Filter radiometer: the selective interferometer

This paper describes a new type of filter radiometer-the Selective Interferometer Filter (SIF)-which has extremely high selectivity to the radiation from a given gas. The basic device is a double output Michelson interferometer, with mirrors fixed at zero geometrical path difference. It is shown that if a cell containing a gas is placed in each of the arms, the ensemble can be made to operate as a highly selective filter, passing radiation only very near the center frequencies corresponding to the absorption lines of this gas. This is a result of direct absorption effects and of the variation of refractive index with frequency and consequent modification of the optical path difference of the two arms of the interferometer. The device may, therefore, after calibration be operated as a highly selective ir radiometer; and a feature of importance in atmospheric and space applications is that it requires no internal moving parts to achieve this selectivity. Details of the device are presented, and a thorough examination of the practical realization of it is given.