Accuracy of a radiometer is adversely affected by scene polarization if the receiving optical system is sensitive to polarization. It is therefore necessary to specify and measure the sensitivity of the system to polarized light. The Mueller-Stokes matrix of an instrument may be determined experimentally and used to predict the effects of the instrument on any beam. The specification of a maximum polarization sensitivity stated in terms of the degree of polarization produced in an unpolarized beam can be experimentally verified even though an unpolarized beam is not available in the laboratory for direct measurement.