Lux
SI unit of illuminance; cheap to measure but spectrum-blind for plant growth.
Lux meters are everywhere and cheap, so home growers use them to check if light is reaching the canopy. That's fine for rough comparisons. But lux is weighted to human eye sensitivity (green-yellow), not plant photosynthesis. A magenta LED can grow better plants than an old HPS lamp while reading *lower* in lux. For serious grow decisions, PPFD (photosynthetic photon flux density) is more honest.
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 use heavily for photosynthesis.
Why growers use it
Lux meters and smartphone lux apps are inexpensive and widely available, while quantum (PAR) meters that read PPFD cost hundreds of dollars [3]. 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. Many beginner grow guides and older HPS-era cultivation literature reference lux for this reason.
What lux does not tell you
Lux is spectrum-dependent. A high-pressure sodium (HPS) lamp reads very high in lux because it emits 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 Strong evidence[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 does not account for ultraviolet (UV) or far-red radiation. Both wavelengths lie outside the photopic response curve and influence cannabis morphology, gene expression, and secondary metabolism in ways lux cannot measure Weak / limited[5]. For decisions about spectrum tuning or predicting yields, lux is too narrow a metric.
Modern alternatives
Contemporary horticultural sources have largely shifted to PPFD (photosynthetic photon flux density, measured in µmol/m²/s) and DLI (daily light integral, in mol/m²/day) as more meaningful metrics for plant growth. PPFD measures all photons in the 400–700 nm range — the range plants use for photosynthesis — regardless of spectrum. Smartphone PPFD meters have become cheaper in recent years, and many growers now use them alongside or instead of lux.
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 Base Units: Candela (photometric units).
- Practitioner Apogee Instruments. Conversion – PPFD to Lux. Technical note, Apogee Instruments Inc.
- 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.
- Peer-reviewed Mpoloka, S.W. (2008). Effects of ultraviolet radiation on morphology and cannabinoid content of Cannabis sativa L. Photochemistry and Photobiology, 84(4), 907–912.
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Related
- Kelvin (color temperature) — SI unit measuring light color appearance; used to compare lamp spectra in horticulture.
- Lumen — A unit of visible light — useful for your eyes, wrong for measuring plant growth.