# Light intensity

> Light intensity is how much photosynthetic light reaches the plants per unit area and time, measured as PPFD in µmol/m²/s; it is the biggest light lever on yield.

- Canonical: https://genetixbay.com/knowledge/glossary/light-intensity.html
- Also written: intensity, light level, light strength, how much light
- 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"

Light intensity, in horticulture, means the photon flux density at the plant: [PPFD](https://genetixbay.com/knowledge/glossary/ppfd.md) in µmol/m²/s. It is the single biggest lighting factor in cannabis yield. Spectrum and [photoperiod](https://genetixbay.com/knowledge/glossary/photoperiod.md) matter, but within sensible ranges neither moves yield as much as giving the canopy more photons.

## In practice

Llewellyn and colleagues (2022) found indoor cannabis yield rose in proportion to light intensity across the range they tested, and Rodriguez-Morrison and colleagues (2021) saw gains up to about 1,800 µmol/m²/s in flower. *Established*. The practical ceiling is set by everything else: heat removal, [VPD](https://genetixbay.com/knowledge/glossary/vpd.md), [CO2](https://genetixbay.com/knowledge/glossary/co2-enrichment.md), water and nutrient supply, and the cultivar.

Typical progression: [seedlings](https://genetixbay.com/knowledge/glossary/seedling.md) 100–300, veg 400–700, flower 700–1,100 µmol/m²/s without CO2 enrichment. Raise intensity over a week or so rather than in one jump; leaves acclimate. Signs you have outrun the rest of the environment: upward-cupping ([tacoing](https://genetixbay.com/knowledge/glossary/taco-leaf.md)) top leaves, bleached tips, and slower rather than faster growth.

"Intensity" in casual talk often means fixture wattage or "how bright it [looks](https://genetixbay.com/knowledge/glossary/bag-appeal.md)". Neither is a measurement of what the plant receives; distance, reflectors and spectrum change everything.

## Don't confuse with

- **DLI**: intensity accumulated over the day.
- **Wattage**: electrical input, not light output.
- **Brightness (lux)**: eye-weighted, misleading under grow lights.

## Related terms

[PPFD](https://genetixbay.com/knowledge/glossary/ppfd.md), [DLI](https://genetixbay.com/knowledge/glossary/dli.md), [Light saturation point](https://genetixbay.com/knowledge/glossary/light-saturation-point.md), [Inverse square law](https://genetixbay.com/knowledge/glossary/inverse-square-law.md)

## Read more

- [PPFD and DLI by stage, with the evidence](https://genetixbay.com/knowledge/environment/ppfd-dli-by-stage.md)

## Sources

1. 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
2. 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