog articles. If daylight factors have attracted the most interest, it will fascinating to see your response to the forthcoming Climate-Based Daylight Modeling article.

Botanical Light Pollution

Red is the New Blue

Ian Ashdown, P. Eng., FIES

Chief Scientist, Lighting Analysts Inc.

[ Please send all comments to ]

Related Posts

Color Temperature and Outdoor Lighting

Light Pollution and Uplight Ratings

Mobile 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.

This is not a newly discovered problem. Botanists were aware of the deleterious effects of incandescent street lighting on trees eighty years ago (Matske 1936), while horticulturalists became aware of the problem with respect to ornamental plants some forty years ago (Cathey and Campbell 1975).

The lighting community can perhaps be excused for not following the latest research in publications such as American Journal of Botany and Journal of Arboriculture, but we were in fact made aware of the issue through publication of an article in Lighting Design and Application (Cathey and Campbell 1974). However, given that the proposed solution then was to avoid using high-pressure sodium (HPS) lamps and instead use less-efficient mercury-vapor lamps with their ghoulish color rendering capabilities … well, we understandably ignored the advice.

Soybeans and Trees

This is not to say that farmers are not aware of the problem. If you are growing soybeans, you quickly learn not to plant them in a field adjacent to HPS roadway lighting (FIG.1). The nighttime illumination – even as little as two to eight lux – can reduce crop yield by 20 to 40 percent due to delayed flowering and ripening (Chen et al. 2009).

Botanical Light Pollution - FIG. 1FIG. 1 – Effect of light pollution on soybean crop. (Source: Chen et al. 2009)

Landscape designers and arborists are also aware of the problem. A publication from Purdue University, for example, lists 65 trees and shrubs that are vulnerable to artificial light (Chaney 2002). Exposure to nighttime illumination, particularly from HPS street lighting, may result in disruption of the plant’s shoot growth, flowering, leaf expansion and abscission, and bud dormancy. In temperate climates, this may make the plants more susceptible to frost, fungal infections, and insect infestations. Again, however, the advice was to avoid using HPS lighting and use mercury vapor lighting instead. For lighting designers, this is pointless advice – mercury vapor lamps were long ago replaced by high-pressure sodium lamps, and these in turn are being replaced by solid state lighting.

… and herein lies today’s issue. LED-based outdoor lighting may – and the emphasis is on the word may – exacerbate the problem from the perspective of wild and domesticated plants. High-pressure sodium lamps emit much more red light than mercury vapor lamps on a per-lumen basis, and white light LEDs may (depending on their correlated color temperature) emit even more. What was once a minor problem for landscape designers and urban arborists may become something that lighting designers will need to consider.

To better understand this issue, we first need to understand the role of photopigments in plant growth and development.


Plants perform their magic of photosynthesis using a photopigment called chlorophyll, but this is only one of many different photopigments plants use to harvest and detect light. Equally important is phytochrome, which regulates a long list of plant functions, including:

  • Seed germination and development
  • Stem elongation
  • Leaf expansion and abscission
  • Photosynthesis development
  • Flowering
  • Ripening
  • Dormancy

Taken together, these functions basically outline the life cycle from seed to adult plant.

The sum of these light-induced changes is called photomorphogenesis. There are other photopigments involved, including blue light-sensitive cryptochromes (Lin 2002) and ultraviolet-sensitive UVR8 (Goto et al. 2006, Kami et al. 2010). However, it is phytochrome that dominates plant growth and development.

Phytochrome itself is an interesting pigment in that it has two states (or isoforms) called Pr and Pfr (e.g., Smith 2000). The Pr isoform strongly absorbs red light, with a spectral peak at about 660 nm (FIG. 2), making it look turquoise-blue when dissolved in solution. This is its biologically inactive state.

When a phytochrome molecule absorbs a red photon, it switches to its Pfr isoform, making it look slightly more greenish. This is its biologically active state, which signals to the plant that red light has been sensed. While in this state, phytochrome has a different spectral absorption distribution (FIG. 2), with a spectral peak at about 730 nm. (Horticulturalists and plant biologists refer to the spectral range of 700 nm to 800 nm as “far-red,” which explains the “fr” subscript.)

When the Pfr isoform absorbs a far-red photon, it reverts to its Pr isoform. Thus, phytochrome performs the function of a resettable biological switch to initiate or terminate photomorphological processes.

Botanical Light Pollution - FIG. 2

FIG. 2 – Phytochrome absorption spectra. (Source: Plants in Action, First Edition[1])

This biological switch behavior has some interesting consequences. While even low levels of red light can initiate many physiological responses, applying far-red light soon thereafter may reset the switch and terminate the response. Light pulses as short as one minute at night – think car headlights on a country road – are enough to induce or prevent the flowering of some plants (Borthwick et al. 1952). Worse, some plants have flower induction thresholds of less than four lux (Botto et al. 1996, Whitman et al. 1998).


Phytochrome may act as a biological switch, but how plants respond to its signaling varies by species and even cultivar. What all plants have in common, however, is photoperiodism, their physiological reaction to the length of the day. Like humans and all other animals, plants have circadian rhythms.

In terms of flowering, plants can generally be divided into three categories: 1) short day; 2) long day; and 3) day-neutral. For short day plants, flowering is initiated, advanced, or promoted when the dark nighttime period is sufficiently long to allow enough phytochrome Pfr to revert to Pr. For long day plants, the opposite is true: flowering is initiated, advanced, or promoted when the dark nighttime period is sufficiently short to increase nighttime levels of phytochrome Pr. As for day-neutral plants, their time of flowering is determined by other environmental cues, such as temperature and moisture.

From the perspective of wild and domesticated plants growing outdoors, artificial light can be a problem. For horticulturalists, however, it can be a boon. Florists have long used incandescent lamps with their copious red and infrared emissions to modify the growth and development of flowering plants in greenhouses. This promotes flowering in long day plants such as asters, azaleas, and fuchsias, while delaying flowering in short day plants such as chrysanthemums, begonias, and poinsettias.

The recent availability of high-power red and far-red LEDs has provided new opportunities for both florists and horticulturalists. Independently switching or dimming these LEDs enables greenhouse operators to precisely control phytochrome as a biological switch. This, combined with the secondary effects of activating cryptochromes using blue light, provides remarkable control of plant growth and development (e.g., Gautam et al. 2015, Islam et al. 2014, Kitazaki et al. 2015, and Lee et al. 2015).

Light Pollution

Outside of the greenhouse environment, however, adding red and far-red radiation to the environment is not a good thing. We can call it what it is: botanical light pollution. For soybean farmers and urban arborists, it may be a nuisance. However, there can also be more insidious and detrimental effects for wild plants and the pollinating insects that depend on them (e.g., Bennie et al. 2016).

The question is, how do we quantify this pollution? It is reasonably easy to quantify astronomical light pollution because we have comprehensive mathematical models of atmospheric physics and optics. However, the best that botanists can do for us is to identify plants as short day, long day, or day neutral.

Pragmatically speaking, we do not need to quantify botanical light pollution in an absolute sense of so many micromoles of radiation per square meter per second or whatever. From a lighting design perspective, the goal is to illuminate an area with so many lumens per square meter while doing our best to prevent wasted spill light. The question then becomes, what is the best light source for plants?

Comparing Light Sources

The phytochrome absorptance spectra (FIG. 2) were obtained by extracting phytochrome from plants and dissolving it in solution for analysis in vitro with a spectrophotometer. When in the plant itself, however, phytochrome is surrounded by other photopigments, especially chlorophyll. Both chlorophyll A and chlorophyll B have absorptance spectra that overlap with those of the phytochrome isoforms (FIG. 3), so it is reasonable to ask whether this influences (or “screens”) the phytochrome absorptance spectra in vivo.

Botanical Light Pollution - FIG. 3

FIG. 3 – Photopigment spectral absorptances.

Fortunately, a variety of studies of the effect of monochromatic radiation on plant growth and development have shown that the absorptance spectra of phytochrome in vitro reasonably predict the plant physiological response. For example, Withrow et al. (1957) studied the “induction and reversion of hypocotyl hook opening” in bean seedlings. A plot of their results as induction and reversion “action spectra” shows a remarkable correlation with the in vitro absorptance spectra of phytochrome (FIG. 4).

Botanical Light Pollution - FIG. 4

FIG. 4 – Typical phytochrome action spectra. (Source: Smith 1977)

Given this, we can use the phytochrome absorptance spectra as a species-independent measure of the effect of red and far-red radiation on plant growth and development (Sager et al. 1988). For a given light source, the probability of a phytochrome molecule absorbing a photon with a given wavelength is determined by the absorptance spectra of the isoform and the relative number of photons with that wavelength.

For a light source, we typically have its relative spectral power distribution (SPD), which is measured in watts per nanometer. However, from the Planck-Einstein relation, we know that a photon’s energy is inversely proportional to its wavelength. Therefore, to determine the relative spectral photon flux distribution, we need only multiply the lamp SPD by the wavelength for each wavelength and normalize the resultant graph. (An example is shown in FIG. 5.)

Botanical Light Pollution - FIG. 5

FIG. 5 – Radiant versus photon flux for a 3000K warm white LED.

With this, we now have the means to compare light sources with different spectral power distributions. Given a reference lamp (say, HPS) and a test lamp (say, a 3000K warm white LED), the calculations consist of:

  1. Multiply the SPD values of each lamp by the CIE 1931 luminous efficiency function V(l) shown in FIG. 6 from 400 nm to 700 nm.
  2. Sum the results of Step 1 to obtain the relative lumens Fref and Ftest generated by the two lamps.
  3. Multiply the SPD values of the test lamp by Fref / Ftest.

The two SPDs now represent the same number of photopic lumens (i.e., luminous flux) emitted by the lamps. With this:

  1. Multiply the SPD values of each lamp by the wavelength to obtain the lamp spectral photon flux distributions from 500 nm to 800 nm.
  2. Multiply the results of Step 4 by the phytochrome Pr spectral absorptance spectrum.
  3. Sum the results of Step 5 to obtain the Pr action values pAref,r and pAtest,r.
  4. Multiply the results of Step 4 by the phytochrome Pfr spectral absorptance spectrum.
  5. Sum the results of Step 7 to obtain the Pfr action values pAref,fr and pAtest,fr.

and finally:

  1. Add the Pr and Pfr action values for each lamp to obtain the lamp phytochrome action values pAref and pAtest.
  2. Divide pAtest by pAref to obtain the relative action value for the test lamp compared to the reference lamp.

Botanical Light Pollution - FIG. 6

FIG. 6 – CIE 1931 luminous efficiency function V(l).

A few explanatory notes:

  1. As shown in FIG. 2, the phytochrome absorptance spectra have secondary peaks in the near-ultraviolet. These are ignored because: a) it is difficult to disentangle the effects of phytochrome from the effects of the blue-sensitive cryptochrome photopigments; and 2) the photomorphological effects of blue light are less pronounced than those resulting from red and far-red radiation. The lower limit of 500 nm was chosen based on the phytochrome absorptance spectra minima.
  2. The spectral peak of Pfr is only 60 percent that of Pr, but the area under each spectral curve between 500 nm and 800 nm is almost the same. Also, phytochrome action spectra for various plant species have shown that equal red and far-red radiant fluences at the spectral peaks of 660 nm and 730 nm have approximately equal effect on the physiological responses. This justifies the final step of adding the two action values.

It must be emphasized that these “action values” are approximate at best, and should not be considered as formally quantifiable metrics. They are introduced here only to explore the potential effects of botanical light pollution.

With this caveat then, the following light sources were selected for comparison:

Light Source Manufacturer Product Code
High-pressure sodium (test) Damar 1782 LU100M
2700K white light LED Lumileds LUXEON Rebel ES LXW9-PW27
3000K white light LED Lumileds LUXEON Rebel ES LXW9-PW30
3500K white light LED Lumileds LUXEON Rebel ES LXW8-PW35
4000K white light LED Lumileds LUXEON Rebel ES LXH7-PW40
5000K white light LED Lumileds LUXEON Rebel ES LXW8-PW40

Table 1 – Comparison light sources.

The HPS lamp SPD was measured in the laboratory with 0.1 nm resolution and averaged to 5 nm bins, while the Lumileds SPDs were digitized from the published datasheet (Lumileds 2014). The equal-lumen SPDs for these light sources are shown in FIG. 7.

Botanical Light Pollution - FIG. 7

FIG. 7 – Equal-lumen spectral power distributions.

Following the above calculation procedure with the HPS lamp as the test source, the relative phytochrome action values are:

Light Source Relative Phytochrome Action
Pr Pfr Pr + Pfr
High-pressure sodium 1.0 1.0 1.0
2700K white light LED 1.7 2.3 1.9
3000K white light LED 1.5 2.0 1.7
3500K white light LED 1.0 1.2 1.1
4000K white light LED 1.0 1.0 1.0
5000K white light LED 0.9 1.0 0.9

Table 2 – Relative phytochrome action values.

From this, it can be seen that while 2700K and 3000K white light LEDs produce the least astronomical light pollution (see related article Color Temperature and Outdoor Lighting), they also unfortunately produce the most botanical light pollution.

It should be noted however that these results apply to Lumileds LUXEON products only. Looking at FIG. 7, it is evident that the 2700K and 3000K products use a different phosphor formulation than the 3500K, 4000K, and 5000K products. Different major LED manufacturers will have their own proprietary phosphor formulations, and so the above results should not be applied to LEDs based solely on their nominal CCTs.

Add More Red

It seems counterintuitive, but one solution to the problem of excess red light generated by low-CCT LEDs is to add more red light.

Some of the early LED modules combined phosphor-coated white and red LED dice in order to compensate for the low-efficiency red phosphors then available. This produced a warm white light with good CIE Ra values, but relatively poor R9 values due to the quasimonochromatic red emissions.

One roadway luminaire manufacturer has recently taken this approach with a new product line that was reportedly designed to comply with the International Dark Sky Association’s Fixture Seal of Approval program requirements for a maximum CCT of 3000K. While the approach works (with a measured CCT of 3145K), the massive spike in red light peaking at 625 nm (see Fig. 8) would seem to be a botanist’s nightmare spectrum.

Botanical Light Pollution - FIG. 8

FIG. 9 – 3000K LED versus 3145K white+red LED equal-lumen spectral power distribution.

Surprisingly, the situation may not be as bad as it appears. First, there is relatively little far-red radiation being emitted. Second, the 625 nm peak occurs where the phytochrome Pr absorptance spectrum is only 50 percent of maximum. This results in a calculated phytochrome action value (relative to the HPS reference lamp) of 0.9 – half that of the 3000K LED.

Light Source Relative Phytochrome Action
Pr Pfr Pr + Pfr
White+red LED 0.9 0.9 0.9

Table 3 – White+red relative phytochrome action values.

Color Filters

Another solution to the problem of excess red light is simply to add a color filter with a sharp cutoff at 625 nm. Red light beyond the cutoff wavelength contributes only ten percent to the luminous flux of a 3000K white light LED, so it is may be a reasonable tradeoff. (The resultant color will, however, be slightly cyan in hue.)

Whether it is possible to develop a suitable dye or coating for the LED optics that is both inexpensive and resistant to fading is, of course, an open question.

Chlorophyll Screening

The preceding analysis necessarily assumes that the phytochrome is not screened by the other plant photopigments, and that the isoform absorptance spectra represent the phytochrome action spectra for any given plant. In practice, this is not necessarily true. Phytochrome is present in very low concentrations in plant tissues. As a result, the much higher concentrations of chlorophyll tend to screen phytochrome by absorbing much of the incident red radiation. (See Fig. 3 for spectral overlapping between phytochrome Pr and chlorophyll A.)

A study by Beggs et al. (1980) demonstrated that if mustard seedlings are treated with the herbicide Norflurazon, the chlorophyll in the plant tissue becomes photobleached, resulting in white rather than green seedlings. With white seedlings, the phytochrome action spectrum had a peak at 660 nm, following the phytochrome Pr absorptance spectrum. With untreated green seedlings, however, the action spectrum was shifted to approximately 630 nm – which is well within the range of the 625 nm LED emission of the white+red LEDs (FIG. 9).

Botanical Light Pollution - FIG. 9

FIG. 9 – Chlorophyll screening of phytochrome Pr action spectrum. (Source: Beggs et al. 1980)


First and foremost, the phytochrome action metric presented in this article is not intended as a formal light source metric in any sense; it was introduced solely as a means of evaluating the potential impact of red and far-red light on both wild and domestic plants.

Second, the effects of applying red and/or far-red radiation will depend on the physiological state of the plant, the physiological response being mediated, and the time of application. Any excess (i.e., artificial) red radiation will convert the Pr isoform in the exposed plant to Pfr , while any excess far-red radiation will convert the Pfr isoform to Pr. Either action will upset the plant’s phytochrome photostationary state (Sager et al. 1988). What effect this will have on a given plant species at any given time of the night and season is unknown.

While phytochrome may function as a biological switch for plants, how individual plants species respond to its signaling will vary. Given that phytochrome mediates so many plant functions, the botanist’s characterization of short day, long day, and day neutral flowering plants is probably about all they will have in common.

If the above analysis has shown anything, it is that by changing roadway and outdoor area lighting from high-pressure sodium to white light LEDs, we may – and again, the emphasis is on may – be upsetting the ecological balance in unexpected ways. By examining what we do know and applying it on a theoretical basis, we can at least be better prepared to respond in the future if we need to.


Beggs, C. J., M. G. Holmes, M. Jabben, and E. Schäfer. 1980. “Action Spectra for the Inhibition of Hypocotyl Growth by Continuous Irradiation in Light and Dark-grown Sinapis alba L. Seedlings,” Plant Physiology 66:615-618.

Bennie, J. T. W. Davies, D. Cruse, and K. J. Gaston. 2016. “Ecological Effects of Artificial Light on Wild Plants,” Journal of Ecology (in press).

Borthwick, H.A., S. B. Hendricks, M. W. Parker, E. H. Toole, and V. K. Toole. 1952. “A Reversible Photoreaction Controlling Seed Germination,” Proceedings of the National Academy of Science 38:662–666.

Botto, J. F., R. A. Sánchez, G. C. Whitelam, and J. J. Casal. 1996. “Phytochrome A Mediates the Promotion of Seed Germination by Very Low Fluences of Light and Canopy Shade Light in Arasbidopsis,” Plant Physiology 110:439-444.

Cathey, H. M., and L. E. Campbell. 1974. “Lamps and Lighting – A Horticultural View,” Lighting Design and Application 4(11):41-52.

Cathey, H. M., and L. E. Campbell. 1975. “Security Lighting and its Impact on the Landscape,” Journal of Arboriculture 1(10):181-187.

Chaney, W. R. 2002. Does Night Lighting Harm Trees? Circular FNR-FAQ-17, Department of Forestry and Natural Resources, Purdue University.

Chen, C.-L., Y.-H. Su, C.-J. Liu, and Y.-C. Lee. 2009. “Effect of Night Illumination on Growth and Yield of Soybean,” Journal of Taiwan Agricultural Research 58(2):146-154.

Gautam, P., M. T. Terfa, J. E. Olsen, and S. Torre. 2015. “Red and Blue Light Effects on Morphology and Flowering of Petunia x hybrid,” Scientia Horticulturae 184:171-178.

Goto, N., T. Kumagai, and M. Koornneef. 2006. “Flowering Responses to Light-breaks in Photomorphogenic Mutants of Arabidopsis thaliana, a Long-day Plant,” Physiologia Plantarum 83(2):209-215.

Islam, M. A., D. Tarkowská, J. L. Clarke, D.-R. Blystad, H. R. Gislerød, S. Torre, and J. E. Olsen. 2014. “Impact of End-of-day Red and Far-red Light on Plant Morphology and Hormone Physiology of Poinsetta,” Scientia Horticulturae 174:77-86.

Kami, C., S. Lorrain, P. Hornitschek, and C. Fankhauser. 2010. “Light-regulated Plant Growth and Development,” Current Topics in Developmental Biology 91:29-66.

Kitazaki, K., S. Watanabe, A. Okamoto, M. Matsuo, S. Furuya, and K. Sameshima. 2015. “Far-red Light Enhances Removal of Pericarps in Tartary Buckwheat (Fagopyrum tataricum Gaertn.) Sprout Production under Artificial Lighting,” Scientia Horticulturae 185:167-174.

Lee, M.-J., S.-Y. Park, and M.-M. Oh. 2015. “Growth and Cell Division of Lettuce Plants under Various Ratios of Red to Far-red Light-emitting Diodes,” Horticulture, Environment, and Biotechnology 56(2):186-194.

Lin, C. 2002. “Blue Light Receptors and Signal Transduction,” The Plant Cell 14:S205-S225.

Lumileds. 2014. LUXEON Rebel ES Datasheet DS61 20140630.

Matzke, E. B. 1936. “The Effect of Street Lights in Delaying Leaf-fall in Certain Trees,” American Journal of Botany 23(6):446-452.

Sager, J. C., W. O. Smith, J. L. Edwards, and K. L. Cyr. 1988. “Photosynthetic Efficiency and Phytochrome Equilibria Determination Using Spectral Data,” Trans. ASAE 31(5):1882-1889.

Smith, H., Ed. 1977. The Molecular Biology of Plant Cells. Berkely, CA: University of California Press.

Smith, H. 2000. “Phytochromes and Light Signal Perception by Plants – An Emerging Synthesis,” Nature 407:585-591.

Whitman, C.M., R. D. Heins, A. C. Cameron, and W. H. Carlson. 1998. “Lamp Type and Irradiance Level for Daylength Extensions Influence Flowering of Campanula carpatica ‘Blue Clips’, Coreopsis grandiflora ‘Early Sunrise’, and Coreopsis verticillata ‘Moonbeam’,” Journal of the American Society of Horticultural Science 123:802–807.

Withrow, R. B., W. H. Klein, and V. Elstad. 1957. “Action Spectra of Photomorphogenic Induction and its Photoinactivation,” Plant Physiology 32:453-462


[1] Plants in Action is a plant physiology textbook published by the Australian Society of Plant Scientists, New Zealand Society of Plant Biologists, and the New Zealand Institute of Agricultural and Horticultural Science. It is freely available online at

Light Pollution and Uplight Ratings

Swatting BUGs

Ian Ashdown, P. Eng., FIES

Chief Scientist, Lighting Analysts Inc.

[ Please send comments to ]

Related Posts

Botanical Light Pollution

Color Temperature and Outdoor Lighting

Mobile Light Pollution

“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.

Those in the lighting industry will be familiar with IES TM-15-11, Luminaire Classification System for Outdoor Luminaires with its BUG (Backlight-Uplight-Glare) rating system (IES 2011); those in the astronomical research community will be familiar with Garstang’s light pollution model (Garstang 1986) and its derivatives. These documents have led to the development of the IDA/IES Model Lighting Ordinance (IDA/IES 2011) by the lighting industry and the lesser-known Pattern Outdoor Lighting Code (Luginbuhl 2010) by the astronomical research community.

Of particular interest to professional lighting designers is the BUG rating system of IES TM-15-11. While the IDA/IES Model Lighting Ordinance (MLO) has seen at best sporadic adoption by individual municipalities and states, BUG ratings are integral to the LEED v4 Light Pollution Reduction credit. While it is only one credit, it may make the difference between, for example, LEED Silver and Gold certification.

Related to this is the IDA’s Fixture Seal of Approval program, which “provides objective, third-party certification for luminaires that minimize glare, reduce light trespass, and don’t pollute the night sky.” While it is not directly related to IES TM-15-11 or LEED v4, outdoor luminaires with this “dark-sky friendly” certification are useful in promoting environmental responsibility in building design.

Curiously, recent changes to this program have removed all references to the BUG rating system, replacing it with the much simpler requirement that the luminaires be full-cutoff, or to quote the IDA FSA Web site, “fixtures must emit no light above 90 degrees.” In other words, after campaigning for lighting pollution control and working with the lighting industry through the Illuminating Engineering Society for the past decade or more, the International Dark-Sky Association apparently no longer recognizes the IES BUG rating system.

What happened here … and why is your humble scribe looking guilty?

History – Astronomical Research

Going back to 1973, the astronomer P. S. Treanor wrote a paper called, “A Simple Propagation Law for Artificial Night-Sky Illumination” (Treanor 1973). In it, he developed an empirical equation for the overhead sky brightness at night due to light pollution from a distant city. As befits the astronomical research community, his equation involved Mie scattering from aerosol particles (dust and smoke), atmospheric density, and extinction coefficients – topics not in the lexicon of most lighting designers. The light source was modeled as a single point source with constant intensity.

In 1986, the astronomer R. H. Garstang wrote a paper called, “Model for Artificial Night-Sky Illumination” (Garstang 1986). Again as befits the astronomical research community, his equations involved Rayleigh scattering from air molecules, Mie scattering from aerosol particles, reflections from the ground, and more. Most important, he empirically modeled the luminous intensity distribution of roadway cobrahead luminaires that were prevalent at the time.

Swatting Bugs - FIG. 1

FIG. 1 – Garstang’s luminous intensity function (green line). (Source: Luginbuhl et al. 2009)

In his own words, however, “The choice of the function … is purely arbitrary … these properties seem to be true for most street lights and for at least some other forms of outdoor lighting.”

Swatting Bugs - FIG. 2

FIG. 2 – Typical cobrahead roadway luminaire

… and never the twain shall meet. The lighting industry has relied on measured luminous intensity distributions to characterize luminaires for nearly a century. It would be unthinkable for a lighting researcher to model such distributions with a “purely arbitrary” function that might “seem to be true.”

In Garstang’s defense, however, a metropolis illuminated with randomly oriented cobrahead luminaires circa 1986 probably did have a composite luminous intensity distribution (i.e., for the entire city) that was reasonably approximated by his empirical function. As evidence of this, recent studies by Duriscoe et al. 2013 and others have mostly validated the sky glow predictions made by Garstang’s model.

That, however, was three decades ago. Things have changed.

History – Lighting Industry

The IDA/IES Model Lighting Ordinance has a long and somewhat contentious history. It was first developed by the IDA without significant input from lighting industry. One of the early drafts defined outdoor luminaires in terms of their wattage, with no reference whatsoever to their luminous flux output. East is East and West is West …

An IES meeting of outdoor lighting industry representatives first saw this proposed ordinance as an existential threat, as recorded in the meeting minutes. Eventually however, it was decided that it was better to work with the astronomical research community rather than to fight it. In 2005 therefore, the Joint IDA/IES Task Force was formed to collaboratively develop the MLO.

This led in turn to the development of the Luminaiure Classification System (LCS), published in IES TM-15-07, with the BUG rating system added in 2009 and subsequently revised in IES TM-15-11. The first public review of the MLO occurred in 2009, the second public review in 2010, and the final Joint IDA-IES Model Lighting Ordinance (MLO) with User’s Guide document was published in June 2011 (IDA/IES 2011). The BUG rating system of IES TM-15-11 is incorporated in the MLO as Table C, Maximum Allowable Backlight, Uplight and Glare (BUG) Ratings.

Referring to IES TM-15-11, it defines six uplight ratings for luminous flux (maximum zonal lumens) emitted above 90 degrees by the luminaire (Table 1). There are two uplight zones, designated UL for vertical angles 90 to 100 degrees and UH for angles 100 to 180 degrees (FIG. 3).

  U0 U1 U2 U3 U4 U5
UH 0 10 50 500 1000 >1000
UL 0 10 50 500 1000 >1000

Table 1 – IES TM-15-11 Uplight Ratings (maximum zonal lumens)

Swatting Bugs - FIG. 3

FIG. 3 – IES TM-15-11 BUG uplight zones (Source: Chinnis et al. 2011)

The question that must be asked, however, is where did these lumen values come from? The only publicly-available documentation appears to be a Leukos paper titled “IES TM-15 BUG Value-Setting and Adjustment Methodology” (Chinnis et al. 2011). One quote from this paper is of particular significance:

“The BUG values were established by the [IDA/IES MLO Task Force] based on professional experience and analysis efforts with a very wide variety of outdoor lighting applications, including variations in ambient brightness, site geometry and function.”

Referring to the astronomical research comment above, it would be unthinkable for an astronomer to specify values in a standard “based on professional experience and analysis efforts” without providing the data needed for impartial and independent verification.

East is East and West is West … as easy as it may be to poke fun at both sides in this matter, it is not constructive. As long as the BUG rating system is being used as a basis for the LEED v4 Light Pollution Reduction credit, there is a need to understand whether the maximum zonal lumens shown in Table 1 are appropriate.

Sadly, they are not.

Measuring Uplight

A year after IES TM-15-11 was published, another Leukos paper titled “Photometric Imprecision Can Limit BUG Rating Utility” investigated the practical issues of measuring luminaires in the laboratory for BUG uplight ratings (Ashdown 2012). The abstract, while extensive, usefully summarizes the results:

“There are, however, limits to what can be measured in the laboratory. IES TM-15–11 requires that a luminaire with an uplight rating of U0 emits zero lumens into the upper hemisphere, while a U1 uplight rating or a G0 glare rating for high viewing angles requires fewer than 10 lumens. Given that the luminaire is emitting thousands of lumens and that the laboratory room surfaces have a diffuse reflectance of at least two percent, it is physically impossible to measure zero lumens, and extremely difficult to measure fewer than 10 lumens. Consequently, a U0 glare rating can only be obtained by physical examination of the luminaire and post-processing of the measured photometric data. Similarly, a U1 uplight rating or a G0 glare rating for high viewing angles is likely the result of data manipulation.”

The paper explained that “post-processing of the measured photometric data” is indeed a common practice in photometric laboratories. If the laboratory technician can clearly see that the luminaire emits no light at or above 90 degrees, it is entirely reasonable to zero out the data for vertical angles greater than zero degrees, as these only record the diffuse interreflections from the laboratory room surfaces.

Of course, it is also possible that the laboratory technician saw that there was some stray light being emitted into the UL zone, but decided that it was probably less than 10 lumens and so reason enough to zero out the data. (Estimating total emitted lumens simply by looking at a luminaire presumably requires professional experience.)

From an engineering perspective, this is an untenable position. The problem is that if you cannot measure something, then it is pointless to divide it into different categories (in this case U0 through U2 uplight ratings).

Calculating Uplight

Kipling’s pessimism aside, it is possible to reconcile the interests of the lighting industry on one hand and the astronomical research community on the other. The approach is simple: given that Garstang’s light pollution model has been validated, it is entirely straightforward to substitute measured luminous intensity distributions for Garstang’s generic and arbitrary distribution (FIG. 1). It did not make sense to do this in 1986, but it certainly does today with the emphasis on BUG ratings. The question to be answered is, what influence do various UL and UH ratings have on light pollution if you assume that the same luminaires are used throughout an entire metropolis?

To be fair, the astronomical research community has addressed this question in several papers, including Aubé et al. (2005), Aubé (2015), Baddiley (2007), and Cizano and Castro (2000). In particular, an open source software program for sky glow modeling called Illumina imports IES LM-63 photometric data files. The problem, however, is twofold: 1) the papers were written for and published within the astronomical research community; and 2) software programs such as Illumina are sophisticated research tools that are designed to answer more pressing questions than whether the BUG uplight rating lumen values are appropriate.

This need not dissuade us, however. Garstang’s light pollution model is not particularly complicated, and it was clearly described in the original paper. It is also not particularly difficult to implement in software – it was after all originally developed to run on a 1980s-era Apple II computer (Garstang 1986). The only difference is that calculations that likely took days to weeks to run in 1986 now execute in a few seconds.

The result is SkyGlowCalc, a program written expressly to answer the above question for the IDA Task Force (FIG. 4). The software was developed on a volunteer basis in the author’s capacity as a member of the Task Force, mostly because the question itself was inherently interesting.

Swatting Bugs - FIG. 4

FIG. 4 – SkyGlowCalc (Source: Lighting Analysts Inc.)

This program is, of course, more than what Garstang envisioned some three decades ago. In addition to importing IES LM-63 photometric data files, it also allows the user to specify common lamp types with their different spectral power distributions (SPDs). The program then calculates the resultant SPDs of the emitted light contributing to sky glow at a remote observing site. As shown in FIG. 4, the wavelength-dependent effects of Rayleigh scattering greatly increase the blue content at the observing site (dashed line). A more detailed discussion of this is presented in the AGi32 blog article, “Color Temperature and Outdoor Lighting.”

For astronomical purposes, the most important output of this program is the sky brightness or its equivalent limiting visual magnitude. The goal was to take the photometric data files of 63 commercial luminaires from the IDA Fixture Seal of Approval program and see what differences in sky brightness there would be, assuming that all the luminaires emitted the same amount of luminous flux and all other parameters were equal (as shown in FIG. 4). The results of this analysis are shown in Table 2.

UL Uplight Rating LPS CIE HP1 3000K LED 5000K LED
U0 24.4 21.4 19.0 18.7
U1 24.4 21.4 19.0 18.7
U2 24.3 21.3 19.3 18.6

Table 2 – Uplight Rating versus Limiting Magnitude

The visual magnitude of the calculated sky glow determines the faintest stars you can see directly overhead at midnight on a moonless night. With the unaided eye, we can see stars as faint as magnitude 6; telescopes gather more light and so allow us to see fainter stars. The scale is logarithmic, with a difference of 0.1 magnitude representing a difference of approximately 10 percent in photometric intensity. These differences are near the limit of our ability to distinguish differences in intensity.

Simply put, not only can we not measure the differences between U0, U1 and U2 ratings in the laboratory with luminaires, we cannot distinguish the resultant differences in sky glow in the night sky.

Shortly after these results were presented to the IDA Task Force, the decision was made to remove BUG rating requirements from the IDA Fixture Seal of Approval program.

Model Lighting Ordinance

As noted above, the BUG rating system is incorporated in the IDA/IES Model Lighting Ordinance (IDA/IES 2011) as Table C. However, there is a twist that is often overlooked (FIG. 5).

Swatting Bugs - FIG. 5

FIG. 5 – Model Lighting Ordinance Table C-2

Put another way, Table C-2 says that different uplight ratings are permitted for different lighting zones, but only for luminaires that are not used for street lighting or area lighting. In other words, only luminaires with U0 ratings are permitted for street and area lighting (which basically includes all significant outdoor lighting)..


Prior to 2007, roadway luminaires were classified as having cutoff, semicutoff, noncutoff, and full cutoff luminous intensity distributions, with “full cutoff” meaning luminaires with no luminous flux emitted at or above 90 degrees vertical, as well as limited intensity at or above 80 degrees (IES 2011). The BUG rating system was developed by the Joint IDA/IES MLO Task Force to address light pollution issues, thereby replacing these mostly empirical definitions.

It seems, however, that we mostly have come full circle – the International Dark Sky Association no longer makes use of the BUG rating system in its Fixture Seal of Approval program. All that is required is that the luminaires do not emit luminous flux above 90 degrees vertical.

This is not an example of backtracking. Rather, it is how science (and hopefully standards development) works. The IDA executive presumably reviewed the above analysis and concluded that the BUG rating system offers no value to light pollution abatement policies. The FSA program requirements were simply updated in accordance with the best available information. (LEED v4 authors, please take note.)

As for Kipling, the problem has always been that the lighting industry and the astronomical research community speak different languages. The International Dark-Sky Association has been accused in the past of “selling out” to the lighting industry in partnering with the Illuminating Engineering Society, but this is unfair. Having reviewed the original MLO drafts in the 1990s, it was painfully clear that neither side understood the other, from technology to terminology. The Joint IDA/IES MLO Task Force did a credible job of bridging this cultural gap over four years, but “professional experience and analysis” can only go so far. SkyGlowCalc was developed solely to assist both sides in finally bridging the communications gap. (The author is himself an amateur astronomer.)

As for the BUG rating system, it must be remembered that its backlight and glare components (except G0)  are still presumed valid, and so it is still useful in environmentally responsible lighting design. It rightfully retains its position in the IDA/IES Model Lighting Ordinance.


The reason why uplight from U0- and U1-rated outdoor luminaires has so little effect on sky glow is simple. Taking the full-cutoff 250-watt metal halide luminaire from IES TM-15-11 as an example, it emits 13,553 lumens downwards. Assuming that the ground has a reflectance (albedo) of 15 percent (Gillet and Rombauts 2001), the amount of light diffusely reflected into the upper hemisphere is 2,033 lumens. The portion of light reflected into the UL zone is 406 lumens, with the remaining 1,627 lumens being reflected into the UH zone. In other words, the luminaire in its natural surroundings has a UL rating of U2 (nearly U3) and a UH rating of U4. Adding a few more lumens of directly-emitted luminous flux will not make any difference.

Together, roadway and outdoor parking luminaires account for over 80 percent of all outdoor lighting on a per-lumen basis (Navigant 2012). If we are to tame light pollution, it must be through a combination of limiting roadway and parking lot illuminance requirements, and perhaps more important employing smart networked lighting technologies to dim or turn off the luminaires when they are not needed.

In the meantime, the twain have hopefully and finally met.


Thanks to Dawn DeGrazio for editorial assistance and historical clarifications.


Ashdown, I. 2012. “Photometric Imprecision Can Limit BUG Rating Utility,” Leukos 9(2):79-88.

Aubé, M., L. Franchomme-Fossé, P. Robert-Staehler, and V. Houle. 2005. “Light Pollution Modelling and Detection in a Heterogeneous Environment: Toward a Night Time Aerosol Optical Depth Retrieval Method,” Proc. SPIE Volume 5890.

Aubé, M. 2015. “Physical Behaviour of Anthropogenic Light Propagation into the Nocturnal Environment,” Philosophical Transactions of the Royal Society B 370(1667):20140117.

Baddiley, C. 2007. “A Model to Show the Differences in Skyglow from Types of Luminaires Designs,” Starlight 2007. La Palma, Canary Islands.

Chinnis, D., M. Mutmansky, and N. Clanton. 2011. “IES TM-15 BUG Value-Setting and Adjustment Methodology,” Leukos 8(1):25-39.

Cinzano, P., and F. J. D. Castro. 2000. “The Artificial Sky Luminance and the Emission Angles of the Upward Light Flux,” Journal of the Italian Astronomical Society 71(1):251.

Navigant Consulting, Inc. 2012. 2010 U.S. Lighting Market Characterization. Washington, DC: U.S. Department of Energy.

Duriscoe, D. M., C. B. Luginbuhl, and C. D. Elvidge. 2013. “The Relation of Outdoor Lighting Characteristics to Sky Glow from Distant Cities,” Lighting Research and Technology 46(1):35-49.

Garstang, R. H. 1986. “Model for Artificial Night-Sky Illumination,” Publications of the Astronomical Society of the Pacific 98:364-375.

Gillet, M., and P. Rombauts. 2001. “Precise Evaluation of Upward Flux from Outdoor Lighting Installations (Applied in the Case of Roadway Lighting),” Proc. International Conference on Light Pollution. Serena, Chile.

IDA/IES. 2011. Joint IDA-IES Model Lighting Ordinance (MLO) with User’s Guide. New York, NY: Illuminating Engineering Society.

IES. 2011. IES TM-15-11, Luminaire Classification System for Outdoor Luminaires. New York, NY: Illuminating Engineering Society.

Kipling, R. 1892. The Ballad of East and West, in Barrack-room Ballads. London, UK: Methuen Publishing.

Luginbuhl, C. B., V. E. Walker, and R. J. Wainscoat. 2009. “Lighting and Astronomy,” Physics Today, December, pp. 32-37.

Luginbuhl, C. B. 2010. Pattern Outdoor Lighting Code (USA). Flagstaff, AZ: U.S. Naval Observatory.

Treanor, P. J. 1973. “A Simple Propagation Law for Artificial Night-Sky Illumination,” Observatory 93:117.