Multiplex spectrometers

For either multiplex (e.g., FT-Raman) or multichannel (e.g., CCD) spectrometers, tM is the appropriate time to insert in Eq. (3.6) or (3.7). Second, a multichannel system can obtain a complete spectrum 1/Nr times faster than a single-channel system, while maintaining the same measurement time and signal strength per resolution element. Of course, both of these statements assume that other variables are equal for both spectrometer types. In Chapter 4, we will see that Im strongly affects SNR, but that the effects are fundamentally different for multichannel as opposed to multiplex spectrometers. 

The analyte shot noise limit illustrates the fundamental Importance of multichannel detection for improving SNR. Equations (4.15) and (4.16) are valid for each resolution element in the system in a single or multichannel spectrometer (but not for a multiplex spectrometer). As noted in Section 3.4 and Eq. (3.9), a spectrometer that can monitor Nk resolution elements simultaneously increases the measurement time for each resolution element by a factor of Nr over the time required for a single-channel system. Since Im = NrIs,... 

Non-dispersive multiplex spectrometers include Hadamard transformation spectrometers and Fourier transform spectrometers and are particularly useful for the case of very stable sources. In both cases the information, such as intensities at various wavelengths, is coded by a multiplex system, so that it can be recorded with a conventional detector. A suitable transformation is then used to reconstruct the wavelength dependence of the information. In Hadamard spectrometry use is made of a codation of the spectrum produced by recombining the information with the aid of a slit mask which is moved along the spectrum [66],... 

The key innovation in passive dispersive (as opposed to interferometric) spectrometers introduced recently is the use of a programmable digital spatial light modulator (SLM) to encode spectral information. The most advanced version of SLM currently is a reflective digital micromirror array (DMA). The feasibihty of DMA-based multiplexed spectrometers has been experimentally demonstrated recently. ... 

Modulation techniques have been shown to be capable of increasing the signal-to-noise ratio for Raman spectroscopy, this being illustrated for the resonance-enhanced inverse Raman effect.Inverse Raman spectroscopy enables spectra of highly luminescent systems to be recorded. A suitable spectrometer has been described in which a resolution of 1 cm was achieved with scan rates dependent only upon the scan speed of the dye laser used for excitation.A 100-fold increased sensitivity was reported with the use of a multiplex spectrometer for... 

Figure 2. A multiplexing spectrometer with several exit slits, allowing much more light to reach the detector.

S-matrices make the best spring balance weighing designs and the best (0,l)-masks for multiplexing spectrometers. 

We have said nothing about the actual construction of a multiplexing spectrometer, but for this the reader is referred to Chapter 5 of ref. 2. In practice it is more convenient to move the mask (for example that shown in Figure 6) continuously across the dispersed light rather than in steps. This produces a linear distortion of the final spectrum and can therefore be compensated for. The analysis of this and a number of other systematic errors is given in ref. 24 and Chapter 6 of ref. 2. 

Filler monochromators are of use only for flame photometry. They make use of interference filters, which often have a spectral bandpass of a few nanometers or less. Multiplex spectrometers include Hadamard transform spectrometers and Fourier transform spectrometers, and are especially useful where very stable sources are needed. Hadamard transform instruments make use of a coding of the spectrum produced by recombining the information with the aid of a slit mask which scans the spectrum [48]. 

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