Select Page
Specifying LED Colors for Horticultural Lighting

Specifying LED Colors for Horticultural Lighting


Whereas human vision relies on five opsins as photoreceptors, most plants have a wide variety of photopigments that are responsive to optical radiation from 280 nm to 800 nm. Beyond photosynthesis, plants rely on this radiation to control photomorphogenesis, phototropism, shade avoidance, and both circadian and circannual rhythm entrainment.

Quasimonochromatic LEDs have proven a boon for botanists in that the molecular genetics of these responses can be elucidated with precisely controlled spectral power distributions (SPDs). In terms of photopigments, cryptochromes, for example, respond to blue light, while phytochrome responds to the R:FR ratio of red (approximately 660 nm) to far-red (approx. 735 nm) light.

The problem is that botanists do not define what is meant by “blue,” “green,” “yellow,” “red,” or “far-red” visible light, while ultraviolet radiation is broadly defined as UV-A and UV-B. Consequently, it is difficult to replicate laboratory experiments without knowing the SPD of the horticultural light source.

This paper proposes an LED “color” specification that represents a given SPD using a small number of radial basis functions, to provide a metric for comparing biologically similar SPDs. It  further introduces a trainable fuzzy logic SPD classifier that can compare biologically similar SPDs for specific horticultural applications.


When the first high-pressure sodium (HPS) lamps were introduced in the late 1960s, they were quickly adopted by commercial greenhouse operators as a means of providing supplemental electric lighting. This made it economically possible to grow vegetables and flowers throughout the year in controlled environments. They had luminous efficacies, ranging from 100 to 150 lumens per watt, they were available in sizes ranging from 400 to 1,000 watts, and they could be incorporated in luminaire housings designed to withstand the heat and humidity of greenhouses.

One disadvantage of HPS lamps is they produce mostly yellow light with fixed spectral power distributions (SPDs). This is not particularly important for plant photosynthesis, as most plants can take advantage of optical radiation within the spectral range of 400 to 700 nm. Horticulturalists often refer to the “McCree curve,” which plots average photosynthesis efficiency versus wavelength for a variety of field-grown crops (McCree 1972). As shown in Figure 1, the spectral output of HPS lamps is near the peak of the McCree curve.

Figure 1. Typical HPS lamp spectral power distribution versus McCree curve.

The problem is that while the yellow light of HPS lamps may be good for photosynthesis, plants have a wide range of photopigments that respond to optical radiation from 280 nm to 800 nm (often referred to as “photobiologically active radiation,” or PBAR). These responses include:

  • Photomorphogenesis – any change in the morphology (i.e., shape) or composition of a plant or its components that is induced by optical-radiation exposure
  • Photoperiodism – response of a plant to daily (circadian) or seasonal (circannual) changes in optical-radiation exposure
  • Photosynthesis – conversion of “photosynthetically active radiation” (PAR) into chemical energy stored as carbohydrates to fuel plant activities
  • Phototropism – any self-actuated change in the orientation of a plant or its components toward or away from optical radiation
  • Secondary metabolite production – organic compounds not directly involved in plant growth, development, or reproduction, including compounds used as medicines, flavorings, pigments, and drugs
  • Shade avoidance – a set of responses to being shaded by other plants, including changes in morphology, flowering times, and allocation of resources

While many of these responses have been known or suspected for decades, it was difficult for botanists to study them in the laboratory without suitable light sources. This changed, however, with the introduction of horticultural luminaires with high-flux quasimonochromatic light-emitting diodes (LEDs). Somewhat serendipitously, the absorption spectra of chlorophyll A and B have peaks that correspond with those of approximately 450-nm InGaN and approx. 660-nm AlInGaP LEDs (Figure 2). Today, the photon efficacy (measured in micromoles of PAR photons per Joule, rather than in lumens) of LED modules is typically greater than equivalent 1,000-watt HPS lamps.

Figure 2. Chlorophyll and b-carotene absorption spectra.

While commercially available horticultural luminaires with blue and red LEDs (producing so-called “blurple” light) are now successfully competing with traditional HPS luminaires, botanists’ attention has turned to the capabilities of multichannel LED luminaires with controllable SPDs. Over 500 academic studies over the past decade have investigated the effects of different wavelength ranges on plants and their absorption by photopigments (Table 1).

Wavelength RangeColor NamePhotopigmentsResponses
280 nm – 315 nmUV-BUVR-8Sec. metabolism
Shade avoidance
315 nm – 400 nmUV-AChlorophylls
Zeitlupe family
Sec. metabolism
400 nm – 500 nmBlueCarotenes
Zeitlupe family
Sec. metabolism
Shade avoidance
500 nm – 575 nmGreenCryptochromesPhotosynthesis
Sec. metabolism
Shade avoidance
575 nm – 610 nmYellow –
Sec. metabolism
610 nm – 700 nmRedChlorophylls
Sec. metabolism
Shade avoidance
700 nm – 800 nmFar-redPhytochromesPhotomorphogenesis
Shade avoidance
Table 1 – Plant Responses to Optical Radiation

The problem is that while UV-A and UV-B are formally defined in the scientific literature (e.g., ISO 2007), the visible color names are colloquial and based on human visual responses. The title of one paper in particular illustrates this issue: “Green light drives leaf photosynthesis more efficiently than red light in strong white light: Revisiting the enigmatic question of why leaves are green” (Terashima 2009). For anyone interested in either replicating the experiments or extrapolating their results, what are “green,” “red,” and “white” light?

The color name “far red,” which refers to the spectral range of 700 nm to 800 nm, is formally defined in terms of horticulture (ASABE 2017). It is important in terms of shade avoidance and photoperiodism, where plants rely on two isoforms of phytochrome to detect the ratio of red to far red (R:FR) optical radiation (Sager et al. 1988), but there is no equivalent definition of “red” (Figure 3). Luminaire manufacturers are now offering products with 660-nm red and 735-nm far-red LEDs to induce or delay flowering in ornamental plants (e.g., Craig and Runkle 2013), but many previous horticultural studies have relied on daylight alone or daylight and incandescent lamps to explore the effects of varying R:FR. How should these studies be interpreted in terms of modern horticultural lighting practices with LED-based luminaires?

Figure 3. Phytochrome absorption spectra.

Characterizing SPDs

Horticultural researchers have recognized that the use of colloquial color names is a problem. Many papers describe their experimental methods in detail, including light source SPD plots, names of specific luminaire products, and occasionally tabulated SPDs. This still leaves open, however, the problem of interpreting the results in terms of other optical radiation sources with similar SPDs.

One proposed product label for horticultural light sources is shown in Figure 4 (Both et al. 2017). By avoiding the use of color names, this proposal eliminates any dependence on the human visual system. However, the arbitrary separation of the PBAR spectral range into 100-nm wide bands ignores the distinct responses of plants to UV-A and UV-B radiation, as well as the response of plants to narrower changes in wavelength. For example, Johkan et al. (2012) provide an example wherein the growth of lettuce under quasimonochromatic radiation from “green” LEDs with center wavelengths of 510 nm, 520 nm, and 530 nm varies markedly depending on the center wavelength for the same photosynthetic photon flux density (Figure 5).

Figure 4. Proposed product label. (Source: Both et al. 2017)
Figure 5. Morphology of red leaf lettuce plants treated with light from a white fluorescent lamp (FL) and green light-emitting diodes (LED). Peak wavelength for each LED was 510 nm (G510), 524 nm (G520), and 532 (G530). Plants were photographed 17 d after sowing. Bars indicate 8 cm. Total photosynthetic photon flux was 100, 200, and 300 μmol·m-2·s-1. (Source: Johkan et al. 2013, Fig. 2).

The spectral absorptance characteristics of the primary plant photopigments chlorophyll A and B, b-carotene, and phytochrome (Figure 2) suggest that their absorptances vary very rapidly with changes in wavelength. However, these data represent the spectral absorptance of the pigment extracts dissolved in solvents (i.e., in vitro). As shown by Moss and Lewis (1952), a combination of the structural complexity of the leaves, screening by other photopigments, and the presence of accessory photopigments have the effect of broadening the spectral absorptance characteristics of the photopigments in vivo. Studies such as those of McCree (1972) have shown that in general, plants are reasonably tolerant of small changes in the center wavelengths of quasimonochromatic radiation. (Johkan et al. [2012] was likely an exception in that photosynthesis probably occurred due to b-carotene rather than chlorophyll A or B, with longer wavelengths of green light being incapable of exciting this photopigment.)

In view of this and other studies, it is clear that any attempt to characterize the SPDs of horticultural luminaires needs to take into consideration the responses of plants to changes in center wavelength of quasimonochromatic light sources, and more generally to horticultural luminaires with both quasimonochromatic and broadband radiation sources.

In relation to this, Maloney (1986) discusses the physical basis of spectral reflectance distributions from natural objects, including organic materials. These distributions are band-limited by molecular interactions and superimposed vibrational and rotational patterns, with the result that the number of parameters needed to adequately represent spectral reflectance distributions in visible light (i.e., 400 nm to 700 nm) is five to seven. Westland et al. (2000) came to a similar conclusion based on statistical studies of reflectance spectra, noting that the spectral reflectance distributions of most natural surfaces form a set of band-limited functions with a frequency limit of approximately 0.02 cycles per nm. This implies that visible light reflectance spectra can be adequately represented using six to 12 basis functions (e.g., Westland and Ripamonti 2004).

A small number of radial basis functions (e.g., Buhmann and Jäger 2000) can therefore be used to approximate a real-valued function (such as an SPD) as a weighted sum of the basis functions. As an example, the set of Gaussian functions , where  , and  for , can be used to approximate any SPD from 350 nm to 800 nm where the functions are separated by 25 nm (Figure 6).

Figure 6. Sum of Gaussian radial basis functions provide flat spectral sensor response.

An advantage of this method is that the set of basis function weights is much smaller than the set of enumerated values for a measured SPD. Rather than referring to “red,” “green,” “blue,” or “white” light, horticulturalists can state the values of a set of basis function weights. Moreover, a useful approximation of the original SPD significant to the needs of horticultural lighting can be reconstructed from these weights.

As an example, Figure 7 shows 13 radial basis functions over the visible light range of 400 nm to 700 nm that are each multiplied on a per-wavelength basis by the SPD of a 4000-K white light LED to yield the basis function weights.

Figure 7. Radial basis function weights for a 4000K white LED SPD.

Figure 8 shows a reconstruction of the LED SPD using a cubic spline curve with the weights as knots. The reconstruction is clearly different from the original SPD, but in terms of predicting plant responses, it is likely adequate.

Figure 8. Spline reconstruction of 4000-K white LED SPD from radial basis function weights.

Horticultural Spectral Sensor

Referring to Figure 6, each basis function can be seen as the responsivity of a radiant flux meter in combination with a Gaussian bandpass filter with a center wavelength of xi. Combining the unweighted outputs of the 19 filtered meters results in a flat response from 375 nm to 775 nm. Presented with an arbitrary SPD, the filtered meter outputs represent the appropriate weighting for the basis functions to approximate the SPD. Moreover, the absolute values of the filtered meter outputs can be used to estimate the absolute spectral irradiance incident on the meters, from which can be calculated the absolute irradiance in watts per square meter and the photosynthetic photon flux density (PPFD) in micromoles per square meter per second.

Figure 9 shows the spectral responsivities of an AS7262 6-channel visible light spectral sensor with a spectral range of approximately 430 nm to 570 nm, as manufactured by ams AG of Premstaetten, Austria. While the spectral responsivities are not ideal Gaussian functions, it is clear that an instrument to directly measure radial basis function weights can be fabricated using existing technology.

Figure 9. Spectral responsivity of ams AS7262 6-channel visible light spectral sensor.

An advantage of this system and method in terms of horticultural light sources is that the spectral power distributions can be unambiguously measured and expressed as a small set of numbers, regardless of the SPD complexity. If the representations of two SPDs are similar, the horticulturalist may be assured that they will likely have the same biological effect on a plant species. As an example, white light fluorescent lamps typically exhibit a combination of continuum and line spectra, whereas white light LEDs typically exhibit a narrow peak emission near 450 nm and a broad continuum from the blends of green- and red-emitting phosphors (e.g., Figure 7). Regardless, if their sets of radial basis function weights are similar, the two light sources may also be regarded as similar with respect to horticultural applications.

Fuzzy Logic SPD Classifier

The unanswered question is, what does “similar” mean in the context of comparing two or more SPD representations? With 13 to 19 radial basis function weights as parameters, it becomes impractical to formulate a table of rules for comparison purposes. The solution to this problem is a fuzzy logic classifier.

Fuzzy logic is often seen as a mathematical means of representing vagueness and imprecise information when making decisions, where input signals are “fuzzified” by mapping their precise values to a set of fuzzy membership functions. Referring to Figure 10 as an example, a radial basis function with weight 0.85 has membership 0.60 in “high” and membership 0.40 in “very high.” (Triangular membership functions are used in this example, but trapezoidal and sigmoid functions may also be used.)

Figure 10. Fuzzy set with five triangular membership functions.

Referring to Figure 7, the fuzzification of the set of 13 radial basis function weights results in Table 2.

Table 2 – Fuzzification of 4000-K White Light LED SPD

WavelengthWeightVery LowLowMediumHighVery High
Table 2 – Fuzzification of 4000K white light LED SPD.

The set of n fuzzified weights for a given SPD is then submitted to a fuzzy if-then rule system. Given any two fuzzified weights x1 and x2 as inputs, a typical fuzzy rule will be:

where there are multiple output classes.

Each rule calculates a “vote” t that is determined by degree of membership m for each fuzzified weight:

and where the fuzzy AND operator is implemented as the minimum of the two membership values.

Once all of the rules have been processed, their votes are aggregated:

for all votes.

This is arguably the simplest possible implementation of a fuzzy logic classifier. There are other methods for calculating and aggregating votes that are likely better for the purpose, but it is the principle that is of interest. What a fuzzy logic classifier accomplishes is a framework for representing expert knowledge of the effect of similar but different SPDs on plant growth and health, taking into consideration the plant species, plant growth stage, plant environmental conditions, and other parameters. In a sense, the fuzzy if-then rules formalize what is known about plant responses to optical radiation (e.g., Table 1) and classify horticultural luminaire SPDs accordingly.


While light-emitting diodes have provided botanists with the ability to generate precisely controlled SPDs for their research, the use of colloquial color names in their published papers has made it difficult to interpret and summarize their research results for the horticultural industry. This paper therefore proposes the use of a small number of radial basis functions to represent SPDs for horticultural lighting purposes, based on the observation that the absorption characteristics of photopigments in vivo limits the need for more-detailed SPDs. A proposal for a horticultural spectral sensor that measures radial basis function weights directly is also introduced.

Finally, a fuzzy logic classifier is proposed as a means of representing expert knowledge gained from horticultural research using fuzzy if-then rules, thereby resolving the problem of determining the similarity of two or more SPDs for horticultural lighting purposes.


ASABE. 2017. ANSI/ASABE S640 JUL2017, Quantities and Units of Electromagnetic Radiation for Plants (Photosynthetic Organisms). St. Joseph, MI: American Society of Agricultural and Biological Engineers.

Both A-J et al. 2017. Proposed product label for electric lamps used in the plant sciences. Hort Technol. 27(4):544-549.

Buhmann, MD, Jäger J. 2000. On radial basis functions. Acta Numerica. 9:1-38.

Craig DS, Runkle ES. 2013. A moderate to high red to far-red light ratio from light-emitting diodes controls flowering of short-day plants. J Am Soc Horticult Sci. 138(3):167-172.

Kuehni RG. 2003. Color Space and Its Divisions. Hoboken, NJ: John Wiley & Sons.

ISO. 2007. ISO 21348:2007(E). Space Environment (Natural and Artificial – Process for Determining Solar Irradiances. Geneva, Switzerland: ISO.

Johkan M et al. 2012. Effect of green light wavelength and intensity on photomorphogenesis and photosynthesis in Lactuca sativa. Environ Exper Botany. 75:128-133.

Maloney L. 1986. Evaluation of linear models of surface spectral reflectance with small number of parameters. J Opt Soc Am. 3(10):1673-1683.

McCree KJ. 1972. The action spectrum, absorptance and quantum yield of photosynthesis in crop plants. Agric Meteorology. 9:191-216.

Moss RA, Lewis WE. 1952. Absorption spectra of leaves. I. The visible spectrum. Plant Physiol. 27(2):370-391.

Sager JC et al. 1988. Photosynthetic efficiency and phytochrome equilibria determination using spectral data. Trans ASABE. 31(5):1882-1889.

Terashima I et al. 2009. Green light drives leaf photosynthesis more efficiently than red light in strong white light: Revisiting the enigmatic question of why leaves are green. Plant Cell Physiol. 50(4):684-697.

Westland S et al. 2000. Colour statistics of natural and man-made surfaces. Sensor Rev. 20(1):50-55.

Westland S, Ripamonti C. 2004. Computational Color Science Using Matlab, Chapter 10. Chichester, UK: John Wiley & Sons.

Ultraviolet Radiation and Viral Misinformation

Ultraviolet Radiation and Viral Misinformation

Let me begin with a smorgasbord of headlines:

These are the first half-dozen of over 200 online articles that were over a period of approximately two weeks following the publication of this Tel Aviv University press release, dated December 14, 2020:

TAU Study Proves that Light Can Kill Coronavirus

with the subtitle, “Groundbreaking research finds UV-LED diodes efficiently and cheaply disinfect social spaces.”

“Groundbreaking” research? This has a touch of hyperbole, but let’s see …

UV-LED Disinfection

On September 10, 2020, the respected Journal of Photochemistry & Photobiology, B: Biology published this paper:

Gerchman, Y., et al. 2020. “UV-LED disinfection of Coronavirus: Wavelength effect,” J. Photochemistry & Photobiology B: Biology 212 (2020) 112044 (DOI: 10.1016/j.photobiol.2020.112044).

The paper is open-access, for which the publisher deserves due credit for making its COVID-19-related research papers freely available.

The paper’s abstract is interesting:

“UV light-emitting diodes (UV LEDs) are an emerging technology and a UV source for pathogen inactivation, however low UV-LED wavelengths are costly and have low fluence rate. Our results suggest that the sensitivity of human Coronavirus (HCoV-OC43 used as SARS-CoV-2 surrogate) was wavelength dependent with 267 nm ~ 279 nm > 286 nm > 297 nm. Other viruses showed similar results, suggesting UV LED with peak emission at ~286 nm could serve as an effective tool in the fight against human Coronaviruses.”

but the introduction is more informative:

“Numerous studies have examined the sensitivity of different microorganisms (including viruses) to UV LED at different wavelengths as detailed in Table 1, for suspended viruses. However, no study to date has examined the efficiency [sic] of UV LEDs at different wavelengths on the inactivation of the human corona virus. Here, we have used the human coronavirus OC43 (HCoV-OC43) as a surrogate to the SARS-CoV-2, to develop a dose-response curve for UV-LEDs at various wavelengths.”

.. and here we need to pause in order to put these statements into context. The authors referenced twelve previous studies in their Table 1, but the key phrase here is “UV LED.” If we generalize this to “ultraviolet (UV) radiation,” there are many more studies of the relationship between wavelength and the efficacy (not “efficiency”) of UV radiation in inactivating viruses. In fact, the first study was published 144 years ago (Downes and Blunt 1877). The virucidal  action spectrum for UV radiation was first established by Rivers and Gates (1928) and Sturm et al. (1932).

Figure 1 shows the action spectra (DIN and IES) for germicidal ultraviolet radiation applications, such as upper-room air and municipal water disinfection, that have been widely adopted by the CIE, IES, ACGIH, NIOSH, DIN, and other standards organizations:

FIG. 1 – Standard germicidal response functions.

What is rarely mentioned is that these action spectra are based on laboratory results with the Escherichia coli bacterium (e.g., Gates 1930). It is not a coincidence that the peak response near 265 nm corresponds with the peak spectral absorptance of deoxyribose nucleic acid (DNA) – UV radiation disrupts the genetic code of viruses, bacteria, and fungi, thereby preventing them from reproducing (e.g., Hollaender and Oliphant 1944).

The peak spectral response of different viruses and other pathogens may therefore vary by perhaps five nanometers or so (e.g., Linden 2001). However, the DIN and IES action spectra remain applicable for practical applications of germicidal UV radiation.

… which brings us back to the current paper of Gerchman et al. (2020). The results presented in the paper are summarized in Figure 2, where the dose refers to the UV irradiance multiplied by the exposure time, and the horizontal “limit of quantification” line represents the dose required to achieve log-three (99.9 percent) inactivation of the virus colony:

FIG. 2 – HCoV-OC43 wavelength response (from Gerchman et al. 2020)

We may compare this to the IESNA germicidal response curve as enumerated in CIE 155:2003, Ultraviolet Air Disinfection, relative to the peak response at 265 nm:

WavelengthGerchman et al.IESNA
267 nm1.00 ± 0.090.990
279 nm0.82 ± 0.030.726
286 nm0.46 ± 0.050.543
297 nm0.19 ± 0.050.268
Table 1 – Spectral response comparison.

The error bars shown in Figure 2 represent standard deviations in the results, which the authors explained as “an artifact due to lack of precision in enumerating the low number of [virus] survivors.” It is refreshingly surprising to see such honesty in published results; this sort of information is usually confined to supplementary material so as to not “confuse” the reader with data that might weaken the paper’s conclusions.

Whether there are errors of fact or the authors used questionable experimental procedures is not a topic that I as a science journalist (among other things) am academically qualified to comment on.  What I will say, however, is that the paper itself has been carefully constructed and well-written, and is a model of academic writing.

So far, so good. However, there are no surprises here – the OC43 coronavirus appears to be somewhat less susceptible to longer wavelengths than E. coli bacteria, but this does not invalidate the applicability of germicidal response functions shown in Figure 1. The research is if anything no more than a confirmation of accepted scientific fact – viruses are susceptible to ultraviolet radiation, with a peak response near the DNA peak spectral absorptance of 265 nm.

The question then is, why did the editors of the Journal of Photochemistry & Photobiology decide that the paper was worthy of publication? While it is impossible to speak on behalf of the editors, one answer is that there is often value in the publication of negative results.

If the research had shown that the OC43 coronavirus was highly susceptible to longer wavelengths of UV radiation, that would have been stunning news that would have more than justified publication of the paper. Instead, the study merely confirmed that the existing standard germicidal response curves are generally applicable to HCoV-OC43, and (presumably) to SARS-CoV-2.

The academic value of the paper is therefore in describing what appears to be a carefully designed and executed series of experiments that yielded negative results. It informs other researchers of what has been done, and so allows them to direct their research efforts elsewhere.

Media Relations

There have to date been over 54 thousand academic papers relating to the SARS-CoV-2 virus that have been published in biomedical and life science journals. The probability of any one article coming to the attention of the public is basically infinitesimal … if it were not for university media relations officers.

The role of the media relations officer is to present the often-arcane details of academic research to the public. For both public and private universities, being seen in positive terms by the public is key to obtaining financial support from both public and private institutions. In other words, the role of a media relations officer is that of a marketing professional.

It is often a difficult job – how do you take a paper with a random title such as, “Genetic diversity of the Plasmodium falciparum GTP-cyclohydrolase 1, dihydrofolate reductase and dihydropteroate synthetase genes reveals new insights into sulfadoxine-pyrimethamine antimalarial drug resistance” (DOI:10.1371/journal.pgen.1009268), and present it to a public more interested in YouTube celebrities and sports figures? You begin your press release with a catchy title such as, “New mutations in malaria parasite encourage resistance against key preventative drug,” but the only qualification for the job is usually a bachelor’s degree in journalism. The media relations officer, through misunderstanding their interview with the researcher or lack of knowledge and experience, may fail, sometimes spectacularly.

Tel Aviv University

I have no desire to name and shame the media relation officer at Tel Aviv University in Israel, but I must say something. Here are a few select quotes for the aforementioned press release of December 14th, 2020:

“Researchers from Tel Aviv University have proven that the coronavirus can be killed efficiently, quickly and cheaply using ultraviolet (UV) light-emitting diodes (UV-LEDs). This is the first study in the world conducted on the disinfection efficiency of a virus from the family of coronaviruses using UV-LED irradiation at different wavelengths or frequencies.”

This may be technically true, but the significance of this study is grossly overrated in calling it, “the first study in the world.”

“In the study, the researchers tested the optimal wavelength for killing the coronavirus, and found that a length of 285 nanometers was almost as efficient in disinfecting the virus as a wavelength of 265 nanometers, requiring less than half a minute to destroy more than 99.9% of the coronaviruses.”

It does not take a mathematician to understand that a relative efficacy of 46 ± 5 percent is not “almost as efficient.”

“The entire world is currently looking for effective solutions to disinfect the coronavirus … The disinfection systems based on LED bulbs, however, can be installed in the ventilation system and air conditioner, for example, and sterilize the air sucked in and then emitted into the room.”

This is true, but it has nothing to do with the research paper. The popular press has been awash with stories about germicidal disinfection using radiation, a technology that has been in commercial use since 1909 (von Recklinghausen 1914). Economics currently favour low-pressure mercury vapour lamps that emit ultraviolet-C radiation at 254 nm, but rapid progress is being made in the development of more efficient and inexpensive ultraviolet LEDs. Again, nothing to do with this paper.

Quoting one of the paper’s authors from the press release, “We discovered that it is quite simple to kill the coronavirus using LED bulbs that radiate ultraviolet light, but no less important, we killed the viruses using cheaper and more readily available LED bulbs, which consume little energy and do not contain mercury like regular bulbs.”

This is … painful. Even if the author meant specifically the OC43 coronavirus and ultraviolet radiation generated by light-emitting diodes rather than mercury-vapour lamps, stating that anything was “discovered” is simply ludicrous.

It should further be noted that the radiant efficacy of commercially-available 285 nm UV-LEDs is currently on the order of one percent. This may be compared to that of mercury-vapour lamps, with efficacies on the order of 40 percent. The advantage of UV-LEDs is that it is much easier to direct their emitted radiation into narrow beams, a requirement for upper-room air disinfection devices.


“Last year, a team of researchers led by Prof. Mamane and Prof. Gerchman patented a combination of different UV frequencies that cause dual-system damage to the genetic load and proteins of bacteria and viruses, from which they cannot recover – which is a key factor that is ignored.”

This is one of those, “Wait, what?” moments, where an entirely different and much more relevant press release could have been written about this technology. The patent application in question is US Patent Application 20200255305, “Method and Device for Water Disinfection,” that uses two sources to simultaneously emit UV-C and UV-B radiation. Patent applications are not peer-reviewed, however, and so the information needs independent verification and much more detail concerning the experiments behind the invention.

The primary problem is that the media relations officer was clearly struggling to understand the issues and write a coherent and informative press release. At best, there appears to have been “a failure to communicate.”

Viral Misinformation

Whatever misinformation and confusion there may have been in the press release, it won the lottery in being selected by the mass media for amplification. Looking at only the first six of some 200 article titles:

99.9% of Covid-19 virus dead in 30 seconds with UV LEDs, says Tel Aviv research.”

No, neither the paper nor the press release made any claims regarding the SARS-CoV-2 virus that causes the COVID-19 disease. It explicitly stated that the HCoV-OC43 virus (which is one of the many viruses responsible for the common cold) was chosen as a surrogate for the SARS-CoV-2 virus, and that “… our future work will confirm these results by testing the impact of LEDs and their combinations on SARS-CoV-2.”

You can kill Covid with a flick of a switch, study shows.”

The paper never suggested such an outlandish idea. It instead focused on the relative dose of ultraviolet radiation at different wavelengths to inactivate (not “kill”) a particular virus. The irradiance levels employed in the experiments would be totally impractical for surface disinfection in the real world.

Tel Aviv research: 99.9% of COVID-19 virus dead in 30 seconds with UV LEDs.”

The paper uses the noun “seconds” just once, referring to “… up to 60 s for 267 and 279 nm and up to 90 s for 286 and 297 nm.” The key metric is dose – ultraviolet irradiance multiplied by exposure time. Whoever wrote this headline simply invented the number as clickbait.

… and so it goes, like some sinister version of the children’s game Chinese whispers. Perfectly reasonable and valid scientific information is endlessly repeated and distorted from paper to press release to mass media articles. Like most such events, the story will have a half-life measured in weeks to a few months before it is forgotten. Unfortunately, the misinformation spreads like a virus, mutating at each step of transmission while driven by the need for favorable press coverage and website advertising revenue … and we are all the poorer for it.


Downes, A., and T. P.  Blunt. 1877.” Research on the effect of light upon bacteria and other organisms,” Proc. Royal Society of London 26:488-500.

Gates, F. L. 1930. “A study of the bactericidal action of ultra violet light: III. The absorption of ultra violet light by bacteria,” J. General Physiology 14:31-42.

Hollaender, A., and J. W. Oliphant. 1944. “The inactivating effect of monochromatic ultraviolet radiation on influenza virus,” J. Bacteriology 48:447-54.

Linden, K. G. 2001. “Comparative effects of UV wavelengths for the inactivation of Cryptosporidium parvum oocysts in water,” Water Science & Technology 34(12):171–174.

Rivers, T., and F. Gates. 1928. “Ultra-violet light and vaccine virus. II. The effect of monochromatic ultraviolet light upon vaccine virus,” J. Experimental Medicine 47:45-49.

Sturm, E., et al. 1932. “Properties of the causative agent of a chicken tumor. II. The inactivation of the tumor-producing agent by monochromatic ultra-violet light,” J. Experimental Medicine 55:441-444.

von Recklinghausen M., 1914. “The Ultra-Violet rays and their application for the sterilization of water,” J. Franklin Institute 178(6):681–704.

Ultraviolet Radiation Terminology

Ultraviolet Radiation Terminology

A word of caution: I am going to be annoyingly pedantic here, but with good reason. The lighting industry has a century-long history of introducing unfamiliar technologies using familiar terminology. We later come to regret our choice of words when it becomes necessary to express precisely what we mean.

Consider, for example, the term luminance. We understand this today to mean “luminous flux per unit solid angle per unit projected source area,” which we express in candela per square meter (where a candela is one lumen per steradian), or sometimes nits. Seventy years or so ago, however, it was common to refer to “brightness,” which today is considered a subjective attribute of a point light sources (and not to be confused with “lightness,” a subjective attribute of an area light source).

The lighting industry at first distinguished between “brightness” and “photometric brightness,” but eventually accepted “luminance” as the companion of radiance. (We will not talk about “helios,” an alternative proposed and rejected in the 1940s.) In the process, it also deprecated alternate units of measurement for luminance, including stilbs, lamberts, apostilbs, skots, brils and foot-lamberts.

UnitConversion Factor
brilπ x 107
foot-lambertπ / (0.3048)2
lambertπ x 10-4
skotπ x 103
Table 1 – Deprecated luminance units

Luminance and candela per square meter – life is so much easier when we can agree on the terminology!

Ultraviolet …

Courtesy of the current pandemic, the lighting industry is becoming all too familiar with a technology that has been in use since the 1930s – germicidal lamps emitting ultraviolet radiation that inactivate bacteria and viruses. Sadly, we are once again using familiar terminology that we will later have to deprecate in order to express what we really mean.

It does not have to be like this, however. It may be as futile an exercise as the legend of King Canute the Great ordering the tide to stop, but we can at least examine what terminology we should be using when discussing ultraviolet radiation as a means of disinfection.

We begin with the term ultraviolet, which designates a region of the electromagnetic spectrum beyond that of visible light with wavelengths of approximately 400 nm to 700 nm. The ultraviolet region is divided by the International Lighting Commission (CIE) into three subregions:

UV-A315 nm – 400 nm
UV-B280 nm – 315 nm
UV-C100 nm – 280 nm
Table 2 – Ultraviolet spectral regions

Here, however, is the point: “ultraviolet” is an adjective. It makes as much sense to refer to “ultraviolet” as it does to refer to “blue” – blue what? Just as we invariably refer to “blue light” in lighting design, we should refer to ultraviolet radiation. (For the record, light is “the natural agent that stimulates sight and makes things visible”; all else – including light – is electromagnetic radiation.)


From here, we move on to the term germicidal. This refers to the effect of UV radiation on pathogens, including viruses, bacteria, and fungi. Ultraviolet photons have enough energy to disrupt the DNA and RNA of bacteria and fungal cells, preventing the organisms from reproducing and thus inactivating them; eventually, they die. Viruses are not technically alive, but in disrupting their genetic code, the UV radiation prevents them from invading living cells and replicating, and thus inactivates them as well.

Unfortunately, this has led to the increasingly popular term germicidal UV, abbreviated GUV. Respecting the previous argument, a better term is ultraviolet germicidal irradiation (UVGI). This has a long history of use in the ultraviolet disinfection community, and the lighting industry should respect it as being unambiguous.


For anyone in the lighting industry, a product that emits visible light is a luminaire. … except that ultraviolet radiation is not light.

There are several types of ultraviolet radiation sources:

Low-pressure mercury-vapor arc lamps

These are basically linear or compact fluorescent lamps without phosphor coatings to convert ultraviolet radiation into visible light, and fused quartz or “soft” glass tubes that are transparent to UV-C radiation. They emit essentially monochromatic UV-C radiation with a wavelength of 254 nm. (The tubes are usually designed to block 185 nm radiation, which can generate toxic ozone.) They are typically used for “upper-room” air disinfection (CIE 2003), HVAC air ducts, and mobile ultraviolet disinfection platforms and robots.

Medium-pressure mercury-vapor arc lamps

These are similar to high-intensity discharge (HID) lamps, but feature broadband radiation centered around 250 nm. They are mostly used for water disinfection purposes.

Pulsed xenon lamps

These are basically high-power electronic flash lamps which emit intense broadband radiation across the entire ultraviolet spectrum with a peak near 230 nm. They are commonly used with mobile disinfection platforms in hospitals.

Excimer lamps

Krypton-chlorine (KrCl*) excimer lamps emit mostly ultraviolet radiation with a peak wavelength of 222 nm, and are being explored as a safer alternative to low-pressure mercury-vapor arc lamps.

Microplasma emitters

Microplasma emitters are based on the essentially the same technology formerly used to produce plasma television screens, but are designed to emit ultraviolet radiation with a peak wavelength of 222 nm.


Ultraviolet-emitting LEDs are becoming increasing available, although to date with exceedingly low radiant efficiencies of one to four percent and peak wavelengths no shorter than 265 nm for commercial devices. However, the roadmap for the development of UV-C LEDs promises much greater efficiencies in the foreseeable future (Krames 2020).

For germicidal purposes, the optimal wavelength for disrupting the DNA of pathogens is 265 nm (Figure 1). Once again, however, this is electromagnetic radiation and not light. It may therefore be preferable to refer to UVGI sources rather than “luminaires.”

FIG. 1 – Germicidal response function for ultraviolet radiation (adapted from CIE 2003).


This should be an easy one. For proper UVGI systems design, we will need to know the intensity distribution of the UVGI sources, just like we have luminous intensity distributions for luminaires and photometric data reports.

Measuring these characteristics in the laboratory is not easy. Integrating spheres measuring two meters or more in diameter are often used to measure the luminous flux generated by luminaires, but the typical barium sulphate (BaSO4) coating is not sufficiently reflective for ultraviolet radiation measurements. Microstructured polytetrafluoroethylene (PTFE), perhaps best known as Labsphere Inc.’s diffuse reflectance standard Spectralon, has acceptable reflectance (> 95 percent at 250 nm), but it must be machined rather than applied as a coating.

Regardless, what is being measured is the radiant intensity distribution of the UVGI sources, with corresponding radiometric data reports. This further implies that these reports should be based on the ANSI/IES TM-33-18 luminaire optical data document specification rather than ANSI/IES LM-63-19 (and earlier editions) and EULUMDAT file formats, which are specific to photometric data (IES 2018). (The Italian UNI 11733:2019 standard is a mirror document to ANSI/IES TM-33-18.) This document specification was developed specifically with ultraviolet radiometry in mind.

More generally, we should always use radiometric units when referring to the performance of UVGI systems:

Illuminance (lm/m2)Irradiance (μW/cm2)
Luminous intensity (cd)Radiant intensity (μW/sr)
Luminance (cd/m2)Radiance (μW/sr-cm2)
Radiant fluence (mJ/cm2)
Table 3 – Photometric versus radiometric terminology

The International Bureau of Weights and Measurements defines radiometric measures in terms of watts, meters, and steradians, but UVGI systems typically express measurements in microwatts (μW) and square centimeters (cm2) for convenience (BIPM 2019).

Radiant Dose

If, as a professional lighting designer, you did a double take on the term radiant fluence, it is understandable – what is this? ANSI/IES RP-16-17 (ANSI 2017) defines “radiant fluence” as “the omnidirectional radiant energy externally incident on an elementary sphere per unit cross-sectional area in time Δt,” or more succinctly:

where is the differential solid angle, da is the cross-sectional area of the sphere, and Δt is the exposure time. (I warned you that I was going to be annoyingly pedantic.)

Fortunately, this metric is much simpler conceptually than its formal definition implies, and it is crucial to an understanding of UVGI system performance.

The SARS-CoV-2 virus is transmitted primarily by respiratory droplets and aerosols that are generated by the simple acts of breathing, talking, sneezing, coughing and singing. The same is true, however, for many other viruses, bacteria, and fungi, including those responsible for the common cold, influenza, measles, chickenpox, and tuberculosis. Respiratory droplets, which are typically larger than 5 μm and consist mostly of water, fall to the ground rapidly after being produced. Aerosols, on the other hand, are relatively dry and may persist in the air for several hours.

Imagine then a droplet or aerosol as a transparent sphere holding the viruses or bacteria in solution. Seen from any direction, the cross-sectional area is defined by the equation for the area of a circle: π x r2, where r is the sphere radius.

Now think of a parallel beam of ultraviolet radiation in this direction. The beam has a measurable radiance L that is expressed in microwatts per steradian per square centimeter (μW/sr-cm2). Conceptually, it is the radiant power of the ultraviolet photons passing through the transparent sphere in the given direction at a specific moment in time.

If we sum the beam radiance for all possible directions, we have the irradiance of the sphere. We can make the diameter of the sphere infinitesimally small, in which case we have the spherical irradiance (aka the fluence rate) of the elementary sphere at a point in free space, measured in microwatts per square centimeter (μW/cm2).

It is not however the irradiance that is important for germicidal action, but rather the radiant dose or fluence. The aerosol particle may drift through the air and thus be exposed to varying levels of spherical irradiance. The longer the particle is exposed to radiation, however, the more likely it is that a UV photon will intersect the DNA or RNA of a virus (which typically has a diameter of less than 100 nanometers) and disrupt it.

Fluence then is the spherical irradiance (measured in μW/cm2) of the particle integrated (i.e., summed) over time (measured in seconds). One joule (energy) is one watt (power) times one second. For UVGI purposes is typically expressed in millijoules per square centimeter (mJ/cm2).

Fluence is defined specifically for aerosols (and more generally points in free space), but the concept of radiant dose also applies to the irradiation of surfaces. For a uniform distribution of radiance (i.e., equal in all directions), the irradiance of a surface is 1 / π times that of spherical irradiance without the surface at that point.

Choosing Radiant Dose

When the UV dose results in a 90 percent disinfection (10 percent survival), it is referred to as D90. For UVGI applications where higher disinfection rates are needed, D99 (i.e., 99 percent disinfection) is often used. To completely disinfect a surgical tool, it is common to expect 99.9999 percent disinfection. There may still be a few active viruses or bacteria that remain, but their density is too low to achieve an infection or replicate in a colony.

The decrease in survival with larger UV doses is exponential – if 10 mJ/cm2 results in 90 percent disinfection, 20 mJ/cm2 results in 99 percent, 30 mJ/cm2 in 99.9 percent, and so on. This is an example of exponential decay, and so it is more convenient to refer to the disinfection in logarithmic units: D90 is log-1 , D99 is log-2, and so on to log-6 for complete disinfection.

There are however complications in determining an appropriate UV dose for inactivating pathogens. Table 4 (adapted from Kowalski 2009) presents the average radiant doses required to achieve D90 disinfection for pathogens:

PathogenTypeWaterSurfaceAir – Low RHAir – High RH
Table 4 – Average UV dose in mJ/cm2 required to achieve D90 inactivation.
(RH – relative humidity, expressed in percent.)

The first complication is that the pathogen species and its environment matters. Hundreds of studies have reported on the susceptibility of many different pathogen species to either monochromatic (254 nm) or broadband UV irradiation, where the pathogens are: a) suspended in water; b) cultured on a growth medium on a surface (e.g., a Petri dish); or c) suspended in airborne droplets or aerosol particles of various sizes. As shown in the table, the relative humidity of the air can have a very significant effect on vegetative bacteria (but not viruses).

The second complication is that Table 4 represents the average dose for D90 disinfection. Depending on the pathogen species and environment, the required dose can vary by a factor of 100 or more from these averages. Designing a UVGI system with an average radiant dose does not guarantee protection from specific pathogens. The choice of narrowband versus broadband radiation sources also matters, especially for bacteria.

The third complication is that once D90 disinfection has been achieved, it is often the case that the surviving 10 percent of the population is an order of magnitude more resistant to UV irradiation. For example, if it takes 10 mJ/cm2 to achieve D90 disinfection, it may take 40 mJ/cm2 to achieve D99 disinfection.. As shown in Figure 2, such pathogens are said to have a “two-step” rare constant for their susceptibility to UV-C radiation.

FIG. 2 – One-step versus two-step pathogen susceptibility to UV-C radiation.

A fourth complication is that when vegetative bacteria and fungi have been exposed to monochromatic UV radiation as a means of disinfection, subsequent exposure to visible light may enable the “killed” cells to repair their DNA and recolonize, thereby increasing their survival rate by 10 to 100 times, especially if the pathogens are suspended in water or present on surfaces in the presence of high relative humidity. (Viruses do not seem to have the complexity needed to effect self-repair of their DNA.) This may be a concern if, for example, the exposed surfaces of a hospital room are decontaminated with a mobile UV disinfection robot, but the room is flooded with direct sunlight thereafter.

A fifth and final complication is that once a surface has been infected with respiratory droplets, any bacteria may find sufficient resources to begin colonizing the surface. The surface may be continuously irradiated by, for example, UV‑C radiation from a microplasma emitter or UV-LED array, but if the irradiance is too low, the surface may not achieve even D90 decontamination, regardless of the exposure time.


There is admittedly a considerable amount of information here that extends beyond the bounds of ultraviolet radiation terminology. It is needed, however, to put the terminology used for decades by the ultraviolet disinfection community into context. Ultraviolet radiation is not visible light, and so the lighting community needs to both understand and respect the terminology when adopting UVGI system design practices for building safety and human health. If we do not use the correct terminology, we risk (and deserve) a plague of apostilbs, brils, lamberts, skots and other annoyances later down the road.


BIPM. 2019. The International System of Units (SI), 9th Edition (

CIE. 2003. CIE 155:2003, Ultraviolet air Disinfection. Vienna, Austria: CIE Central Bureau.

CIE. Undated. CIE International Lighting Vocabulary (

Kowalski, W. 2009. Ultraviolet Germicidal Irradiation Handbook: UVGI for air and Surface Disinfection. Heidelberg, Germany: Springer.

IES. 2017. ANSI/IES RP-16-17, Nomenclature and Definitions for Illuminating Engineering (

IES. 2018. ANSI/IES TM-33-18, Standard for the Electronic Transfer of Luminaire Optical Data. New York, NY: Illuminating Engineering Society.

Krames, M. 2020. “The Rise of UV-C LEDs,” LEDs Magazine 125:36-39.

Spherical Irradiance and Aerosols

Spherical Irradiance and Aerosols

Lighting designers will be familiar with the illuminance of a planar surface, which is measured in lumens per square meter (or foot). The irradiance of a planar surface by a germicidal radiation source is conceptually the same, except that it is measured in watts per square meter (W/m2). Most designers, however, will not be familiar with the concept of the spherical irradiance (or equivalently, fluence rate) of a point in space. This is an essential metric for air disinfection by germicidal lamps, and so it needs to be understood.

Figure 1 shows a parallel beam of UV radiation A with a cross-sectional power density of ΦA watts of power (flux) per square meter (W/m2) irradiating a surface oriented perpendicular to the beam. The beam can be made infinitesimally narrow, thereby irradiating a point on the surface, with irradiance EA = ΦA W/m2.

Figure 1. Surface irradiance.

Another parallel beam B with ΦB watts per square meter intersects the surface with incidence angle θ, the surface irradiance due to this beam being EB = (ΦB cos θ) W/m2. If we sum the contribution of beams from all possible directions above the surface of the plane, we obtain the surface irradiance ES. For germicidal applications, this irradiance is typically measured in microwatts per square centimeter (µW/cm2).

Irradiance is an appropriate metric for pathogens cultured on a plate, but aerosols are miniscule droplets of water suspended in air. If these droplets have been expelled by someone with an infectious respiratory disease such as tuberculosis or influenza, they may contain bacteria or viruses. These pathogens can be inactivated by UV radiation, but there is no planar surface that is being irradiated, and so the concept of irradiance does not apply.

Figure 2 represents the droplet as a transparent sphere S that does not absorb or refract UV radiation. The sphere has radius r, and so has a cross-sectional area π r2. If this sphere is irradiated by a beam A with cross-sectional power density ΦA W/m2, its irradiance is the same as the beam power density (again typically measured in µW/cm2). The diameter of the sphere can be made infinitesimally small, yielding an elementary sphere – a point in space – while the irradiance remains constant.

Figure 2. Spherical irradiance.

If we sum the contribution of beams from all possible directions of an imaginary sphere surrounding the point, we obtain the spherical irradiance of the point in space. This is also referred to as the fluence rate, which, when multiplied by the exposure time (and assuming a constant rate), yields the fluence of the droplet (typically measured in millijoules per square centimeter, mJ/cm2). If the fluence rate varies (as when the droplet moves through a three-dimensional UV radiation field), the fluence is the sum of the fluence rates multiplied by the chosen time intervals, which may vary from milliseconds to hours.

Finally, the term germicidal dose applies to both the irradiance of surfaces and the spherical irradiance of aerosols, but the terms fluence and fluence rate apply only to aerosols.

About Those Standards …

The concept of spherical irradiance may be simple to understand, but only if you ignore the official definitions. The CIE International Lighting Vocabulary has taken the approach that more is better, offering a choice of:

17-742Luminous spherical exposure (see 17-1028)
17-1244Spherical illuminance (see 17-1245)
Photometric Quantities
17-454Fluence (see 17-1028)
17-455Fluence rate (see 17-1245)
17-1023Radiant fluence (see 17-1028)
17-1024Radiant fluence rate (see 17-1245)
17-1028Radiant spherical exposure
17-1245Spherical irradiance
Radiometric Quantities
17-883Photobiological fluence (see 17-1028)
17-884Photobiological fluence rate (see 17-1245)
Photobiological Quantities
17-925Photon fluence (see 17-1028)
17-926Photon fluence rate (see 17-1245)
17-933Photon spherical exposure (see 17-1028)
17-934Photon spherical irradiance (see 17-1245)
Photon Quantities

In other words, spherical irradiance (or fluence rate) and fluence with different spectral weightings such as V(λ) for luminous quantities and photobiological action spectra, including photon quantities for horticultural applications.

Trying to understand the CIE definition of fluence rate can be a challenge:

Quantity defined by the formula:

where dΩ is the solid angle of each elementary beam passing through the given point and Le its radiance at that point.

This makes sense if you think of an “elementary beam” as being an infinitesimally narrow cone (which makes it equivalent to the infinitesimally narrow beam discussed above), but there is the unnecessary complication of defining the beam radiance in terms of point sources. It is conceptually much easier to begin with parallel beams that can be made infinitesimally narrow.

ANSI/IES RP-16-17, Nomenclature and Definitions, on the other hand, is decidedly more spartan:

  • Luminous fluence
  • Luminous fluence rate
  • Radiant fluence
  • Radiant fluence rate
  • Spectral radiant fluence
  • Spectral radiant fluence rate

In addition to being spartan, the IES definitions are both clear and concise:

Fluence rate: The omnidirectional radiant flux externally incident on an elementary sphere about the point, per cross-sectional area of the sphere.

Fluence: The omnidirectional radiant energy externally incident on an elementary sphere about the point, per cross-sectional area of the sphere.

The advantage of these definitions is that they make no mention of radiance or solid angles.

Measuring Spherical Irradiance

CIE-ILV 17-1245, Spherical Irradiance, includes this helpful note:

This is the appropriate radiometric quantity for describing a dose rate for a photobiological or photochemical effect in a scattering medium (e.g., light in skin). It is also the appropriate quantity for describing the irradiation of microorganisms. It is frequently used incorrectly as a substitute for irradiance in some publications.

This leads to the obvious question: how do you measure spherical irradiance?

One approach is to use a spherical actinometer, an instrument that consist of a hollow quartz sphere measuring a centimeter or so in diameter that is filled with a solution of ferrioxalate, persulfate, or iodide/iodate; their transmittance after exposure is linearly proportional to the UV-C fluence (e.g., Kowalski 2009).

Another option is to use a radiometer with a “scalar irradiance” (yet another synonym for fluence rate) collector and a narrowband ultraviolet filter, such as the AMOUR radiometer manufactured by Biospherical Instruments (San Diego, CA). However, such an instrument has a “blind spot” of approximately 80 degrees where the spherical Teflon collector is mounted on its connector shaft.

Figure 3. Spherical irradiance meter. (Source:

UPDATE 20/08/28 – The manufacturer has stated that their AMOUR radiometer is not capable of measuring UV-C radiation due to excess absorption by the Teflon collector.

It may be difficult to measure spherical irradiance, but it is possible to predict its three-dimensional distribution in space using modified lighting design and analysis software, including contributions of direct radiation from UV-C sources and interreflections from surfaces. However, as Pierre de Fermat said regarding his Last Theorem, “I have discovered a truly remarkable theorem … which this margin is too small to contain.” More details to follow …


Thanks to Dawn DeGrazio of the Illuminating Engineering Society for her review and comments.


Kowalski, W. 2009. Ultraviolet Germicidal Irradiation Handbook: UVGI for Air and Surface Disinfection. Heidelberg, Germany: Springer.

Defining Photosynthetic Photon Efficacy

Defining Photosynthetic Photon Efficacy

There has been some discussion online and in presentations recently about the issue of photosynthetic photon flux. The argument goes as follows:

  1. Photosynthetically Active Radiation (PAR) is somewhat arbitrarily defined as optical radiation within the spectral range of 400 nm to 700 nm.
  2. Exposing plants to far-red radiation (defined as 700 nm to 800 nm) results in an increase in the rate of photosynthesis – the Emerson effect that was first noted in 1957 and confirmed by recent research.
  3. Many horticultural luminaire manufacturers are now including far-red (725 nm) LEDs in their products.
  4. The photosynthetic photon efficacy (PPE) of these luminaires is penalized by the definition of PAR because the far-red radiation is not taken into consideration.
  5. The definition of PAR therefore needs to be changed to allow fair comparison of these products.

The one-word answer to this argument is … no.

Photosynthetic Photon Efficacy

ANSI/ASABE S640 JUL2017, Quantities and Units of Electromagnetic Radiation for Plants (Photosynthetic Organisms): defines Photosynthetic Photon Efficacy (PPE) as:

The photosynthetic photon efficacy (Kp) is the photosynthetic photon flux divided by input electric power. The unit is micromoles per second per electric watt (μmol × s-1 × We-1), or micromoles per joule (μmol × J-1).

Ignoring the technical jargon, the key point here is micromoles of photons. Photosynthesis occurs when a photon is absorbed by a photopigment (primarily chlorophyll A or B). In accordance with the Stark-Einstein law (aka the second law of photochemistry), one photon initiates one chemical reaction, regardless of the photon’s wavelength. We must therefore count the number of photons per second (measured in micromoles per second) rather than lumens or radiant watts for horticultural purposes.

Referring to FIG. 1, McCree (1972) measured the relationship between wavelength and photosynthesis to produce the averaged “McCree curve.” He also acknowledged the Stark-Einstein law, which accounts for the blue line between 400 nm and 700m. What this means is that we can ignore the spectral power distribution of any light source within the range of 400 nm to 700 nm. As long as we have a calibrated PAR (aka “quantum”) sensor – which is basically a radiant wattmeter with the spectral response shown in FIG. 1 – we can measure micromoles of photons per second.

Emerson Effect

In a recent paper, Zhen and Bugbee (2020) presented an excellent argument in favor of redefining photosynthetically active radiation to include the spectral range of 400 nm to 750 nm. The title of the paper even includes the phrase, “Implications for Redefining Photosynthetically Active Radiation.”

The authors are unquestionably correct; far-red photons in effect supercharge the process of photosynthesis, and must – not should, but must – be taken into consideration when defining photosynthetically active radiation.

This does not mean however that the PAR metric should be redefined. Quoting from the abstract of Zhen and Bugbee (2020): “Far-red alone minimally increased photosynthesis … far-red photons are equally efficient at driving canopy photosynthesis when acting synergistically with traditionally defined photosynthetic photons.” In other words, if we assume a spectral range of 400 nm to 750 nm, we cannot unambiguously measure the photosynthetic photon efficacy of a light source without knowing its spectral power distribution. That is, without knowledge of the entire spectral power distribution within this range, we cannot predict the rate of photosynthesis.

Figure 1 – McCree curve and PAR sensor response.

To rephrase the issue, the current definition of PAR assumes that the photosynthesis rate of higher plants is linear with respect to incident radiation within the spectral range of 400 nm to 700 nm. There are obviously minimum and maximum irradiance limits to where this assumption applies, but it is necessary in order for the concept of PAR and hence PPE to have any meaning.

The Emerson effect violates this assumption by making the photosynthesis rate nonlinear – add far-red radiation beyond 700 nm and you will change the rate in the manner that depends on the spectral power distribution of the PAR radiation.

This issue does not concern just horticultural luminaires with 725 nm far-red LEDs. Most red-emitting phosphors used in white-light LEDs and red-emitting phosphor-coated LEDs (e.g., Figure 2) have significant emissions in the far-red, and so may invoke the Emerson effect.

FIG. 2 – LED growlight with phosphor red (Source:

Regardless, it remains that the definition of PAR cannot redefined. It is not a matter of penalizing horticultural luminaires with far-red emissions, but of simply having a metric that makes sense.

Photomorphological Considerations

There is an additional complication with far-red radiation. Horticultural luminaires including far-red LEDs typically employ 660 nm red and 725 nm far-red LEDs. These wavelengths correspond nicely with the peak absorptances of the Pr and Pfr isoforms of phytochrome, a plant photoreceptor that is responsible for plant morphology from seed germination to leaf senescence, shade avoidance, and circadian rhythms. By offering luminaires with fixed red to far-red (R:FR) ratios, luminaire manufacturers have only begun to explore the horticultural possibilities of far-red radiation.

Does the current definition of Photosynthetically Active Radiation and hence Photosynthetic Photon Efficacy disadvantage horticultural luminaire manufacturers who include far-red LEDs in their products? In one sense, the answer is yes. However, this is a very narrow view of the issue that focuses on a single metric. The goal should be to educate the customer that despite a possibly lower PPE value for the product, the far-red radiation represents a value-added feature.


ANSI/ASABE S640 JUL2017, Quantities and Units of Electromagnetic Radiation for Plants (Photosynthetic Organisms). St. Joseph, MI: American Society of Agricultural and Biological Engineers.

McCree, K. J. 1972a. “The Action Spectrum, Absorptance and Quantum Yield of Photosynthesis in Crop Plants,” Agricultural and Forest Meteorology 9:191-216.

Zhen, S., and B. Bugbee. 2020. “Far-red Photons Have Equivalent Efficiency to Traditional Photosynthetic Photons: Implications for Redefining Photosynthetically Active Radiation,” Plant Cell Environ. 2020:1-14. DOI: 10.1111/pce.1370.