The total power density emitted from a light source can be calculated by integrating the spectral irradiance over all wavelengths or energies of interest. However, a closed form equation for the spectral irradiance for a light source often does not exist. Instead the measured spectral irradiance must be multiplied by a wavelength range over which it was measured, and then calculated over all wavelengths. The following equation can be used to calculate the total power density emitted from a light source.

where:
H is the total power density emitted from the light source in W m^-2;
F(l) is the spectral irradiance in units of Wm^-2mm^-1; and
dl or Dl is the wavelength.