# Lux

> Lux is illuminance weighted to human eye sensitivity (lumens per square metre); it misreads grow lights because plants use red and blue light the eye underweights.

- Canonical: https://genetixbay.com/knowledge/glossary/lux.html
- Also written: lx, lux meter, lux reading, illuminance
- 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"

Lux (lx) is a photometric unit: lumens per square metre. Photometric units weight light by the human eye's sensitivity, which peaks in green-yellow at 555 nm and falls steeply towards deep red and blue. Plants use exactly those red and blue wavelengths heavily, so lux is the wrong instrument for [photosynthesis](https://genetixbay.com/knowledge/glossary/photosynthesis.md).

## In practice

A cheap lux meter can be useful for one thing: comparing the same lamp type at different heights or positions. It cannot compare an [HPS](https://genetixbay.com/knowledge/glossary/hps.md) with a red-heavy [LED](https://genetixbay.com/knowledge/glossary/led.md), or a white LED with a blurple one, because their lux-to-PPFD ratios differ.

Rough [conversion factors](https://genetixbay.com/knowledge/glossary/conversion-factor.md) (lux per 1 [µmol/m²/s](https://genetixbay.com/knowledge/glossary/ppfd.md)): sunlight about 54, cool white LED about 65–75, HPS about 80–85. A deep red and blue fixture can read low in lux while delivering plenty of PPFD. Phone apps that use the camera to estimate PPFD apply one of these factors and can be badly wrong under a different spectrum.

## Units and formula

- 1 lux = 1 lumen/m²
- 1 [foot-candle](https://genetixbay.com/knowledge/glossary/foot-candle.md) = 10.764 lux
- PPFD ≈ lux ÷ factor for that light source (approximate only)

## Don't confuse with

- **PPFD**: counts photons from 400 to 700 nm equally, regardless of how bright they look.
- **Lumens**: total eye-weighted output of a lamp; lux is lumens per square metre.

## Related terms

[PPFD](https://genetixbay.com/knowledge/glossary/ppfd.md), [Foot-candle](https://genetixbay.com/knowledge/glossary/foot-candle.md), [PAR](https://genetixbay.com/knowledge/glossary/par.md), [Spectrum](https://genetixbay.com/knowledge/glossary/spectrum.md)

## Read more

- [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.