`text(Spectral emissive power :)`
It is defined as the radiant energy (dE) emitted in (`lamda->lamda+dlamda`) interval per second per unit area of the surface per unit wavelength `dlamda` about `lamda`.

Spectral emissive power (`E_lamda`)= `text(Energy)/text(Area x Time x Wavelength)`

Unit `=>J/(m^2xxsecxxoversetoA)`

`text(Total emissive power :)`
It is defined as the total amount of thermal energy emitted per unit time, per unit area of the body for all possible wavelengths.

`E=int_0^(oo)E_lamdadlamda`

Unit `=>` `J/(m^2xxsec)` = `text(Watt)/m^2`

`text(Absorptive power :)`
It is defined as

`a=text(Amount of energy absorbed per sec)/text(Total energy incident per sec)`

The absorptive power of black body is a = 1.

`text(Emissivity :)`
Emissivity of a body at a given temperature is defined as the ratio of the total emissive power of the body to the total emissive power of a perfect black body at that temperature.

`e=E_text(body)/E_text(black body)`

So for a perfectly black body, value of emissivity is 1.