 The irradiance of PAR can be expressed in units of energy flux (W/m2), which is relevant in energy-balance considerations for photosynthetic organisms.However, photosynthesis is a quantum process and the chemical reactions of photosynthesis are more dependent on the "number" of photons than the amount of energy contained in the photons.Therefore, plant biologists often quantify PAR using the number of photons in the 400-700 nm range received by a surface for a specified amount of time, or the Photosynthetic Photon Flux Density (PPFD).