# PPF

> Photosynthetic photon flux: the total number of PAR photons a light fixture emits each second, in µmol/s, regardless of where they land.

- Canonical: https://genetixbay.com/knowledge/glossary/ppf.html
- Also written: photosynthetic photon flux, fixture PPF, total PPF, µmol/s output
- 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"

PPF (photosynthetic photon flux) is the total output of [PAR](https://genetixbay.com/knowledge/glossary/par.md) 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](https://genetixbay.com/knowledge/glossary/photon-efficacy.md) (µmol/J), the single most useful number for comparing fixtures on running cost.

## In practice

A 600 W [LED fixture](https://genetixbay.com/knowledge/glossary/led.md) 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](https://genetixbay.com/knowledge/glossary/ppfd.md) 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](https://genetixbay.com/knowledge/glossary/canopy.md) (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](https://genetixbay.com/knowledge/glossary/photosynthesis.md).

## Related terms

[PPFD](https://genetixbay.com/knowledge/glossary/ppfd.md), [Photon efficacy](https://genetixbay.com/knowledge/glossary/photon-efficacy.md), [PAR](https://genetixbay.com/knowledge/glossary/par.md), [LED](https://genetixbay.com/knowledge/glossary/led.md)

## Read more

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

## Sources

1. Kusuma P, Pattison PM, Bugbee B, 2020. From physics to fixtures to food: current and potential LED efficacy. Horticulture Research 7:56. https://doi.org/10.1038/s41438-020-0283-7