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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.

Figure 2.11
The Basic FT-IR System Concept
Courtesy of Nicolet Inc.

As the movable mirror traverses its programmed path, it will produce a series of maxima and minima
that will be recorded by the sensor, as all the different wavelengths generated by the source pass
through conditions of constructive and destructive interference. In addition, the frequency of this wave
form will be determined by the velocity of the moving mirror which is controllable. In fact, the
interferometer is actually taking a Fourier transform of the incoming signal. An example of an
interferogram obtained from the FTIR is shown in Figure 2.11. It might appear that the resolution of the
interferogram is not very high, and an inverse Fourier transform might not provide a conventional IR
spectrum with very good resolution. In fact one scan from an FTIR instrument, taking about a second,
would give an conventional IR spectrum with resolution equivalent to that obtained by a dispersive
instrument scanning, over a period of 10 to 15 minutes. Nevertheless, the resolution can be improved by
taking a