Ian Ashdown, P. Eng., FIES
Senior Scientist, SunTracker Technologies.
It was all so easy until recently. Plants require light in order to grow, and so we provided them with daylight and/or electric lighting. Given the singular choice of high-pressure sodium (HPS) lamps, we only needed to be concerned about measuring Photosynthetically Active Radiation (PAR) and Daily Light Integrals (DLI).
The introduction of light-emitting diodes (LEDs) and solid-state lighting (SSL) has changed everything. With the ability to independently control the light source spectrum from ultraviolet through visible light to far-red, researchers and growers are discovering that plant species and even cultivars respond differently to different spectral power distributions. From these discoveries are coming “light recipes” for optimal plant growth and health.
Light recipes require more than a pinch of salt and a dash of cayenne, however. We need to measure and quantify the light received by plants, much as professional lighting designers have long measured and quantified light for building interiors and outdoor areas. These designers have numerous of metrics to call upon, all of them based on the human perception of visible light. Unfortunately, plants do not respond to light as we do, and so units of measurements such as lumens, lux, and candela are all but meaningless for horticultural lighting.
Given this, the American Society of Agricultural and Biological Engineers has just announced the publication of ANSI/ASABE S640 JUL 2017, Quantities and Units of Electromagnetic Radiation for Plants (Photosynthetic Organisms). Developed over two years by an international team of experts from industry and academia, this standard brings some much-needed order to the metrics of horticultural lighting.
The document formally defines 33 electromagnetic radiation metrics for horticultural lighting. They are fully compatible with metrics previously defined by standards from the American Society of Agricultural Engineers (precursor of the ASABE), the Illuminating Engineering Society (IES), the Commission Internationale d’Eclairage (CIE), and the International Organization for Standards (ISO). They are, however, specific to the needs of horticulture and plant biology.
What we perceive as visible light spans the electromagnetic spectrum with wavelengths from 400 nm (deep blue) to 700 nm (deep red). Coincidentally, this is the same range over which plant photosynthesis occurs. Outside of this range, plants respond to ultraviolet and far-red radiation. The Pfr isoform of phytochrome, for example, has a peak spectral absorptance of 735 nm, and is responsible for initiating many photomorphogenetic functions. Similarly, the photopigment UVR8 is responsible for sensing excess UV-B radiation (280 nm – 315 nm) and initiating plant stress responses to prevent DNA damage. With this, the metrics are therefore divided into three spectral ranges: ultraviolet (280 nm – 400 m), photosynthetic (400 nm – 700 nm), and far-red (700 nm – 800 nm).
The other division of the metrics is based on radiant versus photon flux. Every photon has a specific wavelength (e.g., 555 nm), and its energy (stated in watts per second, or joules) is inversely proportional to its wavelength. Plant photosynthesis does not care about photon energy, however – the chlorophyll molecule absorbs the photon for its chemical action and releases any excess energy as heat. Thus, horticulturalists and plant biologists are interested in the flow (or “photon flux”) of photons per second, with no regard for wavelength. This flux is measured in micromoles (6.23 ×1017) of photons per second with a broadband “quantum sensor,” typically a silicon photodiode with an optical filter.
Forest ecologists, on the other hand, are often interested in the energy of sunlight incident on the forest canopy, and so they measure electromagnetic radiation in terms of “radiant flux,” stated in watts per second. Here, wavelength matters, with blue light photons having more energy than red light photons. A broadband sensor, again typically a silicon photodiode with an optical filter, is used to measure radiant flux over the spectral range of interest.
It is also important to be able to measure and quantify the spectral power distribution of light sources with a spectroradiometer. In one recent study, for example, a difference of 10 nm in the peak wavelength of green LEDs (520 nm versus 530 nm) had a pronounced effect on the growth and development of red leaf lettuce (Johkan et al. 2012). We therefore have both spectral radiant flux and spectral photon flux, measured in watts per nanometer and micromoles per second per nanometer respectively.
With these divisions, we have the following horticultural lighting metrics defined by ANSI/ASABE S640:
For now, horticulturalists will continue to measure PAR as photosynthetic photon flux density (PPFD) with a quantum sensor, and measure or calculate daily light integrals (integrated daily PPFD). However, ANSI/ASABE S640 is important in that it provides a framework with which to quantify forthcoming light recipes for optimal growth and health of urban agriculture crops.
Looking beyond light recipes, horticultural luminaire manufacturers will be able to quantify the optical performance characteristics of their products, and lighting design software developers will be able to develop products specifically for horticultural lighting design in greenhouses and vertical farms. It all begins, however, with horticultural lighting metrics.
ANSI/ASABE S640 is available for purchase from the ASABE Technical Library (https://elibrary.asabe.org).
Jokhan, M, et al. 2012. “Effect of Green Light Wavelength and Intensity on Photomorphogenesis and Photosynthesis in Lactuca sativa,” Environmental and Experimental Botany 75:128-133.