# PAR

> Photosynthetically active radiation: light in the 400–700 nm waveband that plants use for photosynthesis, counted in photons rather than brightness.

- Canonical: https://genetixbay.com/knowledge/glossary/par.html
- Also written: photosynthetically active radiation, PAR range, ePAR, extended PAR
- Glossary category: Environment and light · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

PAR (photosynthetically active radiation) is the band of light from 400 to 700 nm that drives [photosynthesis](https://genetixbay.com/knowledge/glossary/photosynthesis.md). It is a waveband, not a measurement: the numbers growers quote ([PPF](https://genetixbay.com/knowledge/glossary/ppf.md), [PPFD](https://genetixbay.com/knowledge/glossary/ppfd.md), [DLI](https://genetixbay.com/knowledge/glossary/dli.md)) 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](https://genetixbay.com/knowledge/glossary/far-red.md) 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.

## Related terms

[PPFD](https://genetixbay.com/knowledge/glossary/ppfd.md), [PPF](https://genetixbay.com/knowledge/glossary/ppf.md), [YPFD](https://genetixbay.com/knowledge/glossary/ypfd.md), [Far-red](https://genetixbay.com/knowledge/glossary/far-red.md), [Spectrum](https://genetixbay.com/knowledge/glossary/spectrum.md)

## Read more

- [Light spectrum: blue, red, far-red and UV, what's real](https://genetixbay.com/knowledge/environment/light-spectrum.md)
- [PPFD and DLI by stage, with the evidence](https://genetixbay.com/knowledge/environment/ppfd-dli-by-stage.md)

## Sources

1. McCree KJ, 1972. The action spectrum, absorptance and quantum yield of photosynthesis in crop plants. Agricultural Meteorology 9:191–216.
2. Zhen S, Bugbee B, 2020. Far-red photons have equivalent efficiency to traditional photosynthetic photons: implications for redefining photosynthetically active radiation. Plant, Cell & Environment 43(5):1259–1272. https://doi.org/10.1111/pce.13730