PPFD
Photosynthetic photon flux density: how many usable light photons land on a square meter of canopy each second, in µmol·m⁻²·s⁻¹. It measures intensity at an instant, not the daily total.
PPFD counts only the photons a plant can actually use for photosynthesis, the 400 to 700 nanometer band, and it counts them as particles rather than as brightness. That is why it is the number cultivators design rooms around instead of lux or foot-candles, both of which are weighted for human vision and tell you very little about what a canopy receives.
The notation is worth decoding once, because it reads directly. The µmol part is a count of photons, a micromole being roughly 6 × 10¹⁷ of them. The m⁻² part means per square meter of canopy. The s⁻¹ part means per second. So a canopy sitting at 900 µmol·m⁻²·s⁻¹ is receiving 900 micromoles of usable photons on every square meter, every second.
PPFD is an instantaneous reading, so it varies across a canopy and falls off sharply with distance from the fixture. A single number quoted for a room is usually an average of many measurements taken on a grid at canopy height.
PPFD matters to CO2 enrichment because light and carbon dioxide co-limit photosynthesis. Adding CO2 to a canopy that is short of light changes very little, because light, not carbon, is the constraint.