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Considerable improvement in the performance of background-limited infrared detectors can be obtained over a restricted range of wavelengths by using a cooled filter to reject a large fraction of the background radiation. This principle has been applied in a portable device containing a series of narrow-band and interference filters which, on external command, can successively be brought into use. In this way exceptional detectivity can be realized over a number of separate wavelength regions. Detectivities in excess of 1011 cm/W have been obtained in the 5 to 6 μ region of the spectrum, using filters of 0.6 μ half-width together with a copper-doped germanium detecting element cooled by liquid helium contained in an internal reservoir. Sharper or longer wavelength filters would give still higher detectivity.