Michelson Interferogram

A Fourier transform infrared spectroscopy spectrometer consists of an infrared source, an interference modulator (usually a scanning Michelson interferometer), a sample chamber and an infrared detector. Interference signals measured at the detector are usually amplified and then digitized. A digital computer initially records and then processes the interferogram and also allows the spectral data that results to be manipulated. Permanent records of spectral data are created using a plotter or other peripheral device. 

Figure 10.11—Optical arrangement of a Fourier transform IR spectrometer, a) A 90c Michelson interferometer including the details of the beam splitter (expanded view) b) optical diagram of a single beam spectrometer (based on a Nicolet model). A weak intensity HeNe laser (632.8 nm) is used as an internal standard to measure precisely the position of the moving mirror using an interference method (a simple sinusoidal interferogram caused by the laser is produced within the device). According to the Nyquist theorem, at least two points per period are needed to calculate the wavelength within the given spectrum.

Thus, we need only take a one-sided interferogram. From Eq. 13 we can also see why spectroscopy associated with the Michelson interferometer is called Fourier transform spectroscopy. Movement of the mirror which corresponds to a change in retardation provides a signal which is a function of distance. This signal is then decoded by a Fourier transform to give a spectrum which is a function in the reciprocal space. This is why wavenumbers (cm-1) are such a convenient unit to use with this type of spectroscopy. 

The two main differences between MCFT and conventional FT-Raman are both derived from the characteristics of the CCD, and both are fundamental. First, the resolution depends on the number of CCD elements along the interferogram axis. Since one cannot arbitrarily vary the size of the CCD or the pixel spacing, there is less flexibility than with a Michelson system, where mirror travel and sampling rate are variable. For the configuration shown in Figure 9.17, Eq. (9.8) applies (23) ... 

Figure 25F-8 Formation of interferograms at the output of the Michelson interferometer.

Fourier transform spectrometry [67] makes use of a Michelson interferometer (Fig. 29) to produce the interferogram. With the aid of a beam splitter the radiation is split into two parts each of which is directed to a mirror. When shifting the... 

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