Although many similar design principles apply to both general lighting and horticultural lighting applications, the units used in general lighting (lumens, lux, footcandles and lumens per watt) are heavily biased towards human vision response to light radiation.
Conversely, agricultural lighting systems are better evaluated by using metrics that rate the usefulness of the lighting system’s ability to stimulate photosynthesis. These metrics are therefore associated with absolute photon counts.
Photosynthetically Active Radiation is defined by the CIE as the total photon exposure in the 400 to 700 nm waveband range of radiation that is absorbed by photosynthetic pigments.
Coincidentally, this is almost the same range of wavelengths that is associated with human vision (380-780 nm). However, human vision is most sensitive to the green range of wavelengths (peak sensitivity is at 555 nm).
Photosynthetic Photon Flux, measured in micromoles per second. (A mole is a number equal to the number of atoms in 0.012 kg of carbon-12, and a micromole is one millionth of a mole.) PPF represents the total number of photons in the photosynthetically active range of wavelengths emitted from the light source each second. This metric is used instead of luminous flux (which is measured in total lumens) in lighting applications.
Photosynthetic Photon Flux Density, measured in micromoles per square meter per second. This metric represents the total number of photons in the photosynthetically active range of wavelengths falling on a square meter of a given surface (such as a plant canopy) every second. It is used instead of illuminance (which is measured in lux or footcandles) in agricultural lighting applications.
A measure of fixture efficacy, expressed in micromoles per joule, this metric represents the total number of photons in the photosynthetically active range of wavelengths generated by one joule of electrical energy. This is an electrical efficiency metric that is referenced, instead of lumens per watt, for agricultural lights.
5. Delivered PPF/Watt
A measure of lighting system efficacy, expressed in micromoles that reach the plant canopy per joule, this metric represents the total number of photons in the photosynthetically active range ofwavelengths that reach the plant canopy generated by one joule of electrical energy. Overall, this is the best electrical efficiency metric for evaluating various agricultural lighting systems. Regardless of the LED fixture’s electrical efficiency, the photons must reach the plant canopy in order to be absorbed and induce a photosynthetic, and/or photomorphogenic response.
Tissue Culture Laboratories