Several modern Raman setups consist of a double monochromator in the additive dispersion mode and a multichannel detector system like a charge-coupled device (CCD) camera or a photodiode array. The benefits of multichannel detection for spectroscopy have been discussed widely in the literature in the last decade. However, in comparison to the situation with the single-channel technique (e.g., photomultiplier tube detection), which employs narrow middle slits, the stray light introduces problems when multichannel detector systems are used in combination with grating spectrometers with additive dispersion, since in order to illuminate the whole detector area the middle slit(s) between the two monochromator stages must be opened widely. This arrangement results in a poor stray light rejection. To reduce the latter, researchers have tried several approaches employing optical edge or notch filters. The main drawbacks, however, are the still limited bandwidth and the limited spectral range of these filters. This method also requires a set of such filters for various laser lines used, for instance, in resonance Raman spectroscopic studies. Clearly, the best solution for Raman spectroscopy employing multichannel detection systems is to use a triple monochromator, in which the first two stages are in the subtractive mode, acting as an efficient stray light rejection filter, and the third stage is used as a spectrograph to disperse the frequency shifted light.
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