Gas Chromatography - Tandem Techniques - The Characteristics of Infrared Absorption > IR Spectroscopy Instrumentation > The Fourier Transform IR Spectrometer. > Page 30


Collimated light from a broadband infrared source passes into the optical system and impinges on a beam splitter that comprises a very thin film of germanium.


Courtesy of Nicolet Inc.



Figure 19. The Basic FT-IR System


Approximately 50% of the light passes through the film and is reflected back along its path by a fixed mirror, where half of the light intensity (25% of the original light intensity) is reflected by the same beam splitter, through the sample cell, to the infrared sensor. The other 50% fraction of the incident light is reflected at right angles to its incident path onto a moving mirror. Light from the moving mirror returns along its original path and half of the light intensity is transmitted through the beam splitter, through the sample cell, to the infrared sensor. Thus 25% of the incident collimated light from the source reaches the sensor from the fixed mirror and 25% from the movable mirror. As the path length of the two light beams striking the sensor will differ, there will be destructive and constructive interference and, in fact, the system constitutes a Michelson interferometer.