How many photons are emitted by a light source? To answer this question we need to evaluate the following simple formula at every wavelength in the spectral range of interest and add the values up:
(1)
The Power of Light emitted in is called Spectral Exitance, with the symbol when referred to units of energy. The energy of one photon at a given wavelength is
(2)
with the wavelength of light in meters and and Planck’s constant and the speed of light in the chosen medium respectively. Since Watts are joules per second the units of (1) are therefore . Writing it more formally:
(3)