Knowledge Bank

A new far infrared spectroscopic method using an InSb tunable detector---tuned by a magnetic field---is proposed. If a far infrared source, obeying the Rayleigh-Jeans relation, is used with a grating spectrometer, whose transmittance is independent of wavelength, then the detector output is proportional to $m3$ for a thermal detector, to m for an InSb nontunable detector, and peaks sharply at some fixed m for a tunable detector, where m is the diffraction order. This response is illustrated by absorption spectra of water vapor measured with the tunable InSb detector for various values of the magnetic field. It is proposed to tune the spectral region of the detector synchronously with the motion of the grating, thus eliminating the need for low-frequency-pass filters.