Lighting For Plant Health

I have a current project with a green wall, aka living wall, and other greenery in the space. I’ve been given conflicting information about the lighting requirements I need to meet are and how to measure them, so I did some research. This isn’t definitive, but here’s what I’ve found.

First of all, the measurement units that we’re all familiar with don’t apply to horticulture because the average plant’s response to light is very different from that of the human visual system. We know that the human eye response curve is V(λ) (pronounced vee lambda) which is shown in Figure 1. Our response to electromagnetic energy falls between 380 and 770 nm, with a peak response at 555 nm. In order to measure light the way the human visual system perceived it, V(λ) is folded into the definition of the lumen, the footcandle, etc.

Figure 1 V(λ)

Plants, however, have a response curve called the photosynthesis action spectrum, shown in Figure 2. The wavelengths of light that are absorbed and used by plants are below 520 nm and above 610 nm [i], which roughly equates to the blue and red range of the visible spectrum. Plants need a great deal of red light, a far amount of blue light, and little or no green light.

photosynthetically active spectrum

Figure 2 Average photosynthesis action spectrum of chlorophyll [ii]

So, we can’t talk about the amount of light delivered to plants in a useful way if we’re using lumens and footcandles. The measurement of light for plant health is Photosynthetically Active Radiation (PAR) [iii]. There are PAR calibrated light meters, and digital tools to convert lux/footcandle readings to PAR. Other common measurements are also not relevant to horticulture.

  • Color temperature is a numerical indication of the warmth or coolness of white light, but warmth or coolness are aesthetic criteria and are not relevant in light for plant health.
  • CRI is an indicator of how well a light source allows us to see colors when compared to a reference light source. The response of the human visual system to light is built-in to the CRI calculation. Again, for plant health we are not concerned with seeing the colors of the plants so this metric is not relevant.

 

What kind of light should we provide? Incandescent light has an appropriate balance of red and blue light for plant health, as shown in Figure 3. The power consumption will be high. Fortunately, power consumed by the lighting for plant health is exempt from the energy conservation codes. However, with their short life and high power consumption incandescents are, overall, a poor choice.

Incandescent SPD

Figure 3 SPD for incandescent light of 2800 K, 3000 K, and 3200 K [iv]

High color temperature metal halide lamps have been the horticulture light source of choice for a long time because their SPD provides an appropriate balance of red and blue light (Figure 4). While metal halide lamps are being replaced by LEDs in many applications, I expect they will be available for at least the next decade. For my project, these fixtures would only to be used during the green wall’s growth period in the morning before the space opens to the public. A second set of fixtures with warmer light will be used when the space is open so that I could light the wall in a way that is in balance with the rest of the space during operating hours.

Metal Halide SPD

Figure 4 SPD for a 4200 K metal halide [v]

One of the exciting features of LEDs is that they permit fine-tuning of the emitted spectrum. With LEDs it is possible to create a light source that closely follows the photosynthesis action spectra. This has been shown to “improve factors such as yield, flavor, color, plant growth, and flowering as well as pest and pathogen management and control.”[[vi] The impact has been studied, and results so far have been positive, for leaf lettuce [vii], cucumbers [viii], and tomatoes [ix], among others. At least one study has noted, however, has “concluded that the response of plants to the applied light is individual and depends on the species,” [x]

Therefore, an alternative to metal halide fixtures is multi-colored LED fixtures. Since multi-colored LED fixtures allow users to control the brightness of each color individually one could opt for a fixture with a Red, Blue, White (RBW), a Red, Red, Blue, White (RRBW), or a Red, Blue, Blue, White (RBBW) set of LEDs. This would permit one fixture to provide light for health and accent light. One possible result of a RBW fixture is shown in Figure 5. This is a much better match to the photosynthesis action spectra than incandescent, metal halide, or white LEDs.

Figure 5 Possible RBW LED produced SPD

For the time being, the people responsible for the greenery have asked me to stay with the tried and true metal halide lamps.  In the near future, as metal halide lamps become rarer, and as LEDs become more common in horticulture, I expect we’ll be changing over to LEDs.

 

References

[i] Yingchao Xu, Yongxiao Chang, Guanyu Chen, Hongyi Lin, The Research On LED Supplementary Lighting System For Plants, Optik – International Journal for Light and Electron Optics, Volume 127, Issue 18, September 2016, Pages 7193-7201, ISSN 0030-4026, http://dx.doi.org/10.1016/j.ijleo.2016.05.056.

[ii] The Science of Food Production, http://www.bbc.co.uk/education/guides/z23ggk7/revision/2.

[iii] Torres, Ariana P., Lopez, Roberto G., Measuring Daily Light Integral in a Greenhouse, Department of Horticulture and Landscape Architecture, Purdue University, https://www.extension.purdue.edu/extmedia/ho/ho-238-w.pdf

[iv] Livingston, Jason, Designing Light: The Art, Science, and Practice of Architectural Lighting, Hoboken: John Wiley and Sons, 2014.

[v] TM-30-15 Advanced Calculator, Illuminating Engineering Society, New York: Illuminating Engineering Society, 2015.

[vi] Davis, Philip A. and Burns, Claire, Photobiology In Protected Horticulture, Food and Energy Security 2016: 5(4): 223-238. http://onlinelibrary.wiley.com/doi/10.1002/fes3.97/full

[vii] Filippos Bantis, Theoharis Ouzounis, Kalliopi Radoglou, Artificial LED Lighting Enhances Growth Characteristics And Total Phenolic Content Of Ocimum Basilicum, But Variably Affects Transplant success, Scientia Horticulturae, Volume 198, 26 January 2016, Pages 277-283, ISSN 0304-4238, http://dx.doi.org/10.1016/j.scienta.2015.11.014.

[viii] Brazaityte, A., et.al.,  The Effect Of Light-Emitting Diodes Lighting On Cucumber Transplants And After-Effect On Yield, Zemdirbyste, Volume 96, Issue 3, 2009, Pages 102-118. https://www.scopus.com/record/display.uri?eid=2-s2.0-73949144018&origin=inward&txGid=7294EF1D0E6304BAA77C73981961A69E.wsnAw8kcdt7IPYLO0V48gA%3a2 (Login Required)

[ix] Brazaityte, A., et. al., The Effect Of Light-Emitting Diodes Lighting On The Growth Of Tomato Transplants, Zemdirbyste, Volume 97, Issue 2, 2010, Pages 89-98, https://www.scopus.com/record/display.uri?eid=2-s2.0-78249276864&origin=inward&txGid=7294EF1D0E6304BAA77C73981961A69E.wsnAw8kcdt7IPYLO0V48gA%3a7 (Login Required)

[x] Fra̧szczak, B., et. al., Growth Rate Of Sweet Basil And Lemon Balm Plants Grown Under Fluorescent Lamps And Led Modules, Acta Scientiarum Polonorum, Hortorum Cultus, Volume 13, Issue 2, 2014, Pages 3-13, https://www.scopus.com/record/display.uri?eid=2-s2.0-84898647440&origin=inward&txGid=7294EF1D0E6304BAA77C73981961A69E.wsnAw8kcdt7IPYLO0V48gA%3a12 (Login Required)

A New Lumen Definition For Efficacy Regulations?

Mark Rea and Jean Paul Freyssinier of the Lighting Research Center explain why lighting regulatory policies that combine efficacy and color rendering elements are fundamentally misguided, and how the industry could simply ensure that customers get both energy-efficient and quality light.

Source: CRI should never be used in efficacy regulations but a new lumen definition should (MAGAZINE)

How Bright Are Colored LEDs?

Measuring and describing the brightness of colored LEDs is an increasingly important part of a lighting designer’s practice. They are used more often, and in more types of projects, than ever before. Yet, we don’t have an accurate method for understanding exactly how much light is being produced and how bright it will appear. It’s a problem that the lighting industry needs to solve, and soon.

The human eye does not respond to all wavelengths of light equally. We have the greatest response to the yellow-green light of 555 nm. Our response falls off considerably in both directions.  That is, wavelengths of light do not contribute equally to our perception of brightness. The sensitivity curve of the human eye is called V(λ) (pronounced vee lambda) and is shown below.

The definition of a lumen, the measurement of brightness of a light source, is weighted using V(λ) and essentially assumes that the light source emits light across the visible spectrum – in other words, it produces a version of white light.

Light meters are calibrated to measure white light using V(λ) so that their measurement of brightness corresponds with our perception. Individual colored LEDs emit only a fraction of the visible spectrum, as shown below in the graph of V(λ) and the SPD of a red LED, and that’s the problem.

V(λ) and the SPD of a red LED.

Light meters measure the light that the colored LEDs provide, of course, and this information is included on an LED fixture manufacturer’s cut sheets, but it often makes no sense. For example, an RGBW fixture I’ve arbitrarily selected reports the following output in lumens: Red 388, Green 1,039, Blue 85, White 1,498. Since brightness is additive, the output when all LEDs are at full should be 3,010 lumens. However the Full RGBW output is given as 2,805 lumens! That’s 7% lower than what we expect.

The essential problem is that the colored LEDs give the light meter only a fraction of the spectrum it’s designed to measure. The meter provides a result based on its programming and calibration, but the results are often nonsensical or at odds with our perception. This problem doesn’t affect only architectural lighting designers. Film and TV directors of photography and lighting directors also rely on a light meter’s accurate measurement of brightness in their work, and when using colored LED fixtures the light meter is likely to be wrong. In fact, even white light LEDs can be difficult to measure accurately because of the blue spike in their SPD.

For now, the only way to accurately assess the brightness of colored LEDs is to see them in use. Lighting professionals need to let manufacturers and others know that the current situation is not acceptable, and that an accurate method of measuring and reporting the brightness of colored LEDs is a high priority. Talk to fixture and lamp sales reps, fixture and lamp manufacturers, and decision makers at IES, CIE, NIST and other research and standards setting organizations. There’s a solution out there. We need to urge those with the skills and resources to find it to get going!

DOE Predicts LED Use and Energy Savings

In September the DOE issued, Energy Savings Forecast of Solid-State Lighting in General Illumination Applications (PDF, 116 pages), the latest edition of a biannual report which models the adoption of LEDs in the U.S. general-lighting market, along with associated energy savings, based on the full potential DOE has determined to be technically feasible over time. The new report projects that energy savings from LED lighting will top 5 quadrillion Btus (quads) annually by 2035. Among the key findings:

  • By 2035, LED lamps and luminaires are anticipated to occupy the majority of lighting installations for each of the niches examined, comprising 86% of installed stock across all categories (compared to only 6% in 2015).
  • Annual savings from LED lighting will be 5.1 quads in 2035, nearly equivalent to the total annual energy consumed by 45 million U.S. homes today, and representing a 75% reduction in energy consumption versus a no-LED scenario.
  • Most of the 5.1 quads of projected energy savings by 2035 will be attributable to two commercial lighting applications (linear and low/high-bay), one residential application (A-type), and one that crosses ­both residential and commercial (direc­tional). Connected lighting and other control technologies will be essential in achieving these savings, accounting for almost 2.3 quads of the total.
  • From 2015 to 2035, a total cumulative energy savings of 62 quads – equivalent to nearly $630 billion in avoided energy costs – is possible if the DOE SSL Program goals for LED efficacy and connected lighting are achieved.

Don’t have time for the full report?  Download the report summary.