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.
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 with a red-heavy LED, or a white LED with a blurple one, because their lux-to-PPFD ratios differ.
Rough conversion factors (lux per 1 µmol/m²/s): sunlight about 54, cool white LED about 65–75, HPS about 80–85being reviewed. 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.
- 1 lux = 1 lumen/m²
- 1 foot-candle = 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.
The process in chloroplasts that uses light energy to turn carbon dioxide and water into sugars and oxygen; cannabis uses the C3 pathway.HPS (high-pressure sodium) is a gas-discharge lamp with orange-yellow light peaking near 589 nm; it was the standard flowering light before LEDs.An LED (light-emitting diode) grow light uses solid-state diodes to produce light; modern fixtures are the most efficient grow lights, at roughly 2.5–3 µmol/J.The conversion factor is the multiplier a TDS meter applies to EC to report ppm, commonly 0.5, 0.64 or 0.7; it is why ppm readings differ between meters.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.A foot-candle is an imperial unit of illuminance (one lumen per square foot, about 10.76 lux); like lux, it measures eye brightness, not plant-usable light.Photosynthetically active radiation: light in the 400–700 nm waveband that plants use for photosynthesis, counted in photons rather than brightness.Spectrum is the mix of wavelengths a light source emits, from UV through blue, green, red and far-red; it shapes plant form, while intensity drives most yield.
