For a parallel beam, the photon flow , leaving or passing through an infinitesimal transparent element of surface in a given direction from the source divided by the orthogonally projected area of the element in a plane normal to the given direction of the beam, , , simplified expression: ) when the photon flow is constant over the surface area considered]. The SI unit is . For a divergent beam propagating in an elementary cone of the solid angle containing the direction, the photon radiance is with SI unit . Alternatively, the term can be used with the amount of photons (mol or its equivalent einstein), the SI units then being and , respectively.