Lumens are a unit measuring the total amount of visible light emitted by a source as perceived by the human eye. The human eye is most sensitive to green-yellow light (~555 nm), so lumen measurements are weighted towards this range. Because plant photosynthesis responds to a different range of wavelengths (PAR, 400β700 nm) and the sensitivity curve is different from human vision, lumens are a poor and misleading metric for evaluating grow lights.
Key Facts
- Lumens measure human-perceived brightness, not photosynthetic usefulness
- A high-lumen light can deliver low PAR if its spectrum is biased towards green-yellow
- PPFD (Β΅mol/mΒ²/s) and DLI (mol/mΒ²/day) are the correct metrics for plant lighting
- Marketing grow lights in lumens is misleading β always look for PPFD values at canopy distance
- HPS lights appear very bright in lumens but are less efficient per watt than modern LEDs
- LUX = lumens per square metre β equally misleading for plant growth evaluation