LED Horticultural Lighting
In early 2015, I published an online article which covered LED horticultural lighting research [HERE]. This article concluded that if indoor cannabis growers were looking to achieve optimum yields LED was, in early 2015, not the way to go. At that point in time, research showed that LED horticultural lighting was producing inferior yields to HPS and that LED fixtures costed five times more than HPS fixtures per photon delivered. However, I also noted that LED was a rapidly evolving technology and to keep an eye on things because this could change in the near future. That time has now arrived. LED technology has made huge leaps and bounds forward since early 2015 and today many horticultural LED fixtures are capable of producing as many, or more, PAR watts as 1000-watt double ended (DE) HPS fixtures using about 60 percent of the electrical energy.
Additionally, a quality LED fixture will provide > 50,000 hours of operational use which converts to more than 11-years of plant lighting when the fixture is run 12-hours a day. Comparatively, at the same 12-hours a day operation, HPS bulbs, no matter how good, need to be replaced at least once yearly (every 6-months being more ideal) and it is unlikely that even a top of the line HPS ballast would last anywhere near this 11+ years. Due to these factors (energy savings and long-lifespan) LED, while costlier to initially purchase than HPS, offers return on investment (ROI) in about 2 to 3.5 years for sole light-source applications and 4-7 years for supplemental light applications. Further, LED produces a more ideal spectrum for secondary metabolite synthesis, resulting in higher medicinal oil percentages and better terpene production in cannabis when compared to HPS and other forms of high intensity discharge (HID) lighting. Additionally, a recent July 2020 study showed LEDs took 5-days less to achieve peak ripeness in flowers for harvest. I.e. the use of LED over HPS will shorten the flowering cycle time by 5-days. (see “note” reference at article end)
The material that follows looks in depth at LED horticultural lighting, lighting theory, and the importance of LED now and in the future of indoor and greenhouse (supplemental light) cannabis production.
First off, let’s run through some of the all-important terminology that is used when speaking about horticultural lighting.
Luminaire and Fixture
The terms luminaire and fixture are used interchangeably in the following material. However, luminaire or fixture refers to a complete LED horticultural light unit.
PAR (Photosynthetic Active Radiation)
Photosynthetically active radiation (PAR) is the light within the spectrum of 400 to 700 nm. Increasing energy in the PAR range increases plant photosynthesis until the light saturation point is reached.
Nanometre (nm) or Nanometer (American spelling)
The sun emits an absolutely tremendous amount of energy, generally called electromagnetic radiation or the electromagnetic spectrum. This energy travels to earth in waves, and the wavelengths are measured in metres.
The wavelengths of visible light are tiny, ranging from 400 to 700 billionths of a metre. A billionth of a metre is called a nanometre, or nm. Thus, a nanometre or nm is the measurement of the colour of light that is being put off by either the sun or an artificial light source.
In horticultural lighting a photon is a single particle of light, and can take on a variety of wavelengths. Those photons that are capable of contributing to photosynthesis are considered a photosynthetic photon.
PPFD (Photosynthetic Photon Flux Density)
PPFD (photosynthetic photon flux density) measures the amount of PAR light that arrives at the plant canopy every second, or (more technically) the number of photosynthetically active photons that fall on a given surface each second. PPFD is a spot measurement of a specific location on your plant canopy and it is measured in micromoles per square meter per second (expressed as μmol·m-2·s-1 or µmol m-2s-1 ).
PPF (Photosynthetic Photon Flux)
PPF and PPFD measure the quantity of photons a lighting source admits. The critical difference is that PPFD measures the density of these photons falling on a particular surface (i.e. the plant canopy), while PPF is a measure of the total number of photosynthetic photons released from a light source.
Photosynthetic Photon Intensity Distribution (PPID), (μmol/s·sr)
This term describes the direction and intensity of light leaving a fixture. PPID allows designers to plan the spacing of fixtures in a grow facility, and to understand how different light distribution options in fixtures impact the evenness of light in the work area. See following image of the PPID of just one LED horticultural fixture.
DLI (Daily Light Integral)
Daily light integral (DLI) is the amount of PAR received each day as a function of light intensity (instantaneous light: μmol·m-2·s-1) and duration (day). It is expressed as moles of light (mol) per square meter (m-2) per day (d-1), or: mol·m-2·d-1 (moles per day).
The DLI concept is like a rain gauge. Just as a rain gauge collects the total rain in a particular location over a period of time, DLI measures the total amount of PAR received in a day. Growers can use quantum meters to measure the number of light photons that accumulate in a square meter over the light period. So, for example, most cannabis growers flower at 12/12 or 12 hours of light over a 24-hour period.
Here is the formula for calculating DLI with a quantum (umol/PAR) meter.
μmol m-2s-1 x (,3600 x photoperiod) / 1,000,000 = DLI
– μmol m-2s-1 is a reading, or averaged readings, from the quantum meter
– 3,600 is the number of seconds in an hour
– Photoperiod is the period (in hours) of light exposure per 24 hours
– 1,000,000 is the number of μmols per mole
For example, 38.1 μmol m-2s-1 x 3600 seconds/hour x 12-hour photoperiod / 1,000,000 μmoles/mol = 1.645 mol. m-2d-1 (or 1.6 moles/day = 1.6 DLI).
Another example, 1500 μmol m-2s-1 x 3600 seconds/hour x 12-hour photoperiod / 1,000,000 μmoles/mol = 64.8 mol. m-2d-1 (or 64.8 moles/day = 64.8 DLI).
For those of you who don’t enjoy running equations there are several good DLI calculators online. Just Google “DLI calculator”.
Joule: The joule (symbol J) is the SI unit of energy—a measure of the capacity to do work or generate heat. One joule equals the work done (or energy expended) by a force of one newton (N) acting over a distance of one meter (m).
Cross Illumination: Horticultural lighting engineers apply a practice known as “cross illumination” to achieve uniform lighting across the plant canopy. When looking at grow light spacing in relation to cross illumination, it’s evident that the practice can be reduced to geometry. By applying proper spacing between lights, as well as precise controls with vertical spacing above the canopy, PAR light rays from one fixture mix with PAR light rays from adjacent sources. In the end, these geometric light interplays mix to create uniform coverage for the whole garden canopy.
Interlighting or Intercanopy lighting: Interlighting, which is also referred to as intercanopy lighting, relates to applying part of the light provided to the crop within the canopy through placing artificial light sources inside the crop canopy.
Light Saturation Point: Cannabis Lighting Requirements
Author’s note: In the Following material, I’ll be throwing some PPFD (PAR that arrives at the canopy) as µmol m-2s-1 figures around. These will likely mean almost nothing to growers who haven’t worked with Quantum (PAR) meters. Don’t worry too much about the actual numbers for now. I’ll be covering how to take PPFD as µmol m-2s-1 readings shortly. For now, these numbers will give you something to come back to and compare if and when you measure the PPFD as µmol m-2s-1 in your own grow.
The light saturation point is the light intensity at which the photosynthetic rate reaches its maximum, where more light has no or a negative effect on photosynthesis.
The light saturation point for cannabis has not yet been fully determined, but its net photosynthetic rates at different temperatures (25–40°C) and intensities (up to 2,000 μmol m−2 s−1) were reported. In this study by Chandra, S. et al (2008), no decline in photosynthesis rate was observed at the highest intensity used; however, net photosynthetic rates at 30°C decreased by ~20% from 1,500 to 2,000 μmol m−2 s−1 light intensity. 
Similarly, a 2020 cannabis study by Eaves et al. which compared LED to HPS lighting showed a positive linear relationship between light intensity and yields which continues to at least 1498 μmol m–2 s–1, which is over twice the level provided by an HPS fixture in the grow configuration that is currently the industry standard (high bay 1000-watt DE HPS lighting). (see “note” reference at article end)
Currently, Dr Bruce Bugbee of Utah State University is conducting research with low THC varieties of cannabis (hemp) to establish growth and essential oil production optimums. In this work, at this point in time, Bugbee seems to be suggesting that cannabis has extremely high light requirements with the light saturation point being at or even potentially above 2000 constant μmol m−2 s−1 at canopy level with all other inputs (temp, humidity, CO2, crop nutrition) optimised.
Bruce Bugbee talks about the lighting requirements of cannabis on Youtube. To view this clip, Google “Cannabis Grow Lighting Myths and FAQs with Dr Bruce Bugbee”.
Dr. Allison Justice, a PhD plant scientist and founder of The Hemp Mine, has demonstrated when using LED lighting (Fluence/Osram) that the amount of trimmed cannabis flower that was harvested under different light (PPFD) intensities increased by 1% for every 1.1% light intensity increase between 400 and 600 PPFD at 1500 ppm CO2 and an average temperature of 23.7oC. Between these ranges (400 – 600 PPFD) there was a 51% increase in dry flower yield.
However, as shown in the graph below, the rate of yield gain slows between 700 and 1200 PPFD. Between this range, trimmed flower weight only increases by 1% for every 5.5% increase in light quantity.
This comes down to ‘the law of diminishing returns.’ The law of diminishing returns states that in all productive processes, adding more of one factor of production, while holding all others constant, will at some point yield lower incremental per-unit returns.
In this case, Increasing the PPFD from 400 to 600 causes a much greater increase in dry flower yield than increasing the PPFD from 700 to 1200 μmol m−2 s−1.
See following graph.
Source: Justice, A. Economic Impact of Light Intensity on Yield and Secondary Metabolites, presentation at the Photo x Summit, San Diego 2017 https://2xuwao2gok1v2wn2em9n5ys8-wpengine.netdna-ssl.com/wp-content/uploads/2017/11/Allison-Justice_Economic-Impact.pdf With thanks to Alison Justice for allowing me to reprint this graph.
So, what is the Light Saturation Point for Cannabis?
At this point in time there is no absolute answer to this question. Bruce Bugbee seems to suggest it is higher than Alison Justice’s (2017) and Chandra, S. et al (2008) findings, pointing out that even after 2000 μmol m−2 s−1 (with “all the other inputs optimised”) the photosynthetic curve for cannabis marginally increases. Findings by Chandra, S. et al (2008) surrounding net photosynthetic rates at different temperatures and CO2 levels, concluded, air temperature, CO2 and light for the maximum rate of photosynthesis in cannabis was at 30oC (86oF), 750 μmol mol-1 (CO2) and 1500 µmol m-2s-1 (light) respectively. Keep in mind that the parameters of studies can sometimes limit the data. For example, had Chandra, S. et al (2008), increased CO2 above 750 μmol mol-1 the study may have found that the light saturation point (maximum rate of photosynthesis) for cannabis may have been, inline to Bugbee’s work, >1500 µmol m-2s-1. I.e. light saturation normally occurs when some other factor (typically CO2) is limited.
Similarly, in Allison Justice’s 2017 study the maximum PPFD tested was 1200 µmol m-2s-1 with 1500 ppm of CO2. This study wasn’t set up to establish the light saturation point of cannabis but instead to look at the economic impact of light intensity on yield and secondary metabolites. However, had Allison Justice tested at 1500 µmol m-2s-1 PPFD, inline to the work of Chandra et al (2008), her graph might have shown a slight upwards trend between 1200 µmol m-2s-1 and 1500 µmol m-2s-1 in dry flower and trim weights. Further, the average ambient air temperature of 23.7oC (74.66oF) in this study was too low. I.e. optimum air temperature, while cultivar specific, tends to be somewhere between 25 – 31oC; however, given Alison Justice (2017) was testing under LED lighting, leaf temperature is reduced by about 1.3oC (when compared to HPS) giving us an ideal air temperature of somewhere between of 26.3 to 32.3oC.
Therefore, light and temperature are potentially both limiting factors in the Alison Justice study.
ROI on µmol m-2s-1 also Needs to be Part of the Lighting Equation
Cannabis is a plant adapted to high irradiance levels and warm temperatures. Chandra, S. et al (2008) demonstrated that the highest photosynthetic efficiency was achieved under ∼1,500 PPFD and 25–30°C; however, there is at this point in time limited scientific evidence that supports higher photosynthesis rate equals higher flower yields. It is also questionable whether such a high light intensity (1,500 PPFD) is economically viable in terms of energy costs put into lighting and cooling.
While it is likely that growers can push cannabis yields 3, 4, 5% (?) higher by increasing the PPFD from e.g. 1200 to >1500 µmol m-2s-1 this may prove impractical and cost prohibitive at present for some grow operators.
At this point, the quantity of light requires a larger number of fixtures or higher wattage fixtures – which can mean the capital expense exceeds the value of increased yield. I.e. a small yield gain offset by the additional fixture and energy costs required to achieve this gain may prove counterproductive to the bottom line.
It is, therefore, perhaps necessary to ask what is optimum light intensity for ROI, rather than what is the light saturation point of cannabis? On this level, Alison Justice’s findings prove vitally important.
Horticulturalists generally follow a 1% rule: For most crops, 1% more light means 1% more yield. Cannabis too typically follows this classic rule of thumb; however, there is a point of saturation where the rate of increasing yield begins to diminish.
Allison Justice demonstrated that the amount of trimmed cannabis flower that was harvested increased by 1% for every 1.1% light intensity increase between 400 – 600 PPFD (at 1500 PPM CO2). The rate of yield gain slows between 700 – 1200 PPFD. Between these light levels, trimmed flower weight only increases by 1% for every 5.5% increase in light intensity.
This said, when compared to other agricultural crops, cannabis is probably the most valuable crop on a per acre basis returning over a 100 times per acre than what other agricultural crops, such as cucumber, pepper and tomato, do.
A recent (July 2020) study out of Canada demonstrated that at 1498 PPFD (µmol m-2s-1) under LED, the ROI for cannabis remained highly profitable. Based on this study, it takes about 0.77 W to produce 1 μmol m–2 s–1. In the case of this experiment, plants were grown for 60 days with 12 hours a day of lighting, for a total of 720 hours. Thus, the additional 0.77 W would generate an additional 0.55 kWh of electricity. Assuming electricity is 0.11 US dollars (USD) kWh–1, the additional watts required to produce an additional 2.68 USD of cannabis would cost about 0.06 USD in energy to produce. The authors of this study concluding; “Finally, for all the (light) intensity ranges that we considered, the value of the gain in yields from increasing light intensity far exceeded the cost of using more electricity. (see “note” reference at article end)
These results showed a positive, apparently linear relationship between light intensity and yields which continues to at least 1498 μmol m–2 s–1, which is over twice the level provided by an HPS fixture in the grow configuration that is currently the industry standard (DE 1000-watt HPS high bay lighting). In short, providing high levels of light is highly profitable where cannabis is concerned.
Horticultural LED Lighting– Efficiency
The establishment of LEDs instead of HID lighting systems results in significant energy savings. For example, when comparing LEDs with HPS, Macias et al. reported there was an energy saving of between 41% and 73% on power consumption. 
The more efficient supplemental lighting sources are, the less electric power growers need to finish their crops. Increased energy efficiency can have a significant impact on production overheads and, as a result, the bottom line.
When output and input are compared using like units—such as watts out versus watts in—the resulting ratio is called efficiency. LED package manufacturers often report efficacy in lumens per watt because this is a meaningful metric for human lighting, but it is not applicable for horticultural lighting because it is a measure of photons weighted for human vision based on the human eye response to different colours. Photons cause photosynthesis, not watts. In horticultural lighting, the units are micromoles of photons out per joule of energy in, and when output and input units differ, the resulting value is called efficacy. Thus, the higher the efficiency number/rating, the more efficient the lighting fixture.
The appropriate metric for plant lighting efficiency is photosynthetic photon efficacy (PPE). This is the PAR photon output (unit of micromoles per second, or μmol·s–¹) divided by the input power (watts, or W) to produce that light. Thus, the unit becomes μmol·s–¹·W–¹, and because one watt (W) equals one joule per second (J·s–¹), the ratio can be simplified to μmol·J–¹ (μmol per second/joule per second).
As a benchmark, high-pressure sodium lights put out 1.7 micromoles per joule, similar to LEDs in 2014. Today’s LEDs now can run from an efficacy of 2.6 to 3.1 micromoles per joule, and the technology is still evolving.
Certified, independent test laboratories conduct comprehensive tests on fixtures to characterise their performance. Fixture manufacturers should always be able to provide test results for their fixtures from certified third-party test laboratories. Growers can consult the DesignLights Consortium (DLC) website, designlights.org. The non-profit lists the PPE of fixtures based on third-party testing.
Using this resource, you are able to get reliable information about the various independently tested DLC approved horticultural LED fixtures sold on the market today. By selecting horticultural lighting on the landing page this takes you to various brands. On the left of a given brand you will see “Photon Flux” and under this 400 – 700nm which represents total umol/s output of a brand at PPF (total number of photons released from a light source at source/fixture). Under this you will see “Photosynthetic Photon Efficacy: 400-700 nm, μmol/J (PPE). This represents micromoles per joule or overall efficiency. Additionally, Photosynthetic Photon Intensity Distribution (PPID), (μmol/s·sr) is shown. This term describes the direction and intensity of light leaving a fixture. PPID allows designers to plan the spacing of fixtures in a grow facility, and to understand how different light distribution options in fixtures impact the evenness of light in the grow area.
However, to place some perspective on things, LED lighting fixtures can now produce as much PPF as a 1000-watt DE HPS, which produces about 1800 PPF, using 60% of the wattage. For example, GE recently released their 1000-watt HPS equivalent LED, the Arize Element L1000, 597-watt with a PPF of up to 1827 and an efficiency of 2.7 μmol/J. Similarly, Philips supplies a 600-watt unit with 1800 μmol/s and an efficiency of 2.9 – 3.0 μmol/J.
Efficiency Alone Should Not Determine LED Luminaire Purchasing Decisions
Cannabis is a plant that thrives under high light levels. It would, therefore, be counterproductive to purchase LEDs purely on their efficiency ratings. Luminaire efficacy does not take into account important factors such as the luminaire intensity distribution, optimal luminaire layout, and the number of luminaires that will be required to reach a criterion PPFD. All of these factors are significant when evaluating the overall cost-effectiveness of various horticulture luminaire options.
Not all LED lighting systems are created equal, so it is important for growers to get lighting designs from manufacturers that show average PPFD at defined mounting heights and the light distribution pattern for their plant growth facility. The form factor and light distribution of a horticultural lighting system will influence the number of fixtures needed in a facility, which is another factor that will impact the overall energy efficiency.
The take-home message is that the energy efficiency of horticultural lighting systems is based on several factors, not just one. Using the correct metrics and understanding the factors that influence the energy efficiency of a horticultural lighting system will influence the overall profitability of a grow operation.
The Lighting Research Center (LRC) at Rensselaer Polytechnic Institute has developed a free, easy-to-use online tool that will assist growers to evaluate the performance, efficiency, and economics of a wide variety of horticultural luminaires, typically used in greenhouses and other controlled agricultural environments. This online tool, called the “horticulture luminaire calculator” allows growers to accurately compare several luminaires and select the one that will be most effective for their particular application.
Access the calculator at https://hortcalc.lrc.rpi.edu
The horticulture luminaire calculator allows growers to determine the best arrangement and mounting height of each luminaire they are considering. Using the calculator, growers can determine the number of each type of luminaire that will be needed to light their space to their desired light level, and select the product that will provide the optimum lighting, at the lowest cost. It would take several days to make these calculations using traditional methods. The horticulture luminaire calculator does it in a manner of minutes.
You will need to create a user name and then login when using this calculator.
Heat Output: LED v. HPS
One significant drawback to using HPS lighting is the large amount of radiant heat that is produced from the bulb. The surface temperature of HPS bulbs can reach temperatures above 427oC (800°F), which necessitates adequate distances between the plant canopy and the fixture to avoid damage to plant tissues. The ‘inverse square law’ comes into play as you increase the mounting height of a light fixture above the crop canopy, which greatly reduces the PPF (total number of photons released from the light source) to PPFD (the amount of photosynthetically active photons that actually arrive at the canopy).
The Inverse Square Law
In studies with artificial lighting, light gradients in vegetation are not only dependent on canopy characteristics, but are also strongly driven by the distance between light source/s and crop canopy.
Compared with natural sunlight, plant lighting conditions are dramatically different when grown under artificial light. In the case of artificial light, light intensity strongly decreases with increasing distance between the lamp and plant leaves.
This is because of the inverse square law which states the intensity of light is inversely proportional to the distance from the source.
According to the inverse square law:
1) Light sources emit energy in a 360-degree, circular pattern. In other words, light spreads out evenly and in all directions from its source. As the light spreads from the source its intensity diminishes exponentially. Technically, the inverse square law tells us that light that travels twice as far from the source is spread over four times the area and hence has one fourth the light intensity. For example, if the light intensity from a lamp was measured 0.5 metres away from the lamp and it was 1000 µmol m-2s-1 , then 0.5 metres away from this point it would be 250 µmol m-2s-1 .
2) Light intensity is directly relative to the distance from the light source. Plants grown outdoors under the sun are unaffected by this phenomenon because the sun is a huge, very powerful light source and the distance from the sun to the earth is so great that a matter of a few feet (or for that matter 10 feet) from the top of the plant to the bottom is insignificant. Plants grown indoors under artificial lighting, however, are extremely susceptible to the inverse square law because the light source is relatively small and the light energy is only traveling a few feet so the distance from the top to the bottom of the plant is very relevant to the total distance travelled by the light.
Based on the rules of the inverse square law, the closer a light can be placed to the plant canopy the less the difference between the PPF (photons released from a light source) and PPFD (photons that arrive at the plant canopy).
See following images that demonstrate the Inverse Square Law.
There is a common misconception surrounding LED lighting when it comes to heat produced by the fixture. Many growers believe that LEDs produce less heat than HPS fixtures, which is only true if the LED fixture is driven at a lower wattage. New lighting technologies such as LED generate light more efficiently, or with higher efficacy than HID lamps. In other words, we get more photons per watt. Heat per watt of electricity is constant. Fewer watts are used to produce equal brightness, not that LED light fixtures produce less heat for the wattage that they consume. Therefore, if you have a 1000W LED fixture and a 1000W HPS fixture, they will produce heat in the same general range from a magnitude perspective.
The major difference between LED and HPS systems is how much PAR energy is produced from those 1000 watts and how the heat is radiated from the fixture. As 600-watts of LED will produce the same PPFD as a 1000-watt DE HPS the key is that growers can run less wattage to achieve equivalent PPFD. As less wattage is required to produce the equivalent PPFD from LED when compared to HPS this reduces the amount of overall heat being created in the growing environment by lighting. I.e. wattage equals heat. 1 watt of electricity produces 3.412 British Thermal Unit (BTU) of heat per hour regardless of light type – incandescent, HPS, CDL, LED, fluorescent etc. Therefore, a 1,000W HPS grow light (approx. 1,050W with ballast loss) generates 3,582.6 BTU per hour. Comparatively, a 600 -watt LED will produce the same PPFD as that 1000-watt HPS which generates 2,047.2 BTU (i.e. 600 watts x 3.412 BTU = 2,047.2 BTU). What this means is that LED grow rooms aren’t heated as much by LED lighting when compared to equivalent PPFD from HPS. This results in LED grow rooms running cooler than HPS grow rooms given equivalent PPFD.
Additionally, most heat from HPS lamps is radiated down towards the crop canopy, whereas LEDs produce most of their heat at the connection site of the component where it is assembled to a printed circuit board (PCB). As a result. the heat is typically conducted to the PCB and a heat sink, and removed via upward convection. So, the proximity at which a fixture can be placed near plants without damaging tissue from radiant heat is one of the major benefits of LEDs as a source for horticultural lighting systems. This improves the PPF (total number of photons released from the light source) to PPFD (the amount of photosynthetically active photons that actually arrive at the canopy) when compared to HPS due to the inverse square law (i.e. LED fixtures can be hung closer to the plants reducing loss of PFD to PPFD).
A potential negative to this, however, is that the ambient air temperature needs to be higher under LED lighting to achieve optimal LST (leaf surface temperature) when compared the HPS. One study by Bugbee and Nelson (2015) found the use of LED technology reduces leaf temperature by about 1.3°C, given equivalent ambient air temperatures, compared to HPS technology under typical, indoor growing conditions.
What this and the fact that LED rooms run cooler can mean in colder climates is that extra energy is required to heat green houses and grow rooms beyond what would be required where running HPS. One Netherlands based study by Dieleman JA et al (2016) found when conventional HPS top lighting + LED interlighting was replaced by LED top lighting + LED interlighting , the carbon footprint was reduced by approximately 15%. This reduction was due to the higher efficiency of the LED lights compared to HPS lamps (25% difference), but was offset by the lower heat emission of the LEDs and thereby the higher use of thermal energy to maintain plant temperatures in the winter. Based on this, growers should consider not only the initial capital expenditure costs of LED luminaires themselves, but also the other costs associated with a transition to LED lighting.
Growers in cold climates, for example, will possibly have to compensate for the loss of heat from HPS in the winter – so the additional heating costs will offset some of the LED energy savings.
On the other hand, growers in hot climates (e.g. Australia) will benefit from reduced energy costs as a result of savings on cooling/AC/HVAC.
It is also important to note that in the Dieleman et al (2016) study, the authors assumed an efficiency of 2.3 µmol J-1 for LED lighting. However, developments in the LED industry occur quickly and recent LED luminaires available for greenhouse horticulture have an efficiency of 2.7 µmol J-1. This further reduces the carbon footprint. As LED luminaires become even more efficient (3.0 µmol J-1), the carbon footprint will be reduced by 33% compared to HPS lighting.
Another misconception people have surrounding horticultural LED fixtures is that the inverse square law for LED differs from other light sources such as HPS. However, all EM (electromagnetic) radiation from a point source falls off in intensity with the square of the distance. LEDs can’t magically change this fundamental law of physics. Therefore, the inverse square law applies equally to HPS and LED. `
An Example of the Inverse Square Law and LEDs in Action
One problem growers face with the inverse square law is that no matter how cool LED lights run on the crop canopy, the law of ‘cross illumination’ comes into play.
Horticultural lighting engineers apply a practice known as “cross illumination” to achieve uniform lighting in a room. When looking at grow light spacing in relation to cross illumination, it’s evident that the practice can be reduced to geometry. By applying proper spacing between lights, as well as precise controls with vertical spacing above the canopy, light rays from one fixture mix with light rays from adjacent sources. In the end, these geometric light interplays mix to create uniform coverage for the whole garden canopy. If lights are hung too close to the canopy this may help to reduce light loss (i.e. PFD to PPFD), however, you will also get uneven lighting with some areas receiving high levels of light while other areas are shaded or inadequately lit.
As a real-world example of the inverse square law and cross illumination in action, when looking at the GE Arize Element 601-watt LED Luminaire – a GE equivalent to a 1000-watt DE HPS – this luminaire has a minimum hanging height of 3.5 feet to ensure cross illumination. The Arize Element 601-watt has a PFD of 1820; however, at 3.5 feet above the canopy the GE Arize Element provides a PPFD of 950 µmol m-2s-1. This means that by hanging the luminaire 3.5 feet above the canopy the PPFD is 48.3% less than the PFD.
When overhead lights are used for plant growth, inefficiency occurs due to inability to accurately target light. Light falls between young plants, but as they grow shading occurs, requiring more light to achieve optimal productivity.
One challenge associated with overhead lighting systems is that light intensity decreases with depth within the plant canopy as the inverse square law comes into play and leaves absorb the light. In HPS and overhead LED lighting systems, the top of the canopy is often light saturated, yet the canopy as a whole is light limited. Providing additional light to the lower canopy increases the proportion of light used for photosynthesis without exceeding the point of photosynthetic light saturation. Unlike HPS lamps, LEDs emit little heat and can be placed close to the crop without burning leaves, meaning they are a practical interlighting system in commercial settings. Hawley (2018) demonstrated that supplemental interlighting can increase cannabis bud yield and modify cannabinoid and terpene profiles. The increase in flower yield is assumed to be related to increased photosynthetic photon flux densities (PPFD) compared to production with overhead lighting alone.
What this tells us is there is an argument for providing some of the crops sort after light through overhead LED lighting while also incorporating LED interlighting to provide another portion of the crops light to achieve higher terpene production and a more stable metabolome profile between the upper and lower canopy.
How High to Hang LEDs from The Plant Canopy to Ensure Cross Illumination?
Various LED brands/models have differing light spread from the light fixture. For example, some brands/models may have a PPF equivalent to a 1000-watt DE HPS but have a narrow angle of light spread. These may have a recommended minimum hanging height of 36 – 42 inches (3 – 3 ½ foot) to ensure cross illumination. Other brands may emit light at a wider angle and these may have a recommended minimum hanging height of 24-inches to ensure cross illumination.
The rule to how high to hang LED lights above the canopy is to follow the manufacturers minimum hanging height recommendations. Reputable suppliers have conducted rigorous testing with their lighting, including tests to establish cross illumination.
Young Vegetative Plants Require Less Light than More Mature Plants
During establishment growth, light intensities need to be kept relatively low as the plant is developing leaves and stems that will be used to provide photosynthates during the vegetative growth phase. Steadily increasing light intensity as you transition into the vegetative and reproductive growth phases will increase the rate of photosynthesis, which will provide the plant with more photosynthates used to develop flowers. Plants need time to acclimatise to high light intensities. If you expose plants to high light intensities too early in the crop cycle, you can damage chlorophyll pigments causing photooxidative stress.
However, this one is perhaps a little misunderstood and a 2021 study where clonal plants of ‘Gelato’ were grown for 21 days with canopy-level photosynthetic photon flux densities (PPFD) ranging between 135 and 1430 µmol·m-2 ·s-1 concluded:
“Within the parameters of this investigation, we observed that PPFD levels between 600 and 900 µmol·m-2 ·s-1 appeared to achieve an appropriate balance in optimizing key morphological parameters in vegetative cannabis while minimizing energy use associated with excessively-high ‘LIs’ (Light intensities) and also considering different production strategies. Although the desired morphological and growth attributes of vegetative-stage clonal cannabis plants will be subjective to each genotype and production scenario, the presented LI responses can assist cultivators in optimizing the LI for their individual production goals; balancing the potential economic returns against elevated input costs associated with supplying more PAR to their crops.”
Recommended PPFD µmol m-2s-1 for Different Stages of Growth
|100 – 300||100 – 200||600 – 900||1000 +||1000 +|
LEDS are Dimmable Offering Further Energy Savings
Another unique feature of LED lighting is that the intensity of their light output can be quickly, precisely, and automatically controlled via several methods including pulse-width modulation (PWM) and current control through the driver. The dimmability of LED lights has enabled the development of lighting control strategies that would not be possible with simple on/off control as is the case with HPS. A rule-based approach to controlling supplemental light intensity that utilises the controllability of LED lights is known as dynamic, or adaptive, lighting control.
Dimming LEDs, saves energy at a roughly 1:1 ratio. This means that if you dim LEDs down to 50% of their light output, you save nearly 50% of the associated energy use. Dimming LEDs also makes them run cooler, extending the life of the electronic components in the driver, as well as the phosphor in the LEDs.
Two Tier (Vertical) LED Growing – Doubling Yield in a Given Space
Because LEDs have low heat output onto the plant canopy they can be placed closer to the tops of the plants than HPS and other forms of HID lighting.
This makes them ideal for vertical farming.
A vertical farm replaces traditional single-tier cultivation with a two-tier system, enabling growers to grow one row of plants on top of another.
Many commercial cannabis growers in Nth America use vertical farming for vegetative plants; however, where growing multiples of shorter flowering plants, vertical farming enables growers to produce double the M2 (or 4×4 as per US units of measurement) of plants in the same sized room or warehouse. This system enables growers to double production per M2.
One group (MedMen) In the US replaced a traditional HPS lighting single-tier cultivation approach with vertical LED farming and:
- Reduced the cost per pound of finished product by 75 percent
- Decreased energy consumption per light fixture by 40 percent, going from approximately 1,100 watts per light to 660 watts per light
- Reduced heating, ventilation and air conditioning (HVAC) load by approximately 35 percent per square foot of canopy
- More than doubled production per square foot; reduced land/warehouse requirements by 50 percent
- Increased crop yield by 157 percent while reducing water and fertilizer usage
- Improved consistent, year-round flower quality and chemotype with denser trichrome development
Source: Fluence Science https://fluence.science/cannabis-vertical-farm/
LED Lighting for Vertical Growing
When choosing the best LED fixtures for vertical growing, the physical shape of the light fixture is important; it must be low profile and have a wide even cross illumination at low hanging height.
Light Spectrum and LED Lighting
Light is one of the most important environmental parameters that impacts plant growth and development. It exerts a vast range of effects on photosynthetic activity and photomorphogenic responses throughout the plant’s lifecycle. Close to half of the sun’s total radiation emission reaching the Earth’s surface is visible light, ranging from 400 to 740 nm wavelengths. Visible light is flanked by shorter wavelengths and invisible ultra-violet (UV) electromagnetic radiation (10–400 nm) and by infrared radiation (IR; 700–1 mm); this roughly constitutes the remaining half of the solar radiation incident on the Earth’s surface. These three wavelength regions of the electromagnetic spectrum are the most significant with respect to biological systems. Visible light includes violet (~400–450 nm), blue (~450–520 nm), green (~520–560 nm), yellow (~560–600 nm), orange (~600–625 nm), red (~625–700 nm), and far-red (FR; > 700 nm). The most important part of the light spectrum for plants, PAR (400–700 nm), falls within the visible light range.
Understanding the spectral quality of photosynthesis is critical when selecting a lighting system with proper light quality and quantity for any indoor plant cultivation. Our current understanding of the spectral quality of photosynthesis is mainly based on McCree’s findings in the 1970s. The action spectrum of plant leaves was described as the span of wavelengths from approximately 400–700 nm, over which plants absorb and effectively use radiant light energy for photosynthesis. This brought some definition to what is now commonly known as PAR (photosynthetic active radiation), the measure of that relates the intensity and rate of radiant light energy per surface area emitted by a light source from within the action spectrum of plants.
Cannabis yields strongly correlate with increasing light intensity. However, light intensity does not seem to affect the cannabinoid concentration when cannabis is grown under different light intensities under HPS light. In the studies by Vanhove et al. and Potter and Duncombe, it was concluded that THC concentrations of flower material could be primarily linked to cannabis variety instead of cultivation method. In both studies, an increasing irradiance level correlated positively with flower dry weight; however, medicinal oil production by comparative percentage of dry weight remained the same regardless of irradiance levels.
While it is light intensity that plays the most important role in photosynthesis it is light colour (spectrum) that plays the most important role in photomorphogenesis.
Photomorphogenesis refers to light-mediated development where plant growth is altered in response to light signals. Photomorphogenesis only describes how plant growth patterns respond to light colour, which is a separate process from photosynthesis where light is used as a source of energy to power photosynthesis.  Light colour/spectrum has only a small effect on photosynthesis, but a large effect on plant shape and other factors such as secondary metabolism.
The plant receives signals from the light environment through photoreceptors. Phytochromes, cryptochromes, phototropins, and UVR8 are the most well-studied photoreceptor groups found in higher plants. Phytochromes are the red- and far-red-sensing photoreceptors which regulate, for example, flowering, shade avoidance syndrome behaviour, and germination in many species. Cryptochromes and phototropins are regulated mainly by blue and green wavelengths. UVR8 is responsible for UV-B-induced responses. Short wavelength irradiation has been shown to enhance the plant defense mechanism by inducing metabolic activity, such as phenolic compound synthesis. Phenolic compounds, including anthocyanins, found especially in red-coloured leaves, have been shown to accumulate in lettuce leaves under short-wavelength blue and UV light. Many phenolic compounds are part of the plants’ defense mechanism, which are synthesized under environmental stress. Short-wavelength irradiation and high photon flux irradiance are examples of light-related environmental stress. Several cannabinoids have also been suggested to be involved in the plant defense mechanism and to have antioxidant properties, including Δ-9-tetrahydrocannabinol (THC) and cannabidiol (CBD) as well as cannabigerol (CBG).
By manipulating LED light spectra and stimulating specific plant photoreceptors, it is now proving possible to minimize operation costs while maximizing cannabis biomass and cannabinoid yield, including tetrahydrocannabinol (or Δ9-tetrahydrocannabinol) and cannabidiol for medicinal and recreational purposes.
For example, a 2018 study by Grassi et al. found that THC and CBD production is increased under the wider colour spectrum of LED. This study compared LED to HPS lighting and concluded:
“Our results show that the plant morphology can be manipulated with the light spectrum. Furthermore, it is possible to affect the accumulation of different cannabinoids to increase the potential of medicinal grade cannabis. In conclusion, an optimized light spectrum improves the value and quality of cannabis. Current LED technology showed significant differences in growth habit and cannabinoid profile compared to the traditional HPS light source. “
The authors concluded, cannabis can be manipulated with light spectra and that the lower wavelengths, blue and UVA, could contribute to the higher cannabinoid yield.
Similarly, a 2017 study by Alison Justice found that LED lighting increased total cannabinoid content by 12% when compared to HPS.
Terpene flower content is much lower when compared to flower cannabinoid content but plays a large role in flower quality. Terpenes are a diverse group of aromatic compounds found within the flower trichomes. There are hundreds of terpenes and each have their own distinct aroma. Their inclusion complements the cannabinoid profile, both therapeutically and on the sensory side. Medically, these compounds are used for stress relief and have anti-microbial properties. A few of the commonly noted terpenes are Myrcene, Terpinolene, Limonene, Pinene, Caryophyllene, and Linalool.
Light is one environmental factor that can increase the amount of terpenes. There are two ways of improving terpene profile through lighting: increasing light intensity and optimizing light quality. Simply by increasing light intensity within the canopy during flowering, the grower can increase synthesis of terpene precursors that lead to increased terpene content. Light quality can also improve the terpene profile. For example, by adding a white light component to your spectrum, terpene content can be increased when compared to just using blue and red light.
Earlier Flower Ripening Under LED
A 2020 cannabis study that compared LED to HPS found that crop flowering times were shortened by 5-days under LED when compared to HPS. (see “note” reference at article end)
There are some questionable claims being made about some LED brands with limited research data to support these claims. For example, some LED suppliers assert that LED grow lights produce higher growth rates because their colour spectrum better promotes photosynthesis. This, in fact, is misleading. Studies show that photons within the photosynthetic spectrum of 400 to 700 nm are essentially equally capable of driving photosynthesis and, therefore, plant growth rates.
Light intensity has been shown to have a larger effect on yields than light spectrum. One of the advantages that is highlighted by the manufacturers of LEDs is the possibility to optimize lighting spectra selecting only specific, physiologically reasoned light wavelengths and not to waste the energy for unprofitable spectral parts such as green or yellow. However, the so called “unprofitable” light spectral parts are shown to have significant physiological effects on plants. Removing them from the lighting equation would, therefore, potentially have detrimental effects on plant morphology.
Research by Dr Bruce Bugbee and a team of scientists at Utah State University is expanding our knowledge of PAR. The USU team has evidence that photosynthetic photons extend beyond 400 to 700. In a paper released by Shuyang Zhen and Bruce Bugbee, in mid 2020, the authors note:
“Far-red photons (701–750 nm) are abundant in sunlight but are considered inactive for photosynthesis and are thus excluded from the definition of photosynthetically active radiation (PAR; 400–700 nm). Here, we report the effects of far-red photons on single leaf and canopy photosynthesis in 14 diverse crop species. Adding far-red photons (up to 40%) to a background of shorter wavelength photons caused an increase in canopy photosynthesis equal to adding 400–700 nm photons.
Far-red alone minimally increased photosynthesis. This indicates that far-red photons are equally efficient at driving canopy photosynthesis when acting synergistically with traditionally defined photosynthetic photons. Measurements made using LEDs with peak wavelength of 711, 723, or 746 nm showed that the magnitude of the effect was less at longer wavelengths. The consistent response among diverse species indicates that the mechanism is common in higher plants. These results suggest that far-red photons (701–750 nm) should be included in the definition of PAR.”
Bugbee hesitates to claim he’s redefined PAR, but he says “If we start getting people to think about an expanded range of wavelengths for photosynthesis, that’s a big deal. We are not yet sure how much it should be expanded, but it may be as much as 350 to 750.”
Bugbee says he’s less certain about the lower range, due to less data for UV photons, but the upper end should be at least 730 or 740.
He says there’s an unwarranted emphasis on red/blue or narrow-band light for photosynthesis. As a result, narrow-band light became popular in LEDs, negating the power of the entire spectra of light. “The effect of colour on photosynthesis is way overrated,” he says. “Conversely, the effect of colour on plant shape is underrated.
Regarding photosynthetic effects, Bugbee points to light intensity (aka light quantity). “Photosynthesis is exquisitely sensitive to intensity,” he says
He recommends growers should not become obsessed about spectral quality but instead base purchasing decisions on luminaire efficiency and light intensity. “After selecting an effective fixture, the fractions of blue and far-red photons can be fine-tuned to alter plant shape,” he says.
Bugbee makes a good point. A July 2020 cannabis study that tested spectrally tuned LED horticultural fixtures against LED commercial flood lights found that the LED flood lights produced the same yield given equivalent PPFD as the spectrally tuned horticultural fixtures. The take home message from this is light intensity drives yields while light spectrum drives plant morphology and secondary metabolite production.
Quality and Lifespan of LED Fixtures
There can be significant quality differences between horticultural LED fixtures. A quality LED lighting system is a big investment, so do your homework. Ensure you are considering all factors when considering any LED purchasing decision. In life you typically get what you pay for. Basing your purchasing decision on the cheapest price will very likely turn out to be a costly mistake later on.
Four key factors determine LED fixture quality and life expectancy
- LED junction temperature
- Driver efficiency
- LED drive current
- Optical losses of the fixture
- LED junction temperature
Without sufficient light, plants are not able to adequately photosynthesize and they will not mature properly. To obtain high light levels, high-watt LEDs at fairly high densities are needed. Light output of LEDs degrades mainly based on temperature.
Junction temperature refers to the operating temperature at the actual diode. There are two temperature standards for reporting the efficacy of LEDs: 25 and 85 °C. Efficacy decreases ~10%, as the temperature increases from 25 to 85 °C, a syndrome known as ‘Thermal Droop’ which comes down to a reduction of optical power when the temperature is increased.
The junction temperature of LEDs in fixtures depends on the drive current, ambient temperature, and the heat dissipation (thermal management) of the fixture, but is typically around 85 °C. Better thermal management may increase fixture cost, but it also increases efficacy and longevity of the LEDs.
If LEDs overheat this reduces the lifespan of a LED fixture. Therefore, proper heat management is crucial for LEDs as increased heat affects the performance of the LED fixture in a negative way; effects include a reduction in light efficiency, colour shifts and reduction in lifespan of the LED. Since operating temperature is a major factor in the life span of an LED, fixtures require a means for allowing warm air to escape. There are two ways to cool LED lighting systems in commercial horticultural environments that will impact the photon efficacy of the fixture. Passively cooled fixtures utilize heat sinks to dissipate heat from the circuit board, while actively cooled fixtures rely on fans or water cooling to dissipate heat. Fans that are used to cool fixtures consume energy and will decrease the overall photon efficacy of a fixture. Additionally, if a fan fails during the operation of the fixture, the LEDs on the PCB are likely to overheat and burn out. Even if they don’t fail catastrophically, reduced power output will dramatically reduce the usable life of the LED fixture. This is a very important feature that growers need to consider when comparing LED horticultural lighting systems. That is, because of the relatively high investment needed to replace LED fixtures, it is thought LEDs will be operated to the limit of their lifespan despite the lower PPF in the end-of-life period (like HID lamps). Replacement of individual LEDs is prohibitively expensive and impractical in the field. However, the LED is often not the limiting factor. Power supplies, fans, and other components (sealings, fixtures, enclosures, etc.) in LED fixtures can fail well before the LEDs themselves. It is, therefore, important for any LED fixture fabricator to ensure the supporting electronics for the LEDs are designed with reliability in mind, operating well within operating limits to maximize the lifespan of the fixture to match the lifespan of the LEDs.
Passively cooled fixtures are argued by some to better suited for the harsh conditions of indoor and greenhouse growing environments. Others would tell you passive cooling and fans are the way to go, while others would recommend water cooling which while costly certainly offers the most effective cooling. However, regardless of how efficient your LED fixtures are, the created heat has to be cooled away. The better the cooling, the longer the operational lifespan of the LED fixture.
Incandescent bulbs are deemed as ‘failed’ when the filament breaks and the bulb no longer emits light. LED’s, however, will gradually decrease in light output over an extended period of time. This raises the question, what constitutes a failure point or lifetime of an LED? To solve this, the LED industry developed the IES LM-80 and TM21 standards which specifies how LED manufacturers can test LED components to determining their performance over time. As a result, lifetime predictions where measured in terms of L70, which is the point at which the light source is at 70% of its initial lumens (or a 30% loss in output). As LEDs evolved, the industry adopted additional ratings such as L80 (80% of initial lumens) and L90 (90% of initial lumens). In other words, a fixture rated at 50,000 hours L90 would retain 90% of its light output at 50,000 hours, whereas a fixture rated 50,000 hours L70 would only retain 70% of its output at 50,000 hours. See following graph
Rates of fixture aging can vary greatly among manufacturers. Most manufacturers characterize their fixture lifetime (L70, L90, Q70, or Q90) in terms of LED output depreciation based on a standard LED package test—IES LM-80, which can be interpolated into luminaire lumen maintenance. Projections of lumen maintenance based on LED depreciation cannot exceed six times of the duration that the LEDs were tested, so for a depreciation lifetime claim of 60,000 h the LEDs must have been tested for 10,000 h. Many fixtures that claim extended lifetimes are exceeding the allowable six times interpolation based on LED testing. Fixture lifetimes based on LED depreciation also do not include optical loss mechanisms in the fixture and accelerated aging of the LEDs due to higher temperatures. Also, typical lifetime claims do not consider catastrophic failure of the LED driver, which often fails before the LEDs have reached even L70.
For plants, light maintenance is crucial. Therefore, you should always look at a L90 for LED fixtures that are used in horticulture. This is already 10% less light, a value you would not reach with HPS lamps (they would already be replaced).
- LED drivers
LEDs are semiconductors with light-emitting junctions designed to use low-voltage, constant current DC power to produce light. LEDs have polarity and, therefore, current only flows in one direction. Driving LEDs is relatively simple and, unlike fluorescent or discharge lamps, they do not require an ignition voltage to start. Light output of LED light sources increases with increasing drive current and reduces with decreasing drive current. Too little current and voltage will result in little or no light, and too much current and voltage can damage the light-emitting junction of the LED diode and cause overheating reducing the lifespan of the LED.
Drivers often fail prematurely due to high internal operating temperatures. Battery-like components called electrolytic capacitors are typically the cause of death. Electrolytic capacitors have a gel inside them that gradually evaporates over the lifespan of the driver. High temperatures quicken the evaporation of the gel and shorten the life of the capacitor, causing the driver, and hence the LED, to stop working unexpectedly.
The efficiency of LED drivers ranges from 85 to 95%. LED drivers can be less efficient when they provide dimming, colour control and/or communication functionality.
- Drive current
Drive current is the amount of amperage being sent from the LED driver through the LED array.
Light output of LED light sources increases with increasing drive current.
Some LED manufacturers increase drive current to increase luminaire output without increasing its purchase price. However, while increasing drive current increases light output it also increases heat – and heat is the enemy of the LED. It’s only a matter of time before the repercussions of overdriven products lead to premature repairs and replacements. Despite being lower in initial price per lumen, high drive current products are far from the bargain they seem on paper.
Both junction temperature and drive current density will affect the photon efficacy of LEDs, and in general, the most efficient LED fixture will run their LEDs at low drive currents. However, a lower drive current results in a lower photon output per LED, and the resulting fixture will require many LEDs to achieve a high photon output and thus will be more expensive. Unfortunately, chip area (numbers) is often confidential, and LED manufacturers only report LED specifications at the total LED drive current, not drive current density.
- Optics and Optical losses
The optics in any LED lighting system are crucial elements of that system’s performance, as they alter the directionality and intensity of light from the LED source. Optics in LED lighting can include the spatial distribution of light from the diode itself, and the reflectors, lenses, and holders that cut off or limit output light with mechanical blocking devices. Facilities that install LED lighting systems will use different optics in LED lighting, for example, to control the beam angle of the output light, to create either crisp-edged or diffused light, or to concentrate light in certain areas while limiting it in others.
Photons must impact leaves to be absorbed, and this is an important consideration in fixture design. Early LED fixtures focused photon output over a small area. This facilitated precise photon placement, but caused nonuniform distribution. LED package and fixture design has transitioned to a less-focused photon distribution through the use of optics.
When you look closely at a single LED you will see a small protective dome over the diode. This is called the primary optic which serves to protect and shape the output of the small diode. The light from the LEDs primary optic is generally not suitable for horticultural applications and, therefore, many LED luminaire manufacturers utilise secondary optics to better focus light onto the crop resulting in more uniform mixing of colours and improved photon penetration into the plant canopy.
Optics can produce narrow, wide and asymmetric beam distributions for a different amount of light penetration into the foliage.
A narrow beam angle gives a higher PPFD for plants that require deeper light penetration into the canopy. A wider beam angle has potentially lower PPFD but this can be offset by better cross illumination which means you can lower the luminaire closer to the plants, resulting in high PPFD. Asymmetric distributions can be used to illuminate vertically or off-axis to allow horticultural luminaires to be installed in the ceiling/roof infrastructure while still distributing light to the lower parts of plants. Optics, thus, become important when considering horticultural LED luminaire fixtures. An optimized optical design will direct all possible light onto the target surface and can increase the optical efficiency of a system.
Optics, also, very much determine spatial distribution which refers to the heterogeneity of the PAR light falling on the target plane (the canopy). Many lights are designed with secondary lenses or reflectors to focus most of their output in a narrow cone for higher PAR/PPFD readings. However, just outside this area (still in the light footprint), PAR drops off significantly. This results in an LED grow light appearing more powerful than it really is.
Optics can also diffuse the photons, which reduces efficiency – referred to as optical losses. Optical losses occur when LEDs are mounted in fixtures. The sides of the fixture can obstruct low-angle photons. Protective transparent covers (e.g., glass) transmit up to 92% of the photons and reduce the output by 8%, but this protection can significantly improve the lifetime of a fixture. Fixtures with unprotected LEDs can have 99% optical efficiency, but may have shorter lifetimes in harsh growing environments (e.g. high humidity).
Based on this, it is imperative to keep in mind how the optic will be impacted by exposure to varied operating conditions in the application environment. Often, horticultural lighting fixtures are exposed to water, humidity, and other chemicals used in application. Some, such as hydrogen peroxide, can leave a film on the lens that reduces transmission. If the material cannot withstand the environment, its performance will degrade, negatively affecting the growth of plants. An easily cleaned and maintained material is ideal. A material such as UV glass is highly resistant to abrasive conditions, heat cycling and UV radiation exposure, whereas plastics in the same environment will erode and discolour resulting in severe transmission loss. The loss in transmission can significantly affect the performance of a light fixture, especially in applications like horticultural lighting where consistent and uniform light output are required.
Other Quality Considerations
Luminaires and lighting systems designed for operation in indoor growing facilities are also routinely subject to environmental conditions unlike those commonly found in conventional commercial or industrial applications. For example, to foster plant development, indoor farming environments typically feature increased ambient temperature conditions and higher levels of humidity, requiring that horticultural lighting be designed to reliably operate under these conditions. Horticultural lighting must also be able to safely withstand exposure to increased levels of dirt and dust, as well as water and mist generated by plant irrigation and humidification systems. Plastics and other materials used in the construction of horticultural lighting systems must be designed to hold up under these conditions, as well as prolonged exposure to ultraviolet (UV) radiation.
Unlike conventional luminaires and lighting systems that are typically installed in fixed locations, the positions of horticultural lighting are frequently adjusted in order to optimize plant exposure to light. Therefore, horticultural lighting fixtures may be mounted via adjustable cables or chains, or installed on moveable racks. In either case, an abundance of cables, cords, connectors and plugs may be attached to the fixtures to facilitate movement and to provide the greatest degree of positioning flexibility. This increase in the number of electrical connections can also create unique safety issues that must be addressed in the design of the lighting system itself.
Additionally, as the use of LED-based lighting becomes the preferred technology behind most horticultural lighting systems, considerations regarding the potentially harmful photobiological effects from exposure to LED lighting come into play. Photobiological effects can include skin irritation, as well as irritation of the front surface of the eye and the retina, and can lead to photokeratitis, ultraviolet erythema, cataracts or retinal thermal injury. Because employees in indoor farming operations routinely work in close proximity to lighting fixtures, thereby increasing the potential risk from exposure, LED-based horticultural lighting must thoroughly account for these risks.
HORTICULTURAL LED FIXTURE QUALITY STANDARDS
To date, the LED lighting industry has been sorely lacking in standards developed specifically for quality analysis of the componentry and design of horticultural fixtures. This situation is now changing as bodies such as the Design Lights Consortium (DLC) set about establishing standards that will help consumers and others determine the quality of various fixtures through factors such as:
This is a characterisation of the ability of the device to maintain its output within the given ranges over time.
The flux maintenance point is set at a 10% reduction from initial product output due to the increased sensitivity of plant metabolism to reduced flux. While lengthening a photoperiod may be an option for some growers to achieve desired DLI, feedback from growers has generally indicated that degradation of the lighting system beyond this level results in replacement.
In-Situ Temperature Measurement Testing (ISTMT)
ISTMT must be conducted and provided for the hottest LED in the fixture, and LED-device level drive current must be reported.
Applicants must supply a technical specification sheet for the driver they use in their product, showing the lifetime of the driver based on operating temperature and the temperature measurement point (TMP) for monitoring the operating temperature of the driver. In-situ temperature measurement testing must be conducted, and a report must be provided with the application showing an operating temperature consistent with the driver spec sheet information and demonstrating that the driver will have a lifetime of at least 50,000 hours when operating at or above the highest rated ambient temperature on the fixture’s specification sheet.
Specifically, applicants must characterize the operating temperature of the driver at the fixture’s highest rated ambient temperature.
Products that employ on-board cooling fans must provide a technical specification sheet for each fan type employed in the product. The fan specification sheet must specifically state the lifetime of the fan and a reference operating temperature rating for that lifetime claim. The lifetime must be at least 50,000 hours, at an environmental temperature at or above the fixture’s highest rated ambient temperature.
Electrical Performance/Power Quality:
The DLC requires the testing and reporting of the following to characterize the electrical performance of the device:
Products must have a measured power factor of ≥0.90 at any rated input voltage and maximum designed output power.
Total Harmonic Distortion, current (THDi)
Products must have a measured THDi of ≤20% at any rated input voltage and maximum designed output power.
The DLC requires products to be appropriately safety certified by a relevant safety certification body in the United States or Canada. Specifically, products must be certified by an OSHA NRTL or SCC-recognized body to a set of safety requirements and standards deemed applicable to horticultural lighting products by that safety organization.
Products must have a manufacturer-provided warranty of at least 5 years. The warranty terms and conditions must be provided as part of the submittal for qualification. Terms and conditions must not exclude key components such as the LED, driver, cooling fans (if present), optics, or controls.
The DLC requires that all testing be conducted at appropriately accredited laboratories. Specifically, testing of flux, intensity, and electrical characteristics must be conducted at laboratories that are accredited to ISO 17025 and the appropriate reference test standard by accreditation bodies that are signatories to the ILAC-MRA.
The DLC website can be found here: https://www.designlights.org
Purchase Cost and ROI of LED
Probably the most significant roadblock for widespread adoption of LED lighting is the high purchase cost of LED fixtures. As I write this in June 2020, LED horticultural lighting luminaires cost 3 to 10 times more to initially purchase than HPS fixtures. This puts a lot of growers off switching from HPS to LED. However, failing to switch to LED is arguably short sighted because horticultural LED fixtures are far cheaper to operate in the long-term thanks to significantly reduced power consumption, longer lifespan and elimination of the need to change bulbs every 6-12 months. These factors result in significant savings where the initial investment cost of the LED fixtures is offset by the savings over the course of 2 to 3.5 years for sole-source applications and 4-7 years for supplemental applications (dependent on the DLI/PPFD required, hours of operation per day, local KWh energy cost, the cost per fixture, number of fixtures and the efficiency of the luminaire).
For example, I recently received a quote/ROI analysis on big-name brand (think Philips, GE, Osram/Fluence etc) horticultural fixtures x 350. The fixture concerned outputs 1840 PPF so it provides equivalent PPF to a DE HPS 1000-watt. The operational time was 4350 hours per year as a sole lighting source which equates to 12 hours a day running time over the course of the year. In this case, the payback/ROI occurred in 2.5 years.
Just keep in mind, every grow situation will alter ROI.
This is important point to understand. Return on investment for LED lighting is different for every grow operation.
Payback calculations for LED lighting are only useful when they apply specifically to your situation. Look for a manufacturer/supplier who will provide a custom light plan that considers all the factors specific to your application.
Additionally, factor in other costs when considering ROI. Growers in cold climates, for example, will have to compensate for the loss of heat from HID light sources in the winter – so the additional heating costs will offset a percentage of the LED energy savings increasing the time it takes to realise ROI.
On the other hand, growers in hot climates will likely find reduced energy costs as a result of savings on cooling/AC/HVAC, improving ROI.
Challenges in horticultural LED lighting
Based on the material that we have covered re “the quality and lifespan of LED fixtures” you can possibly tell that there is a lot of physics and electrical engineering that goes into designing and producing a high-quality horticultural LED fixture. You can also perhaps tell that if all factors that influence the quality of the fixture aren’t optimal you could quite easily be purchasing a lemon.
Therefore, it would be remiss of me to write an article on LED horticultural fixtures and not point out that there are often significant quality and performance differences between brands that are commonly marketed through the agricultural and hydroponic retail sectors.
There are, of course, challenges in any new technology and perhaps even more so in LED-based horticultural lighting where experience with LED technology is still evolving, and even the long-involved horticultural scientists are still developing research on optimal light spectrums for plants.
Lighting manufacturers based in Asia have targeted the market with what are often affordable but low-end products, which often come with overinflated claims, but many of the low-end products on the market lack relevant certifications and independent testing. Many growers, especially in the cannabis sector, have been burned with early attempts to deploy LED lighting due to poor fixture performance. Many cannabis growers are still being burned by the sales of substandard products or through sales advice which sees growers purchasing fixtures that aren’t suitable for the task at hand.
Certainly, there are many high-quality LED lighting products on the market as well, including those from reputable companies such as Philips, Osram/Fluence, and GE (among others). GE, Osram and Philips have spent years on horticultural research working with university and dedicated-research-organization teams. For example, Fluence/Osram has conducted a great deal of cannabis research with the likes of Dr Alison Justice and Dr Bruce Bugbee, who is currently working with Fluence/Osram products at the University of Utah.
On the other hand, some Asian manufacturers simply knock off reputable brand name technology, often sacrificing quality aspects along the way, which results in a far cheaper to manufacture, albeit vastly inferior products that ultimately fail to perform at anywhere near the level of the technology which inspired the knockoff.
Then you have the very unethical small percentage of LED manufacturers, wholesalers and retailers who push LED fixtures with what amounts to highly deceptive marketing. For example, I recently had an individual from a Melbourne, Australia hydroponic store tell me that he was getting 75% more yield using his shop LED brand over HPS. I mean, all I can say to this is that if anyone claims something like this, get out of that store as quickly as possible. This is definitely not a person you want to be shopping with… for any product.
A quality 600W LED fixture will certainly yield on par to 1000W HPS (and perhaps even a tad – 1 or 2% more); however, to claim significant yield increases for equivalent PPFD either tells you that the person making such claims either knows nothing about indoor cultivation and light (this alone is a reason not to shop with that person) or they are looking to blind you with bullshit with the aim of having you purchase what is likely a substandard, made in China, LED fixture.
The point being, where LED technology is concerned, it is very much a case of buyer beware.
Important questions you should ask when purchasing LED lighting fixtures for your grow
- Is the brand of LED luminaire associated to a company with a good/proven track record and reputation in horticultural lighting?
- Can the supplier provide comprehensive tech sheets, detailing spectral quantum distribution, PFD to PPFD at given heights above the canopy, the cooling method employed in the fixture, driver lifetime, fan lifetime, compliance standards the fixture meets and comprehensive technical information surrounding junction temperature etc?
- What do I get for my money? A good LED lighting system is a big investment, so do your homework. Ensure you are considering all factors when calculating the cost of ownership. In life you typically get what you pay for. The cheapest price may turn out to be an expensive mistake.
- Has the fixture been independently evaluated? Certified test laboratories conduct comprehensive tests on fixtures to characterize their performance. Fixture manufacturers should always be able to provide test results for their fixtures from certified third-party testing laboratories.
- Has the fixture been approved by the Design Light Corporation or another credible, independent body? If so, visit https://www.designlights.org to look over the specs and check that indeed it has been approved/listed with the DLC.
- Does the supplier seem credible with their marketing claims? Look out for claims that don’t ring true.
- How much PFD does the fixture output?
- How much PPFD from the fixture is available to plants at given heights above the canopy?
- What is the minimum hanging height to ensure uniform cross illumination and how does this impact the PFD to PPFD?
- How much energy is used by the fixture to make this PPFD available to the plants?
- What is the total wattage of the fixture including driver wattage?
- What is the efficiency rating of the fixture?
- What is the L90 of the fixture?
- What is the junction/operating temperature? Junction temperature refers to the operating temperature at the actual diode. The cooler this temperature, the longer the lifespan of the LEDs in the fixture.
- What sort of cooling does the fixture employ to maximise lifespan? Thermal management and junction temperature go hand in hand. The better the cooling, the longer the lifespan.
- In what country is the fixture made? A product made in. e.g. Germany, Holland, the US etc may indicate a higher quality fixture than a product made in China.
- What LEDs does the supplier use in the fixture? Reputable names such as Osram (Munich, Germany), Lumileds (Amsterdam, Netherlands), Samsung (Seoul, South Korea), Cree (North Carolina, USA) etc may indicate a higher quality fixture than a competitor using a lesser known brand of LED in their fixtures.
- Can the supplier help with fixture layout plans/guides to meet the required/target PPFD for your grow?
- Can the supplier furnish an example cost/ROI analysis that demonstrates initial purchase cost of the fixtures and ROI when compared to HPS? Note: Payback calculations for LED lighting are only useful when they apply specifically to your situation. Look for a manufacturer who will provide a custom lighting plan that considers all the factors specific to your application. Be sure to factor in other costs when considering ROI. Growers in cold climates, for example, will have to compensate for the loss of heat from HPS lights in the winter – so the additional heating costs will offset a percentage of the LED energy savings, increasing the time it takes to realise ROI.
- Can the supplier provide references/links to horticultural operations that are using their fixtures? Switched on, professional operations are unlikely to have invested heavily in LED fixtures without doing their homework. You may want to contact some of these references to ask about their experiences with the given brand/fixture.
- What sort of warranty is offered on the fixture/s? A reputable manufacturer will offer a complete system warranty with a rated lifetime of 50,000+ hours and a guaranteed photon flux maintenance under typical operating conditions in a horticultural environment.
- Can the person selling the LED fixture answer these questions adequately or can they direct you to someone who can answer these questions? If not, take your business elsewhere.
Be wary of:
- High Lifetime Claims, E.g. 75,000 hours L90. Projections of lumen maintenance based on LED depreciation cannot exceed six times of the duration that the LEDs were tested, so for a depreciation lifetime claim of 60,000 hours the LEDs must have been tested for 10,000 hours. Many fixtures that claim extended lifetimes are exceeding the allowable six times interpolation based on LED testing. Fixture lifetimes based on LED depreciation also do not include optical loss mechanisms in the fixture and accelerated aging of the LEDs due to higher temperatures. Also, typical lifetime claims do not consider catastrophic failure of the LED driver, which often fails before the LEDs have reached 50,000 hours. If a supplier makes a high lifetime claim ask them what test standards were used to establish the rating. Additionally, ask if any independent tests were conducted and whether the results of this testing are available.
- Higher Light/PAR/Photon Output but Cheaper than Expected Price when Compared to Recognized Quality Luminaires. Some LED manufacturers increase drive currentto increase luminaire output without increasing its purchase price. However, while increasing drive current increases light output it also increases heat – and heat is the enemy of the It’s only a matter of time before the repercussions of overdriven products lead to premature repairs and replacements. Despite being lower in initial price per lumen, high drive current products are far from the bargain they seem on paper.
- Products that do not meet standards set by organisations such as DLC.
- Products which have not been independently quality assessed/assured/approved by independent organisations such as DLC.
MEASURING PAR/PPFD TO ENSURE THE CROPS LIGHTING NEEDS ARE OPTIMAL
Lastly, on the subject of LED lighting, it’s important to monitor/measure the levels of PAR/PPFD that your crop receives and whether this light is evenly distributed across the canopy. This helps growers optimise the levels of light the crop has available for photosynthesis.
PAR light meters, also known as quantum sensors, are used to measure PPFD/PAR (photosynthetically active radiation). Authors tend to use the terms quantum meter, quantum sensor and PAR meter interchangeably; however, whether called PAR meter or quantum meter or quantum sensor these names refer to the same thing. In the following material I will use the term PAR meter.
What is a PAR Meter?
A PAR meter measures the intensity of light, the amount of electromagnetic radiation below a single source, or multiple sources of light. The easiest way to think of a PAR meter is to think of measuring light intensity. A quantum of light is the smallest measurable amount of solar radiation and is called a “photon”. PAR meters measure quantum flux (aka “photon flux”), the number of photons per square meter per second (µmol m-2s-1) within the PAR spectrum of 400 – 700nm.
A PAR meter consists of two main components:
- The sensor itself that measures light (usually a photodiode)
- A meter that displays and often records measurements (either a handheld unit, or computer)
How to use a PAR Meter
To accurately measure light with a PAR meter, it is important that:
- the sensor is level
- the sensor head is clean
- there are no reflective objects near the sensor
- you are not physically interfering with the light source
Remember that a level sensor free of dirt and other light-affecting obstructions will only provide an accurate reading with correct calibration. Sensors generally need re-calibration every two to three years.
Measure PPFD at Multiple Points
PPFD (photosynthetic photon flux density) measures the amount of PAR light that arrives at the plant canopy every second, or (more technically) the number of photosynthetically active photons that fall on a given surface each second. PPFD is a spot measurement of a specific location on your plant canopy and it is measured in micromoles per square meter per second (μmol·m-2·s-1).
This is an essential measurement because it indicates the amount of light available to the plant for photosynthesis.
PPFD measurements should be taken at multiple locations on the illuminated surface (the top of the crop canopy) and mapped. The objective is to have homogeneity over the entire crop area to avoid disparities in plant growth according to light fixture placement.
It is crucial to get this PPFD mapping because single PPFD measurements can be misleading. In fact, most lights have a ‘spatial distribution’ where there can be a non-uniformity of light quantity falling on the canopy surface at different points, under a single luminaire. For this reason, single readings are meaningless as many lights are designed with secondary lenses or reflectors to focus most of their output in a narrow cone for higher PAR readings. However, just outside this area (still in the light footprint), PAR drops off significantly. This results in a grow light appearing more powerful than it really is. It is important to note that studies have shown that when the uniformity of spectral irradiance received by plants is poor, the yield and nutrient composition of plants will be greatly affected.
See following image of spatial distribution of one LED fixture
Source: Pan Jiangen et al. (2020) SPATIAL LIGHT DISTRIBUTION CHARACTERIZATION AND MEASUREMENT OF LED HORTICULTURAL LIGHTS
Other than spatial distribution, multiple PPFD readings at different points on the canopy helps to ensure that LED fixture positioning is optimal to best achieve uniform light/PPFD over the entire crop canopy.
PAR Meter Quality
As with most things in life you get what you pay for. This is true of all scientific testing equipment whether that be an IR thermometer or a PAR meter.
The fact is, there is little point taking PAR measurements if these readings are inaccurate. Therefore, it is advisable that if you are looking to purchase a PAR meter that you do your homework on which meters will give you accurate readings and which units won’t. For example, this Youtube link compares a quality, research-grade Apogee quantum meter to a cheap to purchase Hydrofarm unit. See https://www.youtube.com/watch?v=_H8sDFECfA4 or Google “Youtube Apogee MQ-500 vs Hydrofarm Par Meter – Comparing Light Sources.”
It’s worth noting that in a study by Apogee Instruments comparing eight different PAR meters, including the Hydrofarm LGBQM, it was found:
Kipp & Zonen model PQS 1, LI-COR models LI-190 and LI-190R, and Apogee model SQ-500 quantum sensors had minimal spectral, directional, calibration, and stability errors, and matched each other within about 4 %, suggesting they can be reliably used for accurate photosynthetic photon flux density (PPFD) measurement…. Spectrum LightScout and Active Eye/Hydrofarm LGBQM quantum sensors are not research-grade instruments and should be used with caution when making absolute PPFD measurements. The LightScout had large spectral and calibration errors, and the LGBQM had large spectral and directional errors. The LGBQM was also unstable under electric lights. While the LightScout and LGBQM are low cost, the large errors indicate they can only be used to provide a relative indication of PPFD with time for a given radiation source, if the instability is averaged out for the LGBQM.
So firstly, when purchasing a PAR meter, it is important to understand that a costlier, quality research-grade meter will give you a far more accurate reading than a cheap to purchase product that doesn’t qualify as a research-grade instrument.
Just one quality research-grade PAR meter is the Apogee SQ-500 which represents best in its class for price. Apogee states that their full-spectrum SQ-500 PAR meter “will work great” with LED. The SQ-500 measures between 389 – 682 nm ± 5 nm which pretty much fits into the original McCree definition of PAR (400 – 700nm). However, the founder of Apogee Instruments, Bruce Bugbee, has recently redefined PAR to suggest that far-red photons (701–750 nm) should be included in the definition. As a result, PAR meters should now ideally measure to about 740 – 750 nm in order to accurately measure PAR. For example, when looking at the Fluence PhysioSpec ™ BROAD spectra LED spectral analysis we can see that some far-red spectrum light between 700 – 750 is emitted from the fixture which wouldn’t be accounted for by a meter such as the Apogee SQ-500. See following graph of the Fluence PhysioSpec ™ BROAD spectra spectrum analysis.
This is something to be aware of. PAR meters are improving all the time. In fact, as I was writing this material Apogee released a 340 to 1040 nm PAR “extended range photon flux density (PFD) sensor” for LED lights.
Some Quality Brand Names in Quantum Meters
- Apogee Instruments
- Kipp & Zonen
- Skye Instruments
Factors that Determine PAR Meter Quality
Spectral response and error: The combination of diffuser transmittance, interference filter transmittance, and photodetector sensitivity yields spectral response of a quantum sensor. A perfect photodetector/filter/diffuser combination would exactly match the defined plant photosynthetic response to photons (equal weighting to all photons between 400 and about 740 nm, with no weighting of photons outside this range), but this is challenging in practice. Mismatch between the defined plant photosynthetic response and sensor spectral response results in spectral error when the sensor is used to measure radiation from sources with a different spectrum than the radiation source used to calibrate the sensor.
Don’t purchase any PAR meter unless the manufacturer publishes the sensor’s spectral response curve as part of the specifications.
Temperature error: Temperature error refers to changes in electrical or optical components caused by temperature changes. Sensor temperature response is the change in signal as a function of temperature. Signal output by a sensor should only respond to changes in radiation incident on the diffuser, but electrical or optical components (for example, photodetector, resistor) may have some temperature sensitivity that affects the measurement. Generally, temperature error shouldn’t be a factor in controlled environment greenhouse and indoor grows because optimal temperatures for plant growth match temperature calibration for most PAR meters.
Cosine Error: Improper weighting of radiation incident at non-zero zenith angles. Sensor directional response is the response to radiation incident at different angles. Ideally, a sensor with a hemispherical, or 180°, field of view should accurately measure radiation emanating from the hemisphere above the sensor at any angle of incidence. Lambert’s cosine law states that radiant intensity is directly proportional to the cosine of the angle between the incident radiation beam and a plane perpendicular to the receiving surface. A sensor that measures radiation according to Lambert’s cosine law, meaning it measures radiation accurately at all incidence angles, is said to be cosine-corrected. See following graph that demonstrates the cosine error between a quality Apogee SQ-500 meter v. a cheap to purchase Hydrofarm PAR meter.
Cosine correction: Usually only the most expensive sensors get close to the theoretical response as the angle of incidence of light changes. Do not buy any sensor unless the manufacturer specifies the cosine-correction errors.
Stability error: long-term instability (drift) is caused by changes in sensor components (for example, photodetector degradation), and short-term instability can be caused by electrical interference (for example, measurement in electrically noisy environments). Sensor stability is dependent on the stability of sensor components. If components degrade, the signal output by the sensor will drift. Some degree of long-term drift can be corrected by periodic recalibration of sensors. However, if drift is erratic or rapid, recalibration is not a solution.
Calibration error: inaccurate scaling of the signal output by a sensor to match an accurate PPFD reference or scaling the signal output by a sensor to match an inaccurate PPFD reference. There is not an established PPFD standard, so PAR sensors must be calibrated against a trusted PPFD reference. PAR sensor manufacturers use different PPFD references for calibration.
 Chandra S., Lata H., Khan I. A., Elsohly M. A. (2008). Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions. Physiol. Mol. Biol. Plants 14, 299–306. 10.1007/s12298-008-0027-x, PMID
AND: Chandra S., Lata H., Mehmedic Z., Khan I. A., Elsohly M. A. (2015). Light dependence of photosynthesis and water vapor exchange characteristics in different high Δ9-THC yielding varieties of Cannabis sativa L. J. Appl. Res. Med. Aromat. Plants 2, 39–47. 10.1016/j.jarmap.2015.03.002
 Chandra, S. et al. Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions. Physiol. Mol. Biol. Plants, 14(4)–October, 2008
 Chandra, S. Lata, H. Khan, I. A. and ElSohly, M, A. (2011) Temperature response of photosynthesis in different drug and fiber varieties of Cannabis sativa L. Physiol Mol Biol Plants. 2011 Jul; 17(3): 297–303.
 Macias, H.A.; Ulianov, Y.; Ramos, Y. Illumination benefits using LED high brightness bulb compare to traditional illumination systems. In Proceedings of the 2012 IEEE International Symposium on Alternative Energies and Energy Quality (SIFAE), Barranquilla, Colombia, 25–26 October 2012; pp. 1–5.
 Poorter H., Fiorani F., Stitt M., Schurr U., Finck A., Gibon Y., Usadel B., Munns R., Atkin O. K., Tardieu F., Pons T. L. (2012). The art of growing plants for experimental purposes: a practical guide for the plant biologist. Funct. Plant Biol. xx (in press).
 Jacob A. Nelson, Bruce Bugbee (2015) Analysis of Environmental Effects on Leaf Temperature under Sunlight, High Pressure Sodium and Light Emitting Diodes
 Dieleman JA. (2016) Reducing the carbon footprint of greenhouse grown crops: re-designing LED-based production systems
 Massa G. D., Mitchell C. A., Emmerich J. C., Morrow R. C. (2005). Development of a reconfigurable LED plant-growth lighting system for equivalent system mass reduction in an ALS, in SAE Technical Paper (Rome: ). 10.4271/2005-01-2955
 Hawley D. (2018). The Influence of Spectral Light Quality on Plant Secondary Metabolism and Photosynthetic Acclimation to Light Quality. Ph.D. Guelph University.
 Chandra S, Lata H, Khan IA, Elsohly MA: Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions. Physiol Mol Biol Plants 2008; 14: 299–306. AND Lydon J, Teramura AH, Coffman CB: UV-B radiation effects on photosynthesis, growth and cannabinoid production of two Cannabis sativa chemotypes. Photochem Photobiol 1987; 46: 201–206.
 Vanhove W, Van Damme P, Meert N: Factors determining yield and quality of illicit indoor cannabis (Cannabis spp.) production. Forensic Sci Int 2011; 212: 158–163.
 Potter DJ, Duncombe P: The effect of electrical lighting power and irradiance on indoor-grown cannabis potency and yield. J Forensic Sci 2012; 57: 618–622.
 Hampson AJ, Grimaldi M, Lolic M, Wink D, Rosenthal R, Axelrod J: Neuroprotective antioxidants from marijuana. Ann NY Acad Sci 2000; 899: 274–282.
 Giacoppo S, Gugliandolo A, Trubiani O, Pollastro F, Grassi G, Bramanti P, Mazzon E: Cannabinoid CB2 receptors are involved in the protection of RAW264.7 macrophages against the oxidative stress: an in vitro study. Eur J Histochem 2017; 61: 2749.
 Magagnini, Grassi, Kotiranta. The Effect of Light Spectrum on the Morphology and Cannabinoid Content of Cannabis sativa L. Med Cannabis Cannabinoids 2018;1:19–27
 Justice, A. Economic Impact of Light Intensity on Yield and Secondary Metabolites, presentation at the Photo x Summit, San Diego 2017 https://2xuwao2gok1v2wn2em9n5ys8-wpengine.netdna-ssl.com/wp-content/uploads/2017/11/Allison-Justice_Economic-Impact.pdf
 Hawley, D. et al. Improving Cannabis Bud Quality and Yield with Subcanopy Lighting. HORTSCIENCE 53(11):1593–1599. 2018
 Runkle E. Nelson J. and Bugbee B. LEDs vs. HPS Lamps: A Reality Check, published in GPN Magazine June 2014, www.gpnmag.com
 Kim, H.H., Goins, G.D., Wheeler, R.M. & Sager, J. C. 2004. Green-light supplementation for enhanced lettuce growth under red and blue-light-emitting diodes. HortScience 39: 1617–1622 AND Johkan, M., Shoji, K., Goto, F., Hahida, S. & Yoshihara, T. 2010. Blue light-emitting diode light irradiation of seedlings improves seedling quality and growth after transplanting in red leaf lettuce. HortScience 45: 1809–181
 Zhen, S and Bugbee, B. (2020) Far-red photons have equivalent efficiency to traditional photosynthetic photons: Implications for redefining photosynthetically active radiation
 Kusuma, P., Pattison, P.M. & Bugbee, B. From physics to fixtures to food: current and potential LED efficacy. Hortic Res 7, 56 (2020). https://doi.org/10.1038/s41438-020-0283-7
 Kusuma, P., Pattison, P.M. & Bugbee, B. From physics to fixtures to food: current and potential LED efficacy. Hortic Res 7, 56 (2020). https://doi.org/10.1038/s41438-020-0283-7
 Pan Jiangen et al. (2020) SPATIAL LIGHT DISTRIBUTION CHARACTERIZATION AND MEASUREMENT OF LED HORTICULTURAL LIGHTS
 Zhen, S and Bugbee, B. (2020) Far-red photons have equivalent efficiency to traditional photosynthetic photons: Implications for redefining photosynthetically active radiation
NOTE Reference: Eaves, J, Eaves, S, Morphy, C, Murray, C. The relationship between light intensity, cannabis yields, and profitability. Agronomy Journal. 2020; 112: 1466– 1470.