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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.

Designing an Ultraviolet-C Disinfection System

Designing an Ultraviolet-C Disinfection System

Published 2020/05/29 by the Illuminating Engineering Society as

Germicidal lamps emitting ultraviolet-C (UV-C) radiation have been in use since the 1930s (Wells and Wells 1936). These are most commonly low-pressure mercury-vapor discharge lamps, which are basically fluorescent lamps without a phosphor coating and fused quartz rather than borosilicate glass bulbs. They emit monochromatic radiation mostly at 254 nm, a wavelength that is very effective in disrupting the DNA of viruses, bacteria, and other pathogens.

The photobiological risks of these germicidal lamps are well-known: exposure to UV-C radiation can result in photokeratitis (“snow blindness”), photoconjunctivitis (“pink eye”), and erythema (sunburn). These medical conditions typically only last for a few days, but they can be quite painful. Unlike UV-B radiation (280 nm to 315 nm), UV-C radiation is much less likely to cause long-term cellular damage leading to skin cancer,

More recent germicidal light sources include UV-C light-emitting diodes and pulsed xenon discharge lamps, but there is a newcomer on the block that has gained considerable media attention: far-ultraviolet excimer lamps. Recent medical studies have indicated that, unlike 254 nm radiation, the 207 nm and 222-nm “far-UV” radiation emitted by excimer lamps is likely harmless (e.g., Buonanno et al. 2017, Welch et al. 2018). Excimer lamps have the same germicidal properties as mercury-vapor discharge lamps, but the shorter wavelength radiation cannot penetrate deeply enough into the outermost cells of the eyes and skin to disrupt their DNA.

This leads to the exciting thought that we may be able to design UV-C germicidal systems using far-UV excimer lamps. Unlike mercury-vapor lamps and UV-C LEDs, there does not appear to be any significant photobiological risk (if their residual UV-C emissions are blocked), and so they could be deployed in direct view of the room occupants while disinfecting both the air and contaminated surfaces with their radiation.

Indeed, there are already companies advertising such products, although they do not appear to be commercially available as yet. This does not stop us, however, from asking the question: what does it take to design a UV-C disinfection system using far-UV radiation?

Excimer Lamps

Excimer lamps consist of diatomic molecules that form a plasma when an electrical current passes through them. A combination of krypton and chlorine (KrCl) gases, for example, emits 222-nm radiation, while krypton and bromine emit 207-nm radiation.

Companies such as Ushio and SterilRay manufacture excimer lamps and products for industrial and medical applications, but the lamps are typically comparable to fluorescent lamps in size and form factor (e.g., FIG. 1). However, one company – Eden Park Illumination – has adapted the technology formerly used in plasma television displays to produce thin microplasma lamps intended for general illumination purposes. One of their evaluation products is of particular interest, as it generates nearly monochromatic 222 nm UV-C radiation (FIG. 2).

FIG. 1 – Excimer Wave SterilRay Luminaire. (Source:

Figure 2 – 222-nm microplasma excimer lamp (Source:

The published specifications of this product are not particularly remarkable, but they are useful in that they enable us to evaluate the usefulness of this technology for germicidal applications. Eden Park states that they have achieved a maximum irradiance greater than 25 μW/cm2 in the laboratory, but this is presumably with a much shorter lifetime. (The criterion for lifetime is not defined, but presumably refers to UV-C output power depreciation over time.)

Germicidal Dose

A key characteristic of germicidal lamps of any sort is the UV-C radiation dose (irradiance multiplied by exposure time), expressed in millijoules per square centimeter (mJ/cm2). The required dose depends on both the pathogen species to be eliminated and the desired degree of reduction. For example, eliminating 90 percent of Escherichia coli O157:H7, the bacterium that causes sometimes fatal food poisoning, requires 1.5 mJ/cm2; doubling the dose eliminates 99 percent, tripling eliminates 99.9%, and so forth. This is referred to as log10 (“log-ten”), or more commonly “log,” reduction:

Log10 reductionPercent pathogen elimination
Table 1 – Log10 pathogen reduction.

The International Ultraviolet Association publishes a compilation of dose requirements for many different pathogens, but viruses on average require a dose of about 20 mJ/cm2 for 90 percent reduction when directly exposed to the UV-C radiation (IUVA undated). Most of the studies referenced in the compilation consider 254 nm radiation from low-pressure mercury vapor lamps, but the required dose from 207-nm and 222-nm excimer lamps should be comparable.

The goal, of course, is to provide sufficient UV-C irradiance that the desired log reduction is achieved within an allotted time. With this in mind, it is useful to calculate the expected irradiance versus distance for the 222-nm microplasma excimer lamp (FIG. 3).

Figure 3 – Irradiance from 25 cm2 microplasma lamp versus distance.

It should be noted that the excimer lamp is an area source, and so the inverse square law does not apply in the near-field, or a distance of less than 10 inches. It was also assumed that the lamp has a Lambertian (i.e., cosine) radiant intensity distribution.

This plot is useful in that the irradiance versus distance values will give us a sanity check for the next phase of the design. For example, if we have a requirement for 90 percent virus reduction in 20 seconds, we would need an irradiance of 1,000 μW/cm2, requiring a distance of less than two inches from the lamp. Even if the lamp produced the maximum reported output of 25 μW/cm2, the maximum distance would still be less than five inches.

Doorway Disinfection

A possible design of obvious interest is to place the excimer lamps in the frame of a doorway or entry portal, much like an airport security scanner. Anyone passing through the doorway can – in theory – be disinfected.

In theory … the goal here is not to design a specific disinfection system, but to consider the factors that go into its design. What we learn from this exercise can be used to guide engineering design for commercially-realizable disinfection systems. Conceptually then, we want a system wherein a person walks into the doorway, does a complete 360-degree turn, then continues on after being disinfected by the (presumably) safe 222-nm UV-C radiation.

The doorway shown in Figure 4 has an opening 50 inches wide by 80 inches high. It has four microplasma lamps mounted at 30 inches and 60 inches above the floor, and a fifth lamp mounted overhead. The pseudocolor heat map shows the far-UV irradiance due to these lamps.

Figure 4 – Proposed doorway disinfection system.

It is in general difficult to obtain UV-C reflectance data for most materials. A report published four decades ago summarized the results of studies done between the 1920s and 1940s, but very little information has been published since then (Ullrich and Evans 1976). Still, there is sufficient data available to model the system shown in Figure 3 (Cader and Jankowski 1998 and Nagy 1964):

  • The reflectance of human skin to UV-C radiation is less than one percent.
  • The reflectance of oil-based paints is 5 to 10 percent.
  • The reflectance of white cotton is about 30 percent.
  • The reflectance of etched aluminum is 88 percent.

Given this, we can place a virtual mannequin inside the doorway, rotate it through 90 degrees, and measure the predicted far-UV irradiance at selected target points (FIG. 5). The results are shown in Table 2 and Figure 6.

Figure 5 – Irradiance target points.
1 – Forehead2.013.185.447.257.965.17
2 – Chest0.102.164.686.637.314.18
3 – Abdomen0.011.884.446.537.374.05
4 – Ear8.306.483.430.000.663.77
5 -Shoulder8.429.967.102.831.265.91
6 – Hand24.9412.423.
7 – Calf3.904.262.770.900.922.55
8 – Back (cervix)0.141.574.718.199.314.78
9 – Back (thorax)0.000.504.838.3710.504.84
10 – Back (thorax0.001.223.856.977.584.00
Table 2 – Target irradiance values (μW/cm2).
FIG. 6A – 0.0 degrees rotation
FIG. 6B – 22.5 degrees rotation
FIG. 6C – 45.0 degrees rotation
FIG. 6D – 67.5 degrees rotation
FIG. 6E – 90.0 degrees rotation

The design certainly has a few shortcomings, although we could have expected them from Figure 2. If the goal is to achieve a 90% reduction in pathogens (99.9% is preferable), we would need a far-UV dose of 20 mJ/cm2. With a minimum irradiance value of 2.55 μW/cm2, we would need therefore an exposure time of over two hours!

It gets worse, however. The UV-C doses required to achieve a given reduction in pathogens are determined by irradiating cultures in Petri dishes and test tubes. Disinfecting surfaces in the real world typically requires larger – sometimes much larger – doses. Increasing the number of excimer lamps would of course increase the average irradiance, but this approach has its limits. The American Conference of Governmental Industrial Hygienists recommends a maximum exposure of 3.0 mJ/cm2 of broadband (200 nm – 315 nm) ultraviolet radiation per 8-hour workday, and specifies a spectral weighting function to assess ultraviolet hazards for skin and eye (ACGIH 2013). For 222-nm radiation, the weight factor is 0.12, meaning that a maximum exposure of 25 mJ/cm2 is recommended. At 1,000 μW/cm2, a 30-second exposure would exceed the recommended daily limit. This would be a concern not only for the person being irradiated, but also for any service personnel standing near the doorway for extended periods of time.

It must be acknowledged that the ACGIH spectral weighting function is based on medical studies of photokeratitis and erythema performed prior to 1991, and so do not take recent studies of far-UV exposure into account. Nevertheless, until such time as the spectral weighting function is revised, it remains a standard for UV-C exposure limits.

One issue that will need to be addressed is that krypton-chlorine lamps emit about three percent of their radiation in the region of 230 to 260 nm, as can be seen in Figure 2. There is evidence that far-UV exposure can cause erythema in persons with phototypes I and II skin, and also patients taking photosensitizing drugs (e.g., Woods 2015 and Saadati 2016). This reaction is likely due to the residual UV-C radiation, but it can presumably be blocked by suitably doped fused quartz filters.

There is also a fundamental flaw in this design: the SARS-CoV-2 virus that causes COVID-19 appears to be spread primarily through aerosols generated by coughing, sneezing, and even talking. Even if the doorway disinfection system were capable of properly disinfecting surfaces, it would have no effect on an infected person walking through it.

Security Theater

The unfortunate conclusion is that using microplasma excimer lamps in doorway disinfection systems fails by several orders of magnitude. From an engineering perspective, this is an undesirable outcome. However, we should not be surprised to see such systems in common use in the near future. We have lived with airport security full-body scanners for years. It is widely acknowledged these devices are an example of security theater – the practice of investing in countermeasures intended to provide the feeling of improved security while doing nothing or little to achieve it. Doorway disinfection systems – with the associated annoyance of having to pause for 20 seconds or so before entering a building – may become just as common in our daily lives.

More Research Required

None of the above should be construed as a criticism of far-UV excimer lamps for room disinfection. Based on the evidence to date, they appear to be safer and equally as effective as low-pressure mercury-vapor discharge lamps for upper-room air disinfection and surface disinfection in unoccupied spaces. However, more research is required to determine whether the existing ACGIH dose recommendations for far-UV radiation can be exceeded.


ACGIH. 2013. Ultraviolet Radiation: TLV(R) Physical Agents 7th Edition Documentation. American Conference of Governmental Industrial Hygienists.

Buonanno, M., et al. 2017. “Germicidal Efficacy and Mammalian Skin Safety of 222-nm UV Light,” Radiation Research 187(4):483-491. DOI: 0.1667/RR0010CC.1.

Cader, A., and J. Jankowski. 1998. “Reflection of Ultraviolet Radiation from Human Skin Types,” Health Physics 74(2):169-172.

IUVA. Undated. Fluence (UV Dose) Required to Achieve Incremental Log Inactivation of Bacteria, Protozoa, Viruses, and Algae. International Ultraviolet Association:

Nagy, R. 1964. “Application and Measurement of Ultraviolet Radiation,” American Industrial Hygiene Association Journal 25:274-281.

Narla, S., et al. 2020. “The Importance of the Minimum Dosage Necessary for UV-C Decontamination of N95 Respirators During the COVID-19 Pandemic,” Photodermatology, Photoimmunology & Photomedicine. DOI: 10.1111/phpp.12562.

Saadati, S. 2016. Study of Ultraviolet C Light Penetration and Damage in Skin. Department of Radiophysics, Sahlgrenska University Hospital.  Gothenburg, Sweden.

Ullrich, O. A., and R. M. Evans. 1976. Ultraviolet Reflectance of Paints. American Welding Society.

Welch, D., et al. 2018. “Far-UVC Light: A New Tool to Control the Spread of Airborne-Mediated Microbial Diseases,” Scientific Reports 8:2752. DOI: 10.1038/s41598-018-21058-w.

Wells, W., and M. Wells. 1936. “Air-borne Infection,” J. American Medical Association 107:1069:1703.

Woods J. A., et al. 2015. “The Effect of 222-nm UVC Phototesting on Healthy Volunteer Skin: A Pilot Study,” Photodermatology, Photoimmunology & Photomedicine 31(3):159- 166.

Climate-based Annual Daylight Modelling for Greenhouses with Supplemental Electric Lighting

Climate-based Annual Daylight Modelling for Greenhouses with Supplemental Electric Lighting

This is a preprint of a paper presented by the author at the International Society for Horticultural Lighting (ISHS)’s GreenSys 2019 conference in Angers, France in June 2019, and scheduled for publication in Acta Horticulturae .


Recent advances in LED-based luminaire design have enabled greenhouse operators to temporally control both the photon flux density (PFD) and spectral irradiance incident upon the plant canopy. However, it is difficult to predict the performance and benefits of these luminaires without knowledge of the time-varying PFD and spectral irradiance due to daylight. We have addressed this problem with the development of horticultural lighting design software that incorporates validated climate-based annual daylighting calculations, physically-based modelling of glazing and light diffusion materials, modelling of spectral reflectance from greenhouse crops and surrounding surfaces, and accurate simulation of optical radiation distribution within the greenhouses from direct sunlight, diffuse daylight, and supplemental electric light sources. These measurements can be used to determine daylight availability, monthly Daily Light Integrals, automated shade and energy curtain deployment schedules, and projected electrical energy costs, all in advance of building the physical structures.


Since their commercial introduction in 1964, high-pressure sodium (HPS) lamps have been a mainstay of supplemental electric lighting in greenhouses. With their fixed light outputs and spectral power distributions (SPDs), however, there has been little incentive or opportunity for commercial greenhouse operators with experiment with different “light recipes” for optimum plant growth and health.  Rather, the luminaires are typically turned on at dusk and operated until the desired Daily Light Integral (DLI) for the crop or ornamental plats is achieved.

The introduction of light-emitting diodes (LEDs) for horticultural lighting has completely changed this situation. Many luminaire manufacturers are now offering products with separate SPD settings for promoting vegetative growth and blooming. Some manufacturers are going further by including, in addition to the ubiquitous 450 nm blue and 660 nm red LEDs, ultraviolet-A, green and “white light” LEDs with different correlated color temperatures (CCTs), and also 735 nm far-red LEDs. Going further still, a few products can be dimmed in response to inputs from daylight sensors, and it likely that future products will enable computer control of their SPDs beyond simple “veg” and “bloom” settings.

Together, these studies indicate that successful light recipes may involve daily dynamic changes in both the photon flux density (PFD) and SPDs delivered to crops and ornamentals in greenhouses. However, there is a problem. Most of these studies have been conducted in controlled environment growth chambers.  It is often difficult to translate such laboratory research to greenhouse environments (e.g., Annunziata et al., 2017). Even if light recipes for a given crop or ornamental are developed in a research greenhouse, it is difficult to ensure that all of the requirements are met in commercial greenhouses. Certainly, such simple metrics as DLI are not enough.


Modelling a greenhouse begins with its most important element: glazing.


For the purposes of daylighting, glazing materials have three important optical properties:

Fresnel transmittance.

The optical transmittance of transparent glass and rigid plastic panels (collectively dielectric materials) depends on the angle of incidence q of the incoming light (Figure 1). At normal incidence (i.e., q = 0 degrees), each surface reflects about 4 percent of the light. A single pane has two surfaces, and so the maximum possible transmittance is 92 percent. Double-pane and triple-pane insulated glass panels correspondingly have maximum possible transmittances of 85 percent and 78 percent respectively

What is more important is that the transmittance decreases with increasing angle of incidence, as determined by the “Fresnel equations” (e.g., Ashdown, 2019). This is clearly evident when reflections of the Sun from windows are viewed at grazing angles. Anti-reflection (AR) coatings can improve the transmittance somewhat at normal incidence, but the Fresnel transmittance still dominates at large incidence angles.

Figure 1. The transmittance of transparent glazing depends on the angle of incidence q and the number of panes.

It is also important to note that Figure 1 applies to daylight with a specific angle of incidence. Looking at the graph, it is evident that the transmittance of direct sunlight through the greenhouse glazing panels will depend on the solar position (azimuth and altitude), the building orientation, and the roof panel slope. The solar position varies throughout the day and year, of course, and so any transmittance calculations need to be performed on an hourly basis.

What is less evident is that daylight is comprised of both direct sunlight and diffuse daylight. On a clear summer day at noon, the ratio of direct sunlight to diffuse daylight incident on a surface facing the sun may be 20:1 or so; on an overcast day, there is no direct sunlight. In addition, the amount of daylight diffusely reflected from the ground and incident on vertical surfaces is typically 20 percent or so. The graph shown in Figure 1 is therefore instructive but not useful for calculation purposes.


There is growing evidence that plants use diffuse light more effectively than direct sunlight (e.g., Li and Yang, 2015). Particularly for shade-tolerant plants, translucent glazing results in more even spatial distribution of photosynthetic photon flux (PPFD) within the greenhouse, and also reduces its temporal variation on clear days.

Of course, the analytic modelling method for diffusion materials can also be used to represent greenhouse shade cloth, paint materials, and condensation on otherwise non-diffusing glazing.

Spectral transmittance.

The spectral range of photobiologically active radiation (PBAR) is generally assumed to be 280 nm to 800 nm (ASABE, 2017). This includes ultraviolet-B (280 nm to 315 nm) and ultraviolet-A (315 nm to 400 nm). However, soda-lime glass is opaque to ultraviolet radiation below approximately 320 nm, and so UV-B radiation, while shown to be beneficial to field-grown plants, is not a consideration in greenhouses. Similarly, low-density polyethylene (LDPE) used as an agricultural film for polytunnels, is opaque below 350 nm (Cadena and Acosta, 2014), while polycarbonate is opaque below 390 nm.

Given this, it is reasonable to model spectral irradiance inside greenhouses and polytunnels from 350 nm to 800 nm, where the spectral transmittance of soda-lime glass, LDPE, and polycarbonate is basically constant.

Greenhouse Structure

For most greenhouse designs, the purpose of the greenhouse structure is to support the glazing and possibly fan housings and motorized shades. From the perspective of climate-based daylight modelling, it is the size, position, and orientation of the glazing panels (or film for polytunnels) that is most important.

 While there are many custom greenhouse designs, almost all commercial greenhouses can be classified as having arch, Gothic, Venlo, or sawtooth roofs, while polytunnels can be classified as having either arch or Gothic hoops (Figure 2).

Gothic Tunnel

Figure 2. Four different greenhouse roof styles and two different polytunnel hoop styles determine how direct sunlight and diffuse daylight are transmitted through the roof panels.

While not directly related to daylight modelling, it would clearly be a time-consuming exercise for a typical user (for example, a greenhouse or horticultural luminaire manufacturer) to design and model an entire greenhouse with all the side posts, rafters, support columns, purlins, and cross ties. Fortunately, the simplicity of the framework makes it possible to use parametric design techniques, where the software generates the entire greenhouse structure from a few user-specified parameters. This can include the dimensions and spacing of tables, the placement of horticultural luminaires as supplemental electric lighting, and the specification of motorized shades.

A computer-aided drafting (CAD) model as shown in Figure 3 and needed for the daylighting calculations can be generated from the user-specified parameters in a fraction of a second. Due to the modular nature of greenhouses, even greenhouses as large as hundreds of thousands of square meters can be generated in the same amount of time.

Figure 3. Automatically-generated CAD model of a Venlo greenhouse.

Horticultural Luminaires

For over a century, architectural luminaires have been modeled as point light sources with angular luminous intensity distributions (Figure 4). For more than thirty years, the laboratory measurements have been reported using formatted text files that lighting design software programs can read.

To address this issue, an international standard was developed with specific support for horticultural lighting. Currently published in the United States (IES, 2018) and Italy (UNI, 2019), it is being developed for publication as a worldwide ISO standard. Its features include:

  1. Photon intensity distribution (measured in µmol ´ sr-1 ´ sec-1)
  2. Total photon flux (measured in µmol ´ sec-1)
  3. Spectral power distribution (measured in watts ´ nm-1)
  4. Channel multiplier

If the luminaire allows the LED color intensities to be individually controlled, these can be represented by a “channel multiplier” for each color that represents the channel dimmer setting when the luminaire’s optical characteristics were measured.

Figure 4. Luminaire photon intensity distribution.

Horticultural luminaire manufacturers currently report photosynthetic photon intensity distributions (or a multiplier to convert from lumens to photon flux). However, future light recipes will require more information than this. Accordingly, the spectral range is specified for the photon measurements (minimum and maximum wavelengths) so that it is possible to represent ultraviolet (280 nm – 400 nm), photosynthetic (400 nm – 700 nm), and far-red (700 nm – 800 nm) photon intensity and flux values (ASABE, 2017).

Weather Data

To calculate the daylight incident on a greenhouse, the software needs to know the building’s latitude, longitude, and compass (orientation). With this, it is possible to locate the nearest weather station for which a Typical Meteorological Year (TMY) weather dataset is available. One example is the collection of EnergyPlus TMY3 datasets, representing over 2,500 locations worldwide, although there are other datasets available that have been derived from combinations of historical weather data and weather satellite observations.

Virtual PAR Sensors

To measure the spatial distribution of PPFD on the plant canopy in the greenhouse, it is necessary to specify a horizontal array of virtual PAR (quantum) sensors. Each sensor will then receive direct sunlight, diffuse daylight, and direct photon flux from the luminaires (if any).

There are no restrictions on the position and orientation of the PAR sensors, so they could also for example be placed between the plant rows and oriented to measure vertical rather than horizontal photon flux, including that reflected from the floor and plant leaves.

Daylight Calculations

Once the greenhouse has been modeled and a weather dataset appropriate for the location obtained, the climate-based annual daylight calculations can be performed. Each weather dataset typically has 8,760 hourly records, so there are 4,380 different daylighting scenarios that must be considered.

The daylight calculations occur in two phases. In the first phase, the daylight incident on the exterior of the building is determined. This includes determining:

  • The solar position (altitude and azimuth) for a given time and date;
  • The direct solar irradiance;
  • The spatial distribution of diffuse daylight radiance on the sky dome;
  • The daylight diffusely reflected from the ground; and
  • The daylight SPD.

where the spatial distribution of the diffuse daylight is calculated in accordance with the industry-standard Perez sky model (Perez et al., 1993). The daylight calculation algorithms are detailed elsewhere (Ashdown, 2017).

Daylight SPD

Both direct sunlight and diffuse daylight have SPDs that closely resemble that of a black-body radiator, and so they can be uniquely described by their color temperature, expressed in kelvins (K). Direct sunlight has a color temperature of approximately 5500K, while that of clear blue sky typically ranges from approximately 7500K to 15,000K.

The SPD of daylight with color temperatures greater than 4000K can be calculated using the equations presented in CIE 15:4, Colorimetry (CIE, 2004). For example, the combination of direct sunlight and diffuse daylight on a clear day has a color temperature of approximately 6500K (which is the same white color as a computer display); the corresponding SPD is shown in Figure 6.

For overcast skies, clouds are spectrally neutral and so scatter daylight without changing its SPD. Consequently, a typical overcast sky has a color temperature between 6000K and 6600K (Lee and Hernández-Andréz, 2006). Given this, it is reasonable to assume a color temperature of 5500K for direct sunlight, 10,000K for clear blue sky, and 6500K for overcast sky.

Radiosity calculations

The second phase of the daylight and electric lighting calculations determine the spatial distribution and temporal changes in PPFD within the greenhouse. These calculations use a version of radiative flux transfer equations referred to as the radiosity method, and have been detailed elsewhere (e.g., Ashdown, 1994). Of significance for horticultural lighting design is that even though some 4,380 hourly daylight scenarios must be calculated, the calculation times are on the order of a few seconds to a few minutes, depending on the size of the greenhouse (Ashdown, 2018a).

Automated Shades

Automated shades are a common approach to limit the amount of direct sunlight incident upon the plant canopy. Given this, designated glazing panels in the greenhouse models can be modelled as being both transparent and diffusing (or, for energy curtains, opaque). This has no effect on the daylight or electric lighting calculation times, but it does mean that after the calculations have been completed, the spatial distribution of PPFD within the greenhouse can be accessed on a per-hour basis afterwards with the shades either open or closed.

Virtual Spectroradiometer

Architectural lighting design software models light sources as being “white,” and all surface colors as being combinations of red, green, and blue components. This works well for both lighting calculations and architectural visualizations, but it means that the daylight and luminaire SPDs cannot be represented. (They could, but it would require that the spectral reflectance distributions of all surfaces would need to be known, and greatly increasing the calculation times and memory requirements.)

Fortunately, there are mathematical techniques borrowed from remote satellite imaging that obviate the need for spectral reflectance distributions (e.g., Fairman and Brill, 2004). Instead, given only the red, green and blue components of a color, it is possible to reconstruct a physically plausible SPD. With this, it is possible to implement a virtual spectroradiometer that can be positioned and oriented anywhere in the greenhouse after the lighting calculations have been completed.

Figure 5. Virtual spectroradiometer measuring daylight SPD inside a greenhouse.


Once the daylight and electric lighting calculations have been completed, the most obvious analyses include calculating the predicted monthly DLIs and predicted electrical energy costs for the proposed buildings. However, new tools introduce new opportunities, and CBDM for greenhouses is no exception.

As one example, shade fabrics are available with a wide range of absorption and diffusion characteristics. By modelling different fabrics in software, it is possible to determine which will offer the best performance for different crops, taking into account the monthly DLIs and peak PPFDs rather than simply calculating an example time and date.

Figure 6. Analytic bidirectional scattering distribution function (BSDF) of diffiuion material.

Automated shades and energy curtains are another example. The calculation results can be used to develop automatic shade deployment schedules in response to changing weather conditions. It may be, for example, that the shades are ineffective – something that can be determined during the design phase rather than after construction.

Yet another example is light pollution. Increasing attention is being paid to the negative aspects of greenhouse lighting at night – light trespass onto neighbouring residential properties, increased sky glow (especially on overcast nights with low cloud cover), and ecological disruption for both animals and plants. Greenhouse lighting design software can be used to model and predict these problems. (As one particular example, roof-mounted energy curtains can potentially result in a 20 percent or more reduction in electrical operating costs due to the light being reflected back down onto the plant canopy.)

Finally, a virtual spectroradiometer is the ideal tool for predicting the spectral distribution of photon flux anywhere in the greenhouse. As light recipes become more sophisticated, such a tool becomes increasingly valuable.


As stated in the introduction, the goal of this paper has been to report on the development of climate-based daylight modelling software specifically for greenhouses and polytunnels with optional supplemental electric lighting. The focus has been on the horticultural aspects of the software from a user’s perspective, with as few references to computer science and related topics as possible. To do otherwise would have required at least several textbooks worth of material.

The real goal of this paper has been to introduce what is basically a new tool for greenhouse designers, and to explore the issues that it addresses. This paper will hopefully provide the foundation for further conversations between horticulturalists and software developers responsible for such tools.


The author wishes to thank Peter Socha for his assistance in the research for this paper.

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Greenhouses and Light Pollution

Greenhouses and Light Pollution

Supplemental electric lighting for greenhouses may be essential for extending the growing season in northern climates, but it comes with a not-so-hidden cost: environmental light pollution. The consequences of this pollution may range from irate neighbours in rural areas to municipal bylaws that may prohibit the use of supplemental lighting during certain hours of the night or altogether.

Searching for information on light pollution online will yield plenty of results. However, it soon becomes apparent that it does not apply to greenhouse lighting. Most of the discussion concerns astronomical light pollution, which is of interest to both amateur and professional astronomers, and where the focus is on outdoor lighting for roadways and parking lots. There are also discussions of “light trespass” onto residential properties from adjacent street lighting, which is rarely a concern for commercial greenhouses.

In addition, there are discussions of the effects of “artificial light at night” (ALAN) on nocturnal wildlife, including insects, fish, amphibians, and mammals. This is a complicated topic, as the effects differ between species and genera. There is almost nothing, however, that is specific to supplemental lighting for commercial greenhouses.

This can be a problem, particularly if citizen action committees lobby government agencies for bylaws and regulations. While alleviating light pollution is a laudable goal, it is necessary for all sides – concerned citizens, municipal authorities, and commercial greenhouse operators – to know the facts and discuss the matter accordingly. Beyond this, there needs to be agreement on what can reasonably be done to alleviate any problems.

At present, the primary source of information for citizen groups are publications from the International Dark-Sky Association ( in North America and allied organizations in Europe (e.g., In Canada, the Royal Astronomical Society of Canada has its Light Pollution Abatement Program ( with similar goals. The goal of this article is therefore to look at light pollution in the context of greenhouse supplemental lighting.

Astronomical Light Pollution

Concerns over light pollution began in the 1950s, when the light from large cities began to interfere with astronomical observations made from nearby observatories. (The Dominion Astrophysical Observatory, for example, is only 10 km away from downtown Victoria, BC.) A combination of urban and suburban growth, coupled with the changes from incandescent street lamps to more efficient (and much brighter) mercury vapour and later high-pressure sodium (HPS) lamps, continually exacerbated the problem.

The problem is that even on a clear night, light emitted by roadway and parking lot light fixtures (aka “luminaires”) is reflected from the ground into the night sky. While most of this light escapes into outer space (as we can clearly see when flying over cities at night), a small but significant portion is backscattered down towards the ground by air molecules and airborne dust and smog. The resultant “sky glow” obscures our view of the fainter stars (particularly the iconic Milky Way), and of course interferes with observations by both professional and amateur astronomers.

By itself, a single roadway luminaire does not contribute significantly to sky glow; it is the cumulative effect of thousands to tens of thousands of luminaires in a city that is the problem. Large commercial operations, however, may have thousands of horticultural luminaires and so be equivalent to a small city in terms of light pollution. (Hybrid greenhouses for cannabis production, for example, may have as many as eight hundred 1,000-watt HPS luminaires per acre.)

Spectral Issues

In rural areas away from city smog, the relative amount of backscattered light from roadway luminaires is dependent on the spectral power distribution (or more colloquially, “spectrum”) of the light source. In particular, air molecules (primary nitrogen and oxygen) scatter blue light more effectively than red light (which is why the sky appears blue to us).

Figure 1 shows the spectra of typical high-pressure sodium (HPS) and light-emitting diode (LED) lights used for greenhouse supplemental lighting. The HPS luminaires look yellow-orange to us, while the LED luminaires appear, depending on the ratio of red to blue light (between 1:1 for vegetative growth to 6:1 for reproductive growth), as various shades of purple (or “blurple”). Figure 1 assumes a 1:1 ratio, which is the worst-case scenario for light pollution. The spectra have been scaled such that both light sources produce equal amounts of photosynthetically active radiation (PAR).

FIG. 1 – Relative lamp spectra.

Following IDA and similar publications on astronomical light pollution, concerned citizens and municipal engineers may say something like this: “The amount of backscatter increases as the fourth power of the inverse of wavelength. The blue light of horticultural LEDs will produce much more light pollution than HPS!” This is correct, but it does not explain the situation very well. Figure 2 illustrates what this mean: the amount of backscattered blue LED light is roughly twice that of the backscattered HPS light.

FIG. 2 – Backscatter lamp spectra.

This however does not tell the full story. When our eyes are fully dark-adapted on a starry night, we are most sensitive to blue-green light (actually 505 nm) and less sensitive to yellow and blue. When we take this into account, the increase in astronomical light pollution when changing from HPS to 1:1 blue-red LED lighting is a walloping 3.8 times.

Ecological Light Pollution

Less often talked about but perhaps even more important are the effects of electric lighting on animal and plant life. It is impossible to cover all of these effects here, in part because ecological light pollution is the subject of intense ongoing research by biologists and ecologists. Rodents such as mice are most sensitive to green light and ultraviolet radiation, some migratory birds and bats are most affected by green light, insects respond to ultraviolet radiation, and plants can be impacted by red and far-red light, which disrupts their growth and development. Mammals, reptiles, amphibians, birds, insects … perhaps the best that can be said is that any excess light at night can be considered ecological light pollution, regardless of its spectrum.

While little research has been devoted to the topic to date, there is also the question of whether light pollution from greenhouses (FIG. 3 and FIG. 4) adversely impact birds, bats, and insects. In urban areas, tens of million migratory birds die every year from nighttime collisions with office tower windows. The US Fish and Wildlife Service is more specific – an estimated 6.6 million migratory birds die each year from collisions with communication towers and their warning lights. While migratory animals may not die from collisions with greenhouses, they are likely disoriented by the bright lights and may suffer increased mortality rates due to exhaustion.

FIG. 3 – Greenhouse light pollution near Leamington, ON. (Source: Detroit News)
FIG. 4 – Green house light pollution near Kingsville, ON. (Source: CBS News / Peter Loewen)

Cloudy Nights

Concerned citizens may lobby their municipal councils to “take back the night” so that their children can experience the starry nights they may remember as children, but little attention is paid to overcast nights, when the light pollution can be much worse.

On a clear night, the amount of backscattered light is miniscule – we only notice it because the night sky without light pollution can be exceedingly dark. Low-level stratus clouds, in the other hand, can reflect up to 75 percent of the light reflected from the ground.

For greenhouses, it all depends on the cloud ceiling height (FIG. 4). If it is 2,000 meters, any light from the greenhouse facility reflected from the clouds will be spread over an area of perhaps ten square kilometers and be reasonably unobtrusive. However, if the ceiling height is 250 meters, the area within a radius about 1500 meters will receive up to 60 times the amount of light pollution on the ground. If the greenhouses are near a sensitive ecological area, this can be a problem.

The amount of light pollution on the ground also depends on the average ground reflectance. For most of the year, this is between 5 and 10 percent. During the winter months with snow on the ground, however, it can increase to anywhere from 40 to 90 percent. Figure 5 shows the differences for summer (solid lines) and winter (dashed lines) ground conditions assuming 10 percent and 80 percent ground reflectance respectively.

FIG. 5 – Relative light pollution versus cloud ceiling height.

The visibility of light reflected from the clouds is another matter entirely, as this depends on the cloud opacity and whether the clouds can be seen on the horizon on an otherwise clear night. Brightly illuminated clouds may disorient migratory animals, again possibly increasing mortality rates due to exhaustion. They may also lad to complaints from residents living a considerable distance from the greenhouses.


The simplest solution for most light pollution situations is to use the minimum amount of light needed, and turn the luminaires on only when needed. Unfortunately, this is rarely practical for greenhouse operations. Each crop has specific PAR and daily light integral (DLI) requirements. In response to the suggestion that fewer luminaires could be used or the luminaires turned on for fewer hours at night, each crop has specific requirements related to its circadian rhythms. Like animals, plants need to sleep at night. There may be some flexibility in the supplemental lighting schedule, but likely not enough to make a difference.

What will make a difference are blackout curtains on both the walls and roof of the greenhouse that can be closed at night. The name “blackout” notwithstanding, the preferred color facing inwards is white. In a large greenhouse where the floor area is much greater than the wall area, most of the light from the luminaires will be reflected from the plant canopy and ground through the roof panels. If the average floor reflectance is 10 percent and the curtain reflectance is 80 percent, the plant canopy will receive an additional 20 percent of light.

If a white polyethylene ground cover is used for the greenhouse floor, the average floor reflectance (assuming walkways between the plants) will likely be in the range of 40 to 50 percent. The light will in this case bounce back and forth between the floor and ceiling at least ten times before it is essentially absorbed by the plants, curtains, and floor cover. Each bounce provides further PAR for the plant canopy, so that the end result may be over 100 percent of additional light.

Whatever the amount of additional light received by the plant canopy, it is quite literally recycled light. It reduces the need for supplemental lighting, and can be seen as an operating cost offset to the capital and operating cost of the blackout curtains (which can also function as energy curtains in cold climates).

It is also worth noting that this recycled light is diffuse, and is directed at the plant leaves from both above and below. For many greenhouse crops, this can be beneficial, resulting in stronger stems and less leaf senescence.

Being Proactive

In communities with large concentrations of greenhouses, such as Leamington, ON, light pollution is receiving increasing scrutiny from the press. You know you have a problem when the Detroit Press runs a major article on the light pollution from greenhouses located 25 miles away from the city.

LTO Nederland, the Dutch greenhouse industry organization, has been proactive in mandating screening for 98 percent of the greenhouse if lighting is being used during nighttime hours. If the light levels are greater than 15,000 lux during the hours of 5:00 PM to midnight, the screens need to block 98 percent of the light during the entire night, with the side screens blocking a minimum of 95 percent during nighttime hours.

FIG. 6 – Greenhouse light pollution in the Netherlands.
Copyright 2019 Tom Hegen (

If greenhouse operators wait until their municipalities begin discussing regulations and bylaws, it may put them at a considerable disadvantage. With so little information available to municipal engineers and planners to formulate proposed regulations, it may be advisable to be proactive.


There is no question that supplemental electric lighting in greenhouses can cause significant environmental light pollution. Having facts and figures available when discussing the issue with concerned citizens and municipal authorities is useful; having a solution that may reduce operating costs is a bonus.

Ultraviolet Disinfection for Consumers

Ultraviolet Disinfection for Consumers

In these unfortunate times, it seems that everyone is looking for ways to help with the COVID-19 crisis. From a lighting designer’s perspective, one solution is obvious: ultraviolet radiation. We have known about the disinfectant properties of ultraviolet radiation for nearly 150 years (Downes and Blunt 1877), and we have been using low-pressure mercury arc lamps to kill bacteria and inactivate viruses for the past 85 years (Wells and Fair 1935).

My initial goal in writing this article had been to discuss ultraviolet germicidal irradiation (UVGI) methods and materials, but this work was pre-empted by the Illuminating Engineering Society’s Photobiology Committee and their excellent report, IES CR-2-20-V1, Germicidal Ultraviolet (GUV) – Frequently Asked Questions.

I can highly recommend this publication for any questions related to ultraviolet disinfection and germicidal lamps, as it is written so as to be accessible to both lighting professionals and the general public. However, there is one small but critical omission that needs to be addressed.

Consumer Interests

Many consumers are currently interested in ultraviolet sterilizers for home use. Earlier this month, Forbes published an article titled, “Seven of the Best UV Sterilizers for Phones and Other Household Objects.” These devices included a USB UV sanitizer wand, two UV smartphone cleaning cases, a UV water bottle purifier cap, a portable UV sterilizer for small items such as baby pacifiers, an ultraviolet LED sanitization box, and a 25-watt germicidal lamp. Six of these are reasonable and perfectly safe if used correctly. The seventh … well, we need some background to explain the problem.

The ABCs of Ultraviolet

The Commission Internationale de l’Eclairage (CIE) divides the ultraviolet region of the electromagnetic spectrum into three subregions according to wavelength:

NameWavelength Range
Ultraviolet A (UV-A)315 nm – 400 nm
Ultraviolet B (UV-B) 280 nm – 315 nm
Ultraviolet C (UV-C) 100 nm – 280 nm

Table 1 – Ultraviolet classification

In the natural environment, the only significant source of ultraviolet radiation is sunlight. As the solar spectrum shown in Figure 1 shows, this radiation is only a small fraction of the total radiation received from the Sun. Our atmosphere fortunately absorbs much of the carcinogenic UV-B radiation at sea level, and essentially all of the UV-C radiation, thanks to the Earth’s ozone layer. The risk of exposure to UV-C radiation is therefore entirely from electric light sources.

FIG. 1 – Solar spectrum at sea level.

Now, there are several types of light sources that emit UV-C radiation, but only low-pressure mercury arc lamps and UV-C light-emitting diodes (LEDs) are found in consumer products. We can therefore ignore high-pressure mercury arc lamps, pulsed xenon arc lamps, and krypton-chlorine excimer lamps – this article is about ultraviolet disinfection solution for consumers.


In 1931, F. L. Gates demonstrated that UV-C radiation effectively kills bacteria and protozoa, and inactivates viruses. Using a high-pressure mercury-vapor arc lamp, he produced an “action spectrum” showing effectiveness versus wavelength (Gates 1931). He further noted that this spectrum was very similar to the absorbance spectrum of deoxynucleic acid (DNA), and correctly surmised that the radiation was terminally disrupting the DNA molecules (Figure 2).

FIG. 2 – DNA damage action spectrum (base on E. coli).

The advantage of low-pressure mercury arc lamps is that they emit monochromatic UV-C radiation at 254 nanometers, which is close to the 265 nm peak of Gates’ action spectrum. Even better, these lamps are basically the same as compact and linear fluorescent lamps, but with two important differences: 1) they do not have a phosphor coating to convert the ultraviolet radiation into visible light; and 2) they use fused quartz or soda-lime “soft” glass rather than borosilicate glass for their envelopes. (Fused quartz is transparent at UV-C wavelengths, whereas borosilicate glass is opaque.)

UV-C Exposure Limits

The problem with germicidal lamps is that exposure to UV-C radiation can irritate the eyes and skin. These irritations are technically photokeratitis (“snow blindness” or “welder’s flash”), photoconjunctivitis (“pink eye”), and erythema (sunburn). This medical terminology belies the seriousness of the issue, but a short paper by Trevisan et al. (2006) puts it into perspective.

In 2005, twenty-six medical students attended a 90-minute anatomy lesson in an autopsy room at the University of Padova in Italy. There were three autopsy tables, above each of which was mounted a single 40-watt germicidal lamp. A timer turned the lamps on at 7:00 PM and off at 7:00 AM; the lesson began at 8:30 AM.

Unfortunately for the students involved, the timer failed. They were dressed in protection suits against biological risk, but their eyes and the skin of their faces, scalps and necks were exposed to UV-C radiation for the duration of the lesson. As is common with UV-C exposure, the onset of symptoms occurred on average four hours afterwards. For the eyes, these included a burning sensation, excessive tearing, pain, blurry vision, hindrance to open, conjunctival redness, eyelid swelling, and photophobia. For the skin, the symptoms included acute sunburn, a burning sensation, irritation, and pain. The paper further noted an ocular “foreign body sensation,” which translates into a feeling of having sand constantly in your eyes. These problems lasted two to four days, but six students afterwards reported persistent scurf (flaking of the skin), and one of persistent dryness of the skin.

There are two important points here: 1) the lamps were basically 40-watt linear fluorescent lamps; and 2) the exposure time was only 90 minutes. The horizontal irradiance was later measured and found to vary between 50 μW/cm2 at the level of the autopsy table to 250 μW/cm2 one meter below the germicidal lamps. To put this into perspective, a horizontal illuminance of 500 lux is equivalent to approximately 250 μW/cm2 of visible light.

The American Conference of Governmental Industrial Hygienists recommends a maximum UV-C exposure dose limit for a workday of 3.0 millijoules per square centimeter (ACGIH 2020). For a period of 90 minutes, this translates into an effective irradiance of 0.6 μW/cm2. The students exceeded this recommendation by a factor of 100 to 400 times.

UV-C at Home

You might dismiss the misfortune of the medical students as an unfortunate workplace accident. After all, who would want to have a high-power germicidal lamp in their home?

… which brings us to the seventh UV sterilizer listed in the Forbes article:

FIG. 3 – 25-watt UV germicidal lamp. (Source:

The description of this lamp reads, “UV Germicidal Bulb 25w E26/E27 Screw Socket UV Light Bulb 110 Volt, UVC Ozone Free.” It was priced at $35 on 2020/04/02, but is currently (two weeks later) priced at $75. It is clearly a product that is in demand, although the description is less than reassuring:

“When this bulb lit, will immediately have a special smell in the air, it means it worked, this special smell comes from burnt harmful smalls by the UVC ray, just like the smell in the hospital.”

Th manufacturer explicitly states that this product has a quartz envelope and emits 253.7 nm UV-C radiation, but … 25 watts? Considering the medical distress that the medical students suffered from 40-watt lamps, surely this is incorrect?

Apparently not, for the manufacturer also claims that it will sanitize up to 400 square feet (37 square meters) in 15 to 60 minutes, making it “suitable for basement, bathroom, school, canteen, kindergarten, home, salon, closet, hotel … shoe cabinet, dog/chicken house, toilet …” and so on. The Forbes reviewer helpfully added that, “This UV bulb can be screwed into any standard light socket in your home or office.”

Nowhere on the Amazon Web page is there any mention as to how dangerous the output from a 25-watt germicidal lamp can be. “Wait,” you say, “surely this product has to comply with the relevant government regulations designed to protect consumers from such unseen dangers?”

If you are an American, the Occupational Safety and Health Administration has this to say as a standards interpretation: “… there are no OSHA-mandated employee exposure limits to ultraviolet radiation.” If you are a Canadian, the Canadian Center for Occupational Health and Safety says, “There are no Canadian regulatory occupational exposure limits for UV radiation. Many jurisdictions follow the limits recommended by the American Conference of Governmental Industrial Hygienists (ACGIH).”

There are government standards regulating the emission of UV-B radiation from suntanning beds and ultraviolet emissions from white-light mercury vapor lamps … but apparently nothing for UV-C disinfectant lamps. In other words, dear consumer, caveat emptor.

Wild West

With no regulations to constrain them, manufacturers of high-power UV-C disinfection lamps are free to sell whatever they want to the unsuspecting consumer … and Amazon is assuredly there to help them. Enter “150 watt UV ozone” in the search bar for and you will get over 140 results like this:

FIG. 4 – 150-watt UV germicidal lamp. (Source:

There are a number of reasons to be suspicious about the manufacturer’s claims for this product, but suppose we take the 150-watt electrical input power at face value. We can reasonably assume that the lamp ballast (remember that it is essentially a linear fluorescent lamp) has an efficiency of 90 percent. This means that 135 watts is being delivered to the mercury-vapor arc.

The radiant efficiency of a low-pressure mercury arc is about 45 percent, while the UV-C transmittance of fused quartz is about 90 percent, and so the lamp is emitting 55 watts of UV-C radiation.

The lamp itself, despite its outsized appearance in FIG. 4, is about 80 cm high. If we were to place a 60 cm diameter by 80 cm long tube over this lamp, it would have an inside surface area of roughly 15,000 cm2. The UV-C irradiance of this surface would therefore be 5.5 x 107 microwatts divided by 1.5 x 104 square centimeters, or about 3,650 μW/cm2 – over 6,000 times the ACGIH-recommended limit for 90 minutes’ exposure!

How can you tell whether the unit is activated? Ultraviolet radiation is by definition invisible, but someone sitting next to the unit in the dark, possibly watching television, may eventually notice a pretty violet glow around the lamp electrodes (Fig. 5). This will alert them that they will become mightily uncomfortable two to six hours later.

FIG. 5 – UV germicidal lamp visible emission. (Credit: Wikipedia Deglr6328)

One reason to be suspicious about the manufacturer’s claims for this product is that it supposedly generates ozone. According to the manufacturer, “Ozone has sterilization and disinfection, in addition to formaldehyde and odor, ozone can fill the entire room without being affected by obstructions.”

Whatever the claim of formaldehyde refers to, ozone is a particularly toxic gas, even in minute concentrations of 0.1 parts per million (ppm). It smells like chlorine gas, and it can cause similar (and severe) damage to respiratory tissues. A low-pressure mercury arc emits both 254 nm and 185 nm UV-C radiation. The shorter wavelength radiation is capable of ionizing the molecular oxygen in the air surrounding the lamp and so create ozone, but the quartz or soft glass envelopes of UV disinfection lamps are doped such that they are opaque to 185 nm radiation. Some UV-C lamps do emit ozone, but they are only used for industrial applications such as municipal water disinfection (where it replaces chlorine).

Product safety information? The description for a similar product reads, “Do not look directly at the germicidal light source. Absorbing too much ultraviolet light can cause skin irritation and conjunctival damage.” This is regrettably what we get in the absence of enforced government regulations.

There are reasons to suspect some of the manufacturers’ descriptions of these products (many of which are identical apart from the manufacturer’s name), particularly when one manufacturer states that the product has an “ozone mode” and an “ozone-free mode,” where the latter is suitable for pregnant women and the elderly. How the doping of a quartz envelope designed to block 185 nm radiation from ionizing the surrounding air can be toggled on and off with the simple flip of a switch is left unexplained.

With over 140 products like these to choose from, the average consumer should feel … absolutely petrified.

But rest assured! If you are concerned about your health, searching for “UV disinfection COVID” yields at least 20 UV sterilizer products that are available for purchase. One blatantly states, “Kill 99% COVID-19,” while another features a 10-watt battery-powered flashlight marketed as a “UVC Germicidal Lamp” with “Brightness: about 500 lumens.”

My favorite, however, must be the “UV COVID Indoor Sterilizer” shown in Figure 6. Lighting designers will understandably suspect that this is a theatrical LED luminaire, but the manufacturer assures us that it (somehow) emits “Ultraviolet wavelength of 253.7 nm.” Oh yes, and it can also be used for aromatherapy!

FIG. 6 – Purported UV COVID Indoor Sterilizer. (Source:


We are now living in different times, but one thing will always be the same: wherever there is a dire situation, there will be those among us willing and able to take advantage of our fears and gullibility. Say the words “ultraviolet disinfection” today and you will appreciate what technology has brought us – an online marketplace for charlatans and their wonderous pills that cure all ills.

UPDATE 20/04/23

For Europeans, the relevant standard concerning ultraviolet radiation in the workplace is Table 1.1 of EU-OSHA Directive 2006/25/EC, which specifies a maximum daily exposure to UV-C radiation of 30 J/m2, which is equivalent to the ACGIH recommendation of 3 mJ/cm2. The related publication “Non-binding Guide to Good Practice for Implementing Directive 2006/25/EC, Artificial Optical Radiation” (EC 2010) is highly recommended as an information source.

Worldwide, ISO 15858:2016, UV-C Devices – Safety Information – Permissible Human Exposure applies. This standard is applicable to in-duct UV-C systems, upper-air in room UV-C systems, portable in-room disinfection UV-C devices, and any other UV-C devices that may cause UV-C exposure to humans.

UPDATE 20/04/24

I am indebted to my colleague Dieter Lang of LEDVANCE for the following information and illustrations from his LinkedIn posting, “The Visible Spectrum of Mercury Low Pressure Lamps for Disinfection”:

FIG. 7 – Visible light image of UV-C germicidal lamp. (Source: Dieter Lang / LEDVANCE)

He also provided a useful and informative visible light spectrum for these lamps:

UV-C germicidal lamp visible light spectrum. (Source: Dieter Lang / LEDVANCE)

As Dieter writes, “The color shown in the photograph above is quite close to the real visual impression … of course you need to avoid looking at such a lamp without suitable eye protection.”

With this, I readily acknowledge that Figure 5 is somewhat misleading — it should be evident that high-power UV-C germicidal lamps are activated when viewed in a darkened room. Unfortunately, it was impossible to find photographs of such lamps in use that had not been doctored for advertising purposes (e.g., FIG. 4) by the manufacturer.


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