https://doi.org/10.1351/goldbook.P04639
Number of photons (quanta of radiation, ) per time interval (photon flux), , leaving or passing through a small transparent element of surface in a given direction from the source about the solid , divided by the solid , with the , with the photon and the
angle
angle
and by the orthogonally projected area of the element in a plane normal to the given beam direction, angle
between the normal to the surface and the direction of the beam. Equivalent definition: Integral taken over the hemisphere visible
from the given point, of the expression radiance
at the given point in the various directions of the incident beam of solid angle
angle
between any of these beams and the normal to the surface at the given point.Notes:
- Mathematical definition:
for a divergent beam propagating in an elementary cone of the solidangle containing the direction . SI unit is . - For a parallel beam it is the number of photons (quanta of radiation,
) per time interval (photon flux), , leaving or passing through a small 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, . Mathematical definition in this case: If is constant over the surface area considered, , SI unit is . - This quantity can be used on a chemical amountbasis by dividing
by theAvogadro constant, the symbol then being , the name 'photonradiance, amount basis'. For a divergent beam SI unit is ; common unit is . For a parallel beam SI unit is ; common unit is .