where TBB is the black body temperature, ABB is the area of blackbody, AD is the area of the detector and d is the distance between source and detector. The factor is derived from the analysis of emission of the blackbody radiation from an extended source. The detector remits the part of absorbed radiation. Therefore, this amount of power is to be subtracted from the incident power to obtain the effective power, Peff, which is absorbed by detector, is given as,