Abstract: In a bi-frequency infrared spectrometer having a filter wheel rotating in the path of rays, which filter wheel carries two semicircular filters transparent in different spectral ranges, the influence of the radiation from the filters themselves is eliminated in that the filter wheel has an opaque backplate on the side facing the light source, said backplate having one aperture in the area of each filter, which aperture passes through the path of rays. In this way the detector "sees", at first, only the filter with the radiation emanating from the filter itself, and is subsequently exposed to the measuring beam emanating from the light source and passing through the sample cell, when the aperture is being moved through the path of rays. A clamp circuit keeps the output signal zero until shortly before the aperture passes through the measuring beam. The detector signal is integrated by an integrator during a measuring time interval and a reference time interval.