PPFD (Photosynthetic Photon Flux Density)
Definition: PPFD, or photosynthetic photon flux density, measures how many photons of usable light (400 to 700 nanometers) land on a square meter of canopy each second, expressed in micromoles per square meter per second (µmol/m²/s). It is the most practical number for judging whether a grow light is too weak, about right, or too strong.
How PPFD (Photosynthetic Photon Flux Density) works
Wattage tells you how much power a light uses, and lumens describe brightness as human eyes see it. Neither tells you how much light your plants can actually use. PPFD does, because it counts the photons in the PAR range that arrive at a specific spot on the canopy. One square meter is about 10.8 square feet.
Typical targets
- Seedlings and clones: 100 to 300 µmol/m²/s
- Vegetative growth: 300 to 600 µmol/m²/s
- Flowering, ambient CO2: 600 to 900 µmol/m²/s
- Flowering with added CO2: up to about 1,200 to 1,500 µmol/m²/s
How to measure it
A quantum sensor, often sold as a PAR meter, gives the most accurate reading. Smartphone apps with a diffuser can give a rough estimate. Take readings at canopy height in the center, the corners, and halfway between, because most fixtures deliver much less light at the edges than directly below.
Reading manufacturer charts
Many brands publish PPFD maps at a fixed hanging height. Look at the average across the footprint and the lowest corner value, not just the peak in the middle. A light that hits 1,200 in the center and 300 at the edges will give you uneven buds.
Pushing PPFD past what the plant can use, especially without extra CO2, invites light burn and wastes electricity.