The number of photons in the PAR range hitting a surface per second, measured in μmol/m²/s. The standard metric for light intensity in controlled environments.
Photosynthetic Photon Flux Density (PPFD) measures the number of photosynthetically active photons (400-700 nm wavelength) landing on a specific surface area per second. It is expressed in micromoles of photons per square metre per second (μmol/m²/s).
PPFD is the most meaningful metric for evaluating grow light performance because it directly represents the photon delivery rate available for photosynthesis at the crop canopy level. It differs from lux or lumens, which measure light as perceived by the human eye and do not accurately represent photosynthetically useful radiation.
Typical PPFD targets by growth stage:
PPFD varies across the canopy and should ideally be measured at multiple points. Reputable fixture manufacturers provide PPFD maps showing the distribution of light intensity across a standard footprint.
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