Abstract: The relation of transmittance versus irradiance and numerical aperture (NA) of optical systems is discussed in this paper. Some methods of measuring the transmittance of IR optical systems are introduced, such as the method of integrating sphere, method of broadarea uniformity source, method of full aperture transmittance measurement and method of dual beam paths. The differences between theory and realizability of the above mentioned methods are also put forward. The measurement setup based on the method of broadarea uniformity source is regarded as the one which can measure the transmittance quickly, accurately and reliably, and can achieve a satisfactory effect by comparison with all the methods for transmittance measurement. The IR standard transmittance board was measured by the method of broadarea uniformity source, and two sets of experiment data were obtained. The uncertainty of the instrument which adopts the method of broadarea uniformity source is analyzed. The result indicates that the value of uncertainty is 2.6%.