Single beam spectrum, Fourier spectrometer

The interferogram is actually a series of data points (retardation, intensity) collected during the smooth movement of the mirror. Using a mathematical function known as a Fourier transform, the spectrometer computer is able to deconvolute ( Fourier transform ) all the individual cosine waves that contribute to the interferogram, and so produce a plot of intensity against wavelength, or more usually the frequency in cm that is, the infrared single beam spectrum. All the... 

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.

Many factors affect the photometric accuracy of Fourier transform spectrometers. They include the effect of resolution and apodization on the spectrum, the reproducibility of the single-beam spectra, the accuracy of the zero level, and the linearity of the transmittance (T) scale. Several spectrometer manufacturers have observed that the 100% line on their instmments may be reproduced to better than 0.1% and, as a result, have claimed that their instruments are accurate to 0.1 %T. This is not necessarily the case, however, as we show below. 

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