A passive method of measuring the spectral emissivity and the temperature of materials in the mid- and the far IR is described both in theory and experiment. Theoretical evaluation and real measurements (spectral emissivity and temperature) of selective bodies utilizing IR transmitting silver halide optical fibers are presented. We carried out the theoretical evaluation and measurements for a multiband fiber-optic radiometer, which consists of a silver halide (AgClBr) infrared transmitting fiber, a dual-band cooled infrared detector, and a set of 18 narrow-band infrared filters, covering the 2–14μm spectral range. We performed the measurement on the two selective bodies which had a strong and a weak dependence of emissivity as a function of the wavelength. The body spectral emissivity, the body temperature, and the room temperature were measured simultaneously. The results of the theoretical evaluation agreed with the experimental results. We also discuss the assumptions and the limitations of performing such a measurement for bodies near room temperatures. This method for measuring spectral emissivity has applications in science, industry (semiconductor industry), and medicine.