This paper describes the construction and calibration of a complete optical system, built entirely within a standard helium cryostat, for making absolute measurements of radiation‐induced luminescence and optical absorption in the infrared range out to 25 &mgr;. Helium cooling the entire optical system reduced the thermal background radiation at the detector to approximately 1 × 109photons/cm2‐sec (&lgr;<25 &mgr;). Although helium Dewars for radiation effects have been described previously, to our knowledge no similar instrument for measuring radiation‐induced optical effects has been reported.