Photosynthetically Active Radiation (PAR) is the range of light wavelengths from 400 to 700 nanometres (nm) that plants can use for photosynthesis. PAR defines the portion of the light spectrum that is biologically active for driving photosynthesis, but it does not indicate how much usable light a plant is receiving.
PAR is measured as Photosynthetic Photon Flux Density (PPFD) in micromoles per square metre per second (µmol/m²/s), while the total amount of PAR received over a day is expressed as the Daily Light Integral (DLI) in moles per square metre per day (mol/m²/day). Because PAR refers only to the wavelength range, two light sources can both emit PAR while providing very different amounts of usable light depending on their photon output.
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