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272    Cha pte r  Ni ne


        influence of heat generated noise (dark current). To reduce the noise
        generated in reading out the camera, slow readout speeds are used.
            A next generation of CCD detectors that can be useful in Raman
        imaging is now emerging: the electron multiplied CCD (EMCCD). It
        has been available for some time for UV and VIS excitation, but now
        EMCCDs that have a better sensitivity in the NIR have become
        available. This type of detector is especially useful for high-speed,
        low-signal applications that are limited by the readout noise of the
        CCD. By amplification of the signal, the readout noise that increases
        with higher readout speed becomes much smaller compared to the
        signal noise (shot noise). However, the amplification process itself
        also generates noise and there is an additional (spurious) noise
        induced when shifting the charge toward the readout register, so for
        applications that are limited by signal noise this is not an improve-
        ment, although they may benefit from enabling a higher read out
        speed.



   9.3 Imaging Techniques
        Several approaches to Raman imaging have been invented to simul-
        taneously record spatial and spectral information, as depicted in
        Figure 9.4. Raman imaging data consists of 2 spatial (x, y) and one
        spectral dimension—the Raman intensity as a function of Raman fre-
        quency. Most modern Raman imaging methods employ a multi-chan-
        nel CCD that can record two dimensions of the three-dimensional
        information. Raman imaging systems can be differentiated by the way
        they collect the third dimension. Most Raman imaging systems use a
        spectrograph coupled to a CCD and a scanning stage for either
        two-dimensional point scanning, one-dimensional line scanning,
        or spatial multiplexing. 27



         Point    Spectrometer  CCD





         Line
                  Spectrometer  CCD



    Widefield
                             CCD
                 LCT Filters


   FIGURE 9.4 Point, line, and widefi eld Raman spectroscopic imaging.
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