PAR (Photosynthetically Active Radiation) is the range of light wavelengths β from 400 nm (violet) to 700 nm (red) β that plants can use for photosynthesis. PAR is not a unit of measurement but a defined spectral range. The intensity of PAR light at a surface is measured in PPFD (Β΅mol/mΒ²/s). Understanding PAR is fundamental to evaluating grow lights, because light outside the PAR range (UV, far-red, infrared) is not used for photosynthesis in the same way.
Key Facts
- PAR range: 400β700 nm β visible light that drives photosynthesis
- Blue light (400β500 nm): promotes compact vegetative growth and chlorophyll synthesis
- Red light (600β700 nm): drives photosynthesis most efficiently; promotes flowering
- Green light (500β600 nm): partially absorbed by plants; penetrates deeper into canopy
- Far-red (700β800 nm): not classic PAR, but triggers shade avoidance and the Emerson effect
- Lux and lumens measure light as perceived by the human eye β irrelevant for plant growth