The Advanced Sensor Evaluation and Test (ASET) facility is used to characterize infrared sensor performance under conditions simulating those of outer space (viz., low radiant back-ground and high vacuum). One function of the facility is to provide to the sensor under test a well collimated beam having controllable irradiance covering seven (or more) decades of dynamic range, with spectral character approximating that of a 300K temperature Planckian source. The sensor response to these stimuli yields a sensor response function, provided that the stimulus (i.e., the test beam spectral irradiance) is accurately known over the wavelength region of interest (4 pm to 24 μM). To obtain this information a Spectroradi-ometer Assembly (SRA) was designed and built. It is based on a Czerny-Turner grating monochromator with beam collecting optics, and a Si:As photoconductive detector. The task of covering the dynamic range of interest translates to a dynamic range of nine (or more) decades of detector signal. The unavailability of a linear detector necessitates a complicated calibration procedure. Further, the only "standard" on which the calibration can currently be based is a simulated blackbody. Because of limitations imposed by the space available, the spectral irradiance provided by such a reference is much greater than that normally obtaining in the test beam, thus further increasing the complexity of the calibra-tion. The approach taken in the current calibration is to first calibrate the SRA using a blackbody as a "standard" source, taking steps to account for detector nonlinearity. Then, using the calibrated SRA, the spectral irradiance of the test beam is determined. The results are given as spectral transfer functions of the ASET optical system, and samples will be presented. In conclusion the results of a detailed error analysis will be given.