PPF (photosynthetic photon flux) is the total output of PAR photons from a fixture, in micromoles per second (µmol/s). It is measured in an integrating sphere, so it describes the lamp, not your canopy. Divide PPF by the fixture's wattage and you get photon efficacy (µmol/J), the single most useful number for comparing fixtures on running cost.
In practice
A 600 W LED fixture rated at 1,620 µmol/s has an efficacy of 2.7 µmol/J. If every photon landed evenly on a 1.2 × 1.2 m (4 × 4 ft) canopy of 1.44 m², the average PPFD would be about 1,125 µmol/m²/s. In reality some photons hit the walls or miss the canopy, so expect a lower average; reflective tent walls recover part of the loss.
PPF tells you how much light you bought. PPFD tells you how much light the plants get. You need both: a high-PPF fixture hung badly can still leave the corners dim.
Units and formula
- PPF in µmol/s.
- Average PPFD (ideal) ≈ PPF ÷ canopy area (m²).
- Efficacy (µmol/J) = PPF (µmol/s) ÷ input power (W).
Don't confuse with
- PPFD: photons per square metre per second at the canopy, measured with a quantum sensor.
- Wattage: power drawn from the wall, which says nothing about light output on its own.
- Lumens: the human-eye version of output, irrelevant to photosynthesis.
