# PPFD

> Photosynthetic photon flux density: the number of PAR photons landing on one square metre each second, in µmol/m²/s; the standard measure of light intensity at the canopy.

- Canonical: https://genetixbay.com/knowledge/glossary/ppfd.html
- Also written: photosynthetic photon flux density, PAR reading, µmol/m²/s, umol/m2/s, light density
- 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"

PPFD (photosynthetic photon flux density) is the rate at which [PAR](https://genetixbay.com/knowledge/glossary/par.md) photons strike a surface, in micromoles per square metre per second (µmol/m²/s). It is what a quantum (PAR) meter reads, and it is the number to use when setting light height and dimmer levels. Full summer sun at noon is roughly 2,000 µmol/m²/s.

## In practice

Typical indoor targets without added [CO2](https://genetixbay.com/knowledge/glossary/co2-enrichment.md):

| Stage | PPFD (µmol/m²/s) |
|---|---|
| Clones and [seedlings](https://genetixbay.com/knowledge/glossary/seedling.md) | 100–300 |
| Veg | 400–700 |
| Flower | 700–1,100 |
| Flower with CO2 at 800–1,200 ppm | up to about 1,500 |

Cannabis keeps converting extra light into yield further than most crops. Rodriguez-Morrison and colleagues (2021) saw flower yield rise roughly linearly up to about 1,800 µmol/m²/s, and Llewellyn and colleagues (2022) found yield proportional to intensity. Past about 1,000 µmol/m²/s, though, heat, [VPD](https://genetixbay.com/knowledge/glossary/vpd.md), CO2 and nutrition must keep up or the extra light shows up as [bleaching](https://genetixbay.com/knowledge/glossary/light-stress.md) instead of weight. *Established*.

Measure a grid of points (centre, corners, edges) at canopy height, not one reading under the middle of the fixture.

## Units and formula

[DLI](https://genetixbay.com/knowledge/glossary/dli.md) (mol/m²/day) = PPFD × hours of light × 0.0036.

## Don't confuse with

- **PPF**: total fixture output in µmol/s, not per area.
- **DLI**: the daily total; the same PPFD gives a higher DLI on longer days.
- **Lux**: an eye-weighted brightness unit that misleads under grow lights.

## Related terms

[DLI](https://genetixbay.com/knowledge/glossary/dli.md), [PPF](https://genetixbay.com/knowledge/glossary/ppf.md), [PAR](https://genetixbay.com/knowledge/glossary/par.md), [Light saturation point](https://genetixbay.com/knowledge/glossary/light-saturation-point.md), [Light intensity](https://genetixbay.com/knowledge/glossary/light-intensity.md)

## Read more

- [PPFD and DLI by stage, with the evidence](https://genetixbay.com/knowledge/environment/ppfd-dli-by-stage.md)
- [Heat and light stress: taco leaves, bleaching and foxtails](https://genetixbay.com/knowledge/environment/heat-light-stress.md)

## Sources

1. Rodriguez-Morrison V, Llewellyn D, Zheng Y, 2021. Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment. Frontiers in Plant Science 12:646020. https://doi.org/10.3389/fpls.2021.646020
2. Llewellyn D, Golem S, Foley E, Dinka S, Jones AMP, Zheng Y, 2022. Indoor grown cannabis yield increased proportionally with light intensity, but ultraviolet radiation did not affect yield or cannabinoid content. Frontiers in Plant Science 13:974018. https://doi.org/10.3389/fpls.2022.974018
3. Chandra S, Lata H, Khan IA, ElSohly MA, 2008. Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions. Physiology and Molecular Biology of Plants 14(4):299–306. https://doi.org/10.1007/s12298-008-0027-x