Number of photons (quanta of radiation, ) per time interval (photon flux), , incident from all upward directions on a small element of surface containing the point under consideration divided by the area of the element. SI unit is . Equivalent definition: Integral, taken over the hemisphere visible from the given point, of the expression the photon radiance at the given point in the various directions of the incident beam of solid angle and the angle between any of these beams and the normal to the surface at the given point.
- Mathematical definition: . If the photon flux is constant over the surface considered, . Equivalent definition: .
- This term refers to a beam not scattered or reflected by the target or its surroundings. For a beam incident from all directionsphoton fluence rate () is an equivalent term.
- This quantity can be used on a chemical amount basis by dividing by the Avogadro constant, the symbol then being , the name 'photon irradiance, amount basis', SI unit is ; common unit is .
PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 396