The amount of EM radiation emitted by a real object of temperature T at wavelngth \lambda is
where \epsilon is the emissivity and \L_\mathrm{BB}(\lambda,T) is the amount of EM radiation emitted by a black-body.
Real objects absorb less than a black body and therefore can re-emit less (typically some 80 to 98%) of the radiation received. The emitting ability of real objects is defined by the emissivity.