# Photosynthetically Active Radiation (PAR) Units



## nobogart (Mar 11, 2013)

The Photosynthetically Active Radiation(PAR) measure of radiant power is important in evaluating the effect of light on plant growth.  In 1972 it was shown by K.McCree (Agric.Meteorol., 10:443, 1972) that the photosynthetic response correlates better with the number of photons than with energy.  This is expected because photosynthesis isa photochemical conversion where each molecule is activated by the absorption of one photon in the primary photochemical process.

PAR is defined in terms of photon (quantum) flux, specifically, the number of moles of photons in the radiant energy between 400 nm and 700 nm.  One mole of photons is 6.0222 x 1023rd power photons (Avagadros Number).
The Photosynthetic Photon Flux Density(PPFD), i.e., the photon irradiance, is expressed in moles per square meter and per second (formerly,Einsteins per square meter and per second)


----------

