Photosynthetic Photon Flux (PPF) is the metric used to measure PAR. It is a total count of light energy (in photons) from 400nm to 700nm. 

PPF is measured in micromols (μmol). Micromols are 10-6 of a mol (A mol is a unit of measurement). The measurement is classified in time, revealing how many seconds it takes the light to reach the measurement tool. PPF is expressed as μmol/sec.

The calculation for PPF is:

The PPF measurement has become a popular metric for measuring the power of a grow light source; however, PPF has two fundamental flaws. First, the wavelengths from 300nm to 400nm and 700nm to 800nm are excluded in the measurement. Second, all photons are weighted equally regardless of wavelength. The McCree Curve clearly shows plants respond to energy outside the 400nm to 700nm PAR range. Furthermore, plants respond differently to energy within the PAR range as not all photons carry the same weight or provide the same benefits. In order to truly compare light sources, you must look at all pieces of data, not just the PPF.