Radiant power, , leaving or passing through a small transparent element of surface in a given direction from the source about the solid angle, divided by the solid angle and by the orthogonally projected area of the element in a plane normal to the given beam direction,
- Mathematical definition: for a divergent beam propagating in an elementary cone of the solid angle containing the direction . SI unit is .
- For a parallel beam it is the radiant power, , of all wavelengths 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 the radiant power is constant over the surface area considered, . SI unit is .
- Equivalent to , where is the spectral radiance at wavelength.
PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 408