Doped silicon and germanium are used as extrinsic photoconductors for IR astronomy from ground-based, balloon-, and airborne observatories and from space platforms. The detectors of the Infrared
Space Observatory (ISO) will be capable of observing at the sensitivity limits imposed by natural sky background radiation. For the first time, ISO will use stressed Ge:Ga detectors on a satellite covering the wavelength range out to 240 μm. Preflight testing has shown that ISO's calibration will have to correct for high-energy radiation effects and delayed flux response of its detectors. For future far-IR projects, these problems might be solved by blocked-impurity-band detectors. Several other difficulties in today's detector assembly production and readout technology also deserve improvements.