There are two main infrared (IR) spectrophotometers: dispersive IR spectrometers and Fourier transform infrared (FTIR) spectrometers. In a dispersive IR spectrometer, a beam of infrared radiation produced by a hot wire is divided into two parallel equal-intensity beams using mirrors. One beam passes through the sample, while another is a reference beam. The beams then move through the monochromator, which separates the radiations into a continuous spectrum of different frequencies. The monochromator consists of a rapidly rotating beam chopper that alternates the sample and reference beams before they reach a diffraction grating. Next, the radiation passes through the thermocouple detector that measures the ratio between the two beams' intensities. The signal from the detector is amplified, and the resulting spectrum is drawn using the recorder. Dispersive IR spectrometers record the spectrum in the frequency domain, meaning the spectrum is captured as the frequency of the radiation changes with the rotation of the diffraction grating.

In an FTIR spectrometer, the radiation produced by the source is passed through a beam splitter. A beam splitter is a mirror placed at an angle of 45° to the incoming radiation and separates the beam into deflected and undeflected radiations. These radiations are later allowed to fall on fixed and moving mirrors. It is important to note that the motion of the two mirrors changes the path length of the beams, and the radiation reflected by the mirrors is combined in the beam splitter. Thus, the produced combined beam contains both constructive and destructive interference patterns due to differences in the path length of the radiations.

1. Constructive interference occurs when the peaks of two waves align, leading to an increase in the amplitude of the combined wave.
2. Destructive interference happens when one wave's peak aligns with another's trough, canceling each other out and resulting in reduced amplitude.

The resulting combined beam, called an interferogram, is passed through the detector. The detector detects the interferogram modified by the sample, and the FT process separates the individual absorption frequency from the interferogram.

FTIR spectrometers are preferred over dispersive IR spectrometers due to their faster operation and greater sensitivity.