Resolution and spectral coverage

The practical consequences of Eq. (8.6) lie in both resolution and spectral coverage. High dispersion (small dv/dl) yields high resolution (small Av range per pixel or slit width), according to ... 

As described in Section 8.2.1.1, there is a trade-off between resolution and spectral coverage for a dispersive CCD spectrometer. For a given grating... 

The figures of merit of primary interest to analytical chemist that are discussed are noise characteristics and powers of detection, linearity and dynamic range, spectral stripping and interpolative correction for variable underlying background, compromise between resolution and spectral coverage and application to nonideal "real" samples. 

The early Fourier transform spectrometers, which were nearly all home made, came into being to measure spectral features which were clearly too weak to measure with a grating or prism spectrometer and bolometer combination. They were small instruments having modest resolution and wavelength coverage. 

Light sources can either be broadband, such as a Globar, a Nemst glower, an incandescent wire or mercury arc lamp or they can be tunable, such as a laser or optical parametric oscillator (OPO). In the fomier case, a monocln-omator is needed to achieve spectral resolution. In the case of a tunable light source, the spectral resolution is detemiined by the linewidth of the source itself In either case, the spectral coverage of the light source imposes limits on the vibrational frequencies that can be measured. Of course, limitations on the dispersing element and detector also affect the overall spectral response of the spectrometer. 

The FT-Raman spectrometer used HP-532/1-50/1-4CH is a commercially available system from Kaiser Optical Systems. The system contains a green LASER with a wavelength of 532 nm and an output power of 35 mW. The power ouq>ut at the probe head is about 10 mW. The focused probe head has a sapphire window and can be used up to 280 °C and 60 bar pressure. The probe head is connected with a 100 pm optical fiber with a CCD detector for the Raman scattered light and an FT-spectrometer. The FT-spectrometer has a spectral coverage from 100 to 4400 cm and a spectral resolution of 5 cm [3]. 

Figure 8.2. Raman shift coverage and resolution for two different gratings (300 and 1200 lines/mm) and a 25 mm wide CCD in a 250 mm spectrograph. For a constant number of CCD elements, resolution improves as spectral coverage decreases.

Figure 8.7 shows an experimental example of the trade-off of spectral coverage and resolution for three gratings in a 330 mm spectrograph and... 

---