Abstract: In a ratio-type double-beam infrared spectrophotometer, an output signal of the photodetector is frequency discriminated to derive therefrom a component having a fundamental frequency f corresponding to at least one of the sample and reference beam intensities and another component having a frequency 2f corresponding to at least the other beam intensity, and these frequency components are computed to determine the ratio of the sample beam intensity to the reference beam intensity. According to the invention, the component of fundamental frequency f is synchronously rectified into a rectified signal which is further differentiated into a differential value, the differential value is subtracted from the rectified signal to give a signal which is compensated for rapid changes of spectral absorption due to the presence of H.sub.2 O and CO.sub.2 in the beam paths during wavelength scanning and is used as the f-component signal.

Abstract: A double-beam spectrophotometer for spectral analysis of a sample in the infrared region is provided in which to eliminate errors in measurement of the absorbance of the sample caused by undesired thermal radiation from the sample itself, first and second sectors are used for division and recombination of beam paths and coordinated such that a detector which receives a beam along the combined beam path produces output signals consisting of components having a frequency f associated with the cycle of operation of the sectors and components having a frequency 2f, those components having frequencies f and 2f are independently derived out of the detector output signals, and the ratio of the components is computed, thereby obtaining the ratio of intensity of sample beam to reference beam independent of the undesired thermal radiation.

Abstract: In a double-beam spectrophotometer intended for spectral analysis in the infrared region, inputs to a light detector are switched to dark, reference light, sample light, and reference light in this order, and those components in an output signal of the detector which have a fundamental frequency corresponding to one cycle of the switching and a frequency twice the fundamental frequency undergo arithmetic operation to determine the ratio of the sample to reference light intensity, which is representative of the transmittance of the sample.