PAR (Photosynthetically Active Radiation)
Definition: PAR, or photosynthetically active radiation, is the band of light between 400 and 700 nanometers that plants use to drive photosynthesis. It describes which wavelengths count, not how much light arrives. Growers use PAR as the foundation for intensity metrics like PPFD and DLI when comparing grow lights.
How PAR (Photosynthetically Active Radiation) works
PAR is often used loosely, as in “this light has great PAR,” but strictly it names a range of wavelengths. The useful numbers come from measuring photons inside that range.
PAR and its related terms
| Term | What it describes | Unit |
|---|---|---|
| PAR | The 400 to 700 nm wavelength band | nanometers |
| PPF | Total PAR photons a fixture emits per second | µmol/s |
| PPFD | PAR photons landing on an area per second | µmol/m²/s |
| DLI | PAR photons landing on an area per day | mol/m²/day |
| Efficacy | PPF produced per watt of electricity | µmol/J |
Colors inside PAR
Blue light (400 to 500 nm) tends to keep plants compact. Red light (600 to 700 nm) drives much of photosynthesis and flowering. Green light (500 to 600 nm), long assumed useless, penetrates deeper into the canopy. Full-spectrum white LEDs cover all three.
Just outside the range
Far-red light (700 to 750 nm) affects stretching and flowering responses, and newer research counts it in an extended range called ePAR. UV-A sits just below 400 nm. Neither is part of classic PAR, so some fixtures list them separately.
Why lumens mislead
Lumens are weighted toward the yellow-green light human eyes see best, so a bright-looking bulb can be a weak plant light. When shopping, compare PPF, efficacy (quality LEDs commonly reach 2.5 µmol/J or more), and the PPFD map for your grow tent size.