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photon number
number of photons
quotient of radiant energy and photon energy 
where 
is the frequency of the corresponding electromagnetic wave
Note 1 to entry: Photon number can also be expressed by the time integral of the photon flux, 
.
Note 2 to entry: The photon number has unit one.
Note 3 to entry: This entry was numbered 845-01-29 in IEC 60050-845:1987.
Note 4 to entry: This entry was numbered 17-930 and 17-829 in CIE S 017:2011.