Emissive power is the rate at which radiation is emitted per unit area. It is a measure of heat flux. The total emissive power from a surface is denoted as  and can be determined by,where  is in units of steradians and  is the total intensity.
The total emissive power can also be found by integrating the spectral emissive power over all possible wavelengths. This is calculated as,where  represents wavelength.
The spectral emissive power can also be determined from the spectral intensity, as follows,
where both spectral emissive power and emissive intensity are functions of wavelength.