The standard measure that quantifies the energy available for photosynthesis is "Photosynthetic Active Radiation" (aka "Photosynthetic Available Radiation") or PAR. Contrary to the lumen measure that takes into account the human eye response, PAR is an unweighted measure. It accounts with equal weight for all the output a light source emits in the wavelength range between 400 and 700 nm. PAR also differs from the lumen in the fact that it is not a direct measure of energy. It is expressed in "number of photons per second", whose relationship with "energy per second" (power) is intermediated by the spectral curve of the light source. One cannot be directly converted into the other without the spectral curve.

The reason for expressing PAR in number of photons instead of energy units is that the photosynthesis reaction takes place when a photon is absorbed by the plant, no matter what the photon's wavelength (or energy) is (provided it lies in the range between 400 and 700 nm). That is, if a given number of blue photons is absorbed by a plant, the amount of photosynthesis that takes place is exactly the same as when the same number of red photons is absorbed. For convenience, number of photons is usually reported in the literature in micromole units, or microEinsteins. One microEinstein is equivalent to 6.02 1017 photons. Another important difference is that usually PAR is quoted as an illumination measure akin to lux, thus related to the receiving surface. PAR is typically reported in microEinstein/second/m2.