Acquiring the Data Dispersive Spectrophotometers

A large proportion of spectral data is acquired by dispersive spectrophotometry. The discussion that follows is restricted to instruments that use a diffraction grating as the principal dispersive element. The sense of the following also applies to systems that use a prism. In general, we treat systems using photosensitive detectors and fixed-position slits. Scanning is achieved by rotation of the diffraction grating. 

To place the discussions of this and the following chapter in a clear context, we describe a specific system model. Application discussions may be seen in the light of a specific instrument. To use deconvolution techniques effectively, the instrument as well as the data acquired must be analytically well characterized. 

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. 

As the beam leaves the prism predisperser, it is focused on the entrance slit of the grating monochromator. The slit is curved, has variable width, and opens symmetrically about the chief ray (optical center line of system). The monochromator itself is of the off-axis Littrow variety (James and Sternberg, 1969 Stewart, 1970 Jennings, 1974) and uses a double-pass system described by McCubbin (1961). The double-pass aspect of the system doubles the optical retardation of the incident wave front and theoretically doubles the resolution of the instrument. The principal collimating mirror is a 5-m-focal-length, 102-cm-diam parabola. 

From the exit slit, the beam is directed to either of two detectors by the movable mirror behind the exit slit. Typical detectors are InSb, a photovoltaic indium antimonide detector operated at 77 K, and He Ge, a photo-conductive mercury-doped germanium detector maintained at 15 K by a closed-cycle helium refrigerator 

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