The radiometric errors introduced by the optical components of a high performance imaging infrared radiometer have been quantified both theoretically and experimentally. The excellent agreement obtained has enabled an algorithm to be implemented within the system processing electronics to "correct" the apparent temperature of the displayed, internal temperature references for such radiometric errors. Cor-rections have been evaluated both for the scanner unit with a wide angle window and with an optional x20 telescope. The radiometer under discussion is the Dual Waveband Imaging Radiometer (DUWIR) which has proved to be a very successful data collection tool due to its unique combination of a high performance imager offering simultaneous 3-5μm and 8-12μm TV compatible outputs together with internal tempera-ture references to provide real-time temperature calibration. The prototype has been used extensively for IR signature collection over the past three years and production units are now in use in several countries.