PPFD
Photosynthetic photon flux density: the number of photons from 400 to 700 nanometers reaching one square meter each second, expressed in µmol·m⁻²·s⁻¹.
PPFD measures the photon flux within photosynthetically active radiation, the 400 to 700 nanometer waveband. It counts photons rather than weighting light for human vision, which is why lux and foot-candles are not interchangeable with PPFD.
The unit µmol·m⁻²·s⁻¹ means micromoles of photons per square meter per second. PPFD is an instantaneous measurement and varies with fixture output, distance, angle, obstructions, and position across the canopy.
A room-level PPFD is usually summarized from a grid of canopy-height measurements. Daily light integral adds those measurements over the photoperiod Photoperiod The duration and timing of light and darkness in a repeating daily cycle. Photoperiod-sensitive cannabis uses night length as a flowering signal. to describe the total photons delivered per day.