Littrow spectrometer

For clarity and precision in the following discussion, the 5-m Littrow spectrometer at the University of Tennessee at Knoxville will serve as the prototype (Jennings, 1974). The optical diagram is presented in Fig. 1. The system is used as an absorption spectrometer most of the discussion also applies to acquisition of emission spectra. 

The two most-used large Littrow spectrometers are those produced by Bausch and Lomb and by Adam Hilger, Ltd. The instruments have similar specifications however, the Hilger unit combines all three wavelength... 

The data illustrated in Fig. 4(a) are methane absorption lines (0.02 cm-1 wide) observed with a four-pass Littrow-type diffraction grating spectrometer. For these data also, 256 points were taken. The data were obtained at low pressure, so that Doppler broadening is the major contributor to the true width of the lines. The straightforward inverse-filtered estimate with 15 (complex) coefficients retained is shown in Fig. 4(b). Figure 4(c) shows the restored function. The positions and intensities of the restored absorption... 

Glass prisms do not require the special attention given quartz since optical rotation and birefringence do not occur in glasses. However, glass prisms for spectrometers commonly are made similar to those of quartz. The 60° prism is common, as is a Littrow (half-prism) type. Prisms usually are used near the position of minimum deviation. For most uses the 60° prism appears to be close to the optimum design. 

Two basic designs of prism spectrometer are commonly used for spectrochemical analytical purposes, the Cornu and the Littrow types. Many of these instruments are presently in use, although grating spectrometers are gradually replacing prism instruments. 

FIGURE 4-2. Schematic diagram of a Littrow prism spectrometer. 

For a prism spectrometer with a Littrow mounting, the spectral slit width is s = Sg + So + ... 

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