Photosynthentically Active Radiation (PAR)

Definition - What does Photosynthentically Active Radiation (PAR) mean?

Photosynthetically active radiation, abbreviated PAR, is that part of the visible spectrum of light that plants utilize to perform photosynthesis. It occurs at wavelengths between 400 and 700 nanometers (nm). Blue and violet light are at the lower end and red light is at the higher end. Plants utilize different wavelengths of light during different phases of their development. During the vegetative stage of development, cannabis utilizes light with PAR values generally in the 400-500 nm range. During the flowering phase, they need light with PAR values above 620 nm. Growers can measure the amount of PAR their lights are emitting with relatively inexpensive PAR meters.

MaximumYield explains Photosynthentically Active Radiation (PAR)

Novice cannabis growers may easily become confused trying to determine what type of grow light to use as their output may be listed in lumens and not PAR. Lumens are a measure of light that humans can see, while PAR is more accurate to refer to the light that plants can "see". To further complicate the matter "PAR" may also refer to another lighting term, Parabolic Aluminized Reflector which may be used with some LED lights. It should also be noted that photosynthetically active radiation values are affected by the distance the light source is from the plant they are illuminating and this should be taken into consideration when mounting any grow lights.

This definition was written in the context of Cannabis
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