The capability of the Low Background Infrared (LBIR) facility at the National Institute of Standards and Technology to spectrally calibrate infrared detectors was demonstrated with the spectral calibration of arsenic doped silicon blocked impurity band (BIB) detectors. The BIB detectors were calibrated over the 2 micrometer to 30 micrometer range, using light from a monochromator with a nominal 2% bandwidth. Photon fluxes used for the calibration ranged from 1013 photons/s/cm2 to 1014 photons/s/cm2. The large area detectors (10 mm2) calibrated in this paper were very linear up to 2.5 X 1014 photons/s/cm2, where they showed a 1% drop in signal from linearity. The calibrations contained less than 6 1% standard component of random noise uncertainty, and there was about a 6 5% standard component of uncertainty arising from systemic effects that will be discussed in detail. The calibrations were performed in ultra- high vacuum in a 20 K background environment by making direct intercomparisons between the power measured by an absolute cryogenic radiometer and the response measured by a detector irradiated by the same beam. A detailed description of measurement methodology and system apparatus is given. Detector linearity and uniformity are also discussed. The LBIR facility can now provide calibrated BIB detectors as transfer standards as well as evaluate and calibrate customer's large area detectors and detector arrays provided the detectors stay within certain physical limitations.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.