Define and explain spectral emissive power.

Explanation

Spectral emissive power : ..At a given temperature, the amount of radiant energy emitted per time per unit surface area, in unit frequency interval about a given frequency (f) is called spectral emissive power `(W_(f))` of the given surface at the given temperature, corresponding to that frequency,..
It is clear that the sum of the spectral emissive powers for all the frequencies gives total emissive power.
`W=sum_(f)W_(f)`
The magnitude of `W_(f)` depends on the temperature material of the surface and frequency (f).