PAR (photosynthetically active radiation) is the band of light from 400 to 700 nm that drives photosynthesis. It is a waveband, not a measurement: the numbers growers quote (PPF, PPFD, DLI) are all counts of PAR photons. Plant scientists count photons because photosynthesis is a photon-driven reaction; energy or perceived brightness is the wrong currency.
In practice
"PAR" is often used loosely to mean "the reading on my quantum meter". Strictly, a quantum sensor measures PPFD within the PAR band. When a fixture sheet says "PAR output", look for the unit: µmol/s is PPF (fixture total), µmol/m²/s is PPFD (at a point on the canopy).
Research since 2020 shows far-red photons from 700 to 750 nm drive canopy photosynthesis about as well as PAR photons when they are mixed with shorter wavelengths. That is why some specifications now report ePAR (extended PAR, 400–750 nm). A fixture rated in ePAR will look better on paper than the same fixture rated in PAR, so compare like with like. Established for the far-red finding in lettuce and other crops; cannabis-specific canopy data are thinner.
Units and formula
| Quantity | Band | Unit |
|---|---|---|
| PAR | 400–700 nm | (waveband, no unit) |
| ePAR | 400–750 nm | (waveband, no unit) |
| PPF | PAR | µmol/s |
| PPFD | PAR | µmol/m²/s |
Don't confuse with
- Lux: weighted to the human eye, not to plants.
- PPFD: a measured flux of PAR photons at a surface; PAR itself is only the band.
