The
photon irradiance
, at
wavelength
per unit
wavelength interval.
The SI unit is

, but a commonly used unit is

.
Alternatively, the term can be used with the amount of photons
(mol or its equivalent
einstein), the SI unit then being

and the common unit

.
Source:
PAC, 1996, 68, 2223
(Glossary of terms used in photochemistry (IUPAC Recommendations 1996))
on page 2275