Lux
The SI unit of illuminance, widely used by cannabis growers as a rough proxy for how much light a plant is receiving.
Lux is a real, defined unit — but it's a unit designed for human eyes, not plants. Growers use it because lux meters are cheap and phone apps can fake it, but for serious cultivation decisions you want PPFD (µmol/m²/s) instead. Lux readings under LEDs, HPS, and sunlight aren't directly comparable because each light source has a different spectrum. Treat lux as a rough field check, not a horticultural standard.
Definition
One lux equals one lumen per square meter. It is the SI derived unit for illuminance — the amount of luminous flux falling on a surface, weighted by the photopic response of the human eye [1][2]. Because the weighting curve peaks around 555 nm (green-yellow) and falls off sharply in the blue and far-red, lux undercounts wavelengths that plants actually use heavily for photosynthesis.
Why growers use it
Lux meters and smartphone lux apps are cheap and ubiquitous, while quantum (PAR) meters that read PPFD cost hundreds of dollars. As a result, lux became a folk standard in home cannabis cultivation for sanity-checking light levels at canopy height. Anecdote
For a fixed light source, lux scales linearly with PPFD, so it can be useful for relative comparisons — e.g., "is the corner of my tent getting half as much light as the center?" That use case is legitimate.
What lux does not tell you
Lux is spectrum-dependent. A high-pressure sodium (HPS) lamp reads very high in lux because it dumps energy near the eye's peak sensitivity, but a magenta-tinted LED with strong blue and red output can deliver more photosynthetically active radiation while reading lower in lux [3][4]. Conversion factors between lux and PPFD differ for every light source: roughly 0.0185 µmol/m²/s per lux for HPS, 0.0150 for sunlight, and variable for white LEDs [4]. There is no universal multiplier.
Lux also says nothing about UV or far-red, both of which influence cannabis morphology and cannabinoid expression Weak / limited.
Used in articles
You'll see lux referenced in beginner grow guides, lamp spec sheets, and older HPS-era cultivation literature. Modern horticultural sources have largely shifted to PPFD and DLI (daily light integral) as more meaningful metrics for plant growth.
Sources
- Government Bureau International des Poids et Mesures (BIPM). The International System of Units (SI), 9th edition, 2019.
- Government National Institute of Standards and Technology (NIST). SI Units – Luminous Intensity and related photometric units.
- Peer-reviewed Sager, J.C., Smith, W.O., Edwards, J.L., Cyr, K.L. (1988). Photosynthetic efficiency and phytochrome photoequilibria determination using spectral data. Transactions of the ASAE, 31(6), 1882–1889.
- Practitioner Apogee Instruments. Conversion – PPFD to Lux. Technical note, Apogee Instruments Inc.
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