When measuring the irradiance of PAR, values are expressed using units of energy (W/m), 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 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). Values of PPFD are normally expressed using units of mol⋅m⋅s. In relation to plant growth and morphology, it is better to characterise the light availability for plants by means of the Daily Light Integral (DLI), which is the daily flux of photons per ground area, and includes both diurnal variation as well as variation in day length.
PPFD used to sometimes be expressed using einstein units, i.e., μE⋅m⋅s, although this usage is nonstandard and is no longer used.