The theory of the bichromatic pyrometer is outlined and its response is calculated for different filter pass bands of different band widths. The ratio of the response at one colour to the response at the other is calculated as a function of the temperature of the source and the results are presented graphically. A range of pass bands in the visible spectrum, having sharply defined cut-off frequencies and uniform transmission in the pass frequencies, is considered and the effect of the width of the pass bands is estimated in each case. One result is given for an infra-red pyrometer with both pass bands wide and accommodated in the sensitive region of typical lead sulphide photoconductive cells. From these results it is clear that the two pass bands may be close to each other in frequency and that their band widths may be large, without an appreciable loss in sensitivity of the pyrometer. Positive recommendations may then be made that the design of a bichromatic pyrometer should concentrate on energy collection and should therefore adopt two wide pass bands both situated in the red part of the spectrum.