Multichannel Techniques

The frequency distribution of noise is characterized by a power spectrum. There a two types. First, white noise, whose noise power is independent of the frequency. This noise arises from the statistics of electrons or photons and of the thermal energy of conductors. White noise can be reduced by extending the measuring time. Second, excess low frequency noise, flicker, or 1// noise is due to fluctuations, drift and schlieren. It can be reduced by modulation. All types of noise are reduced by multiplex procedures, multichannel techniques, and multiple recording (Schrader, 1980). 

There are different types of spectrometers with detector arrays. Due to the limited number (256, 512 or 1024) of available detector elements compared to the number of resolvable spectral elements, the spectrometers permit either low resolution over the entire spectral range, or high re.solution within a limited range. Alternatively, the technique described by Knoll et al. (1990) may be used They employed scanning a multichannel instrument, which combines the merits of the scanning technique with those of the multichannel technique. 

Figure 8.45. Spectram of calcium silicate obtained with the scanning multichannel technique. A very wide shift range was covered during continuous grating scanning and CCD readout. The features at 6000 to 9000 cm are due to photoluminescence, not Raman scattering. (Adapted from Reference 38.)...

One new field of application which the greatly increased sensitivity of optical multichannel techniques has opened up is to biological molecules, through both transparent and resonance Raman scattering. Infrared CD is likely to be inapplicable to most biological samples in aqueous media, so it is in biochemistry and biophysics that ROA will probably come into its own. 

No mulhplex or multichannel technique was available for the measurement of Raman spectra hence, the instruments were based on scanning monochromators with a single PMT detector. 

V Deckert, W Keifer. Scanning multichannel technique for improved techniques for improved spectrochemical measurements with a CCD camera and its application to Raman spectroscopy. Appl Spectrosc 46 332-338, 1992. 

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