Single-channel dispersive spectrometer

In the absence of fluctuation noise, the multiplex advantage of FT-IR spectrometry over a single-channel dispersive spectrometer with the same optical throughput and efficiency (often referred to as Fellgett s advantage) is given by the square root of the number of resolution elements in the spectrum, M, where M =... 

As with VCD, the first ROA instruments were built around single-channel scanning dispersive spectrometers [18,19,76,77], Photomultipliers with dualchannel photon-counting electronics were used to record the spectra. Scanning rates were no faster than 1 cm l per minute because of the requirement to accumulate at least 10 7 counts per spectral location, and preferably 10 . Applications with these instruments were limited to samples with favorable Raman scattering and the goals of these early studies were simply to explore the nature of ROA spectra and to improve measurement techniques. Several reviews... 

The most simple dispersive spectrometer (Fig. 12.2) comprises a source, a monochromator and a detector. The monochromator, made up of an entrance slit, an output slit and prisms or gratings, is u,sed to separate the light into its basic components. The role of the slit system is to enhance the spectral resolution and compensate for intensity variations. The transmission infrared spectrum of the sample is the recording of the light intensity transmitted as a function of the wave-numbers w hich are scanned in front of the monochromator output slit by rotating the dispersive element. In the infrared domain, the wave-numbers are always recorded sequentially, due to the single-channel nature of the detectors. This recording is compared to that of the reference or the source in order to deduce the absorption due to the sample. 

FT-IR spectrometers cannot be built as double-beam instruments. Unlike dispersive instruments, FT-IR spectrometers acquire single channel spectra of sample and reference and their ratio is calculated afterwards (Fig. 4.2). Sample and reference may automatically be replaced by a sample slider, or the IR beam may be switched between sample and reference by flip-mirrors. In the case of higher accumulation numbers, the instrument switches repeatedly between sample and reference scan. 

Among wavelength-dispersive spectrometers, a distinction can be made between single-channel instruments and multi-channel spectrometers. In the former type of instrument, a single dispersive crystal/detector combination is used to sequentially measure the X-ray intensity emitted by a sample at a series of wavelengths when this sample is irradiated with the beam from a high power (2—4 kW) X-ray tube. In a multi-channel spectrometer, many crystal/detector sets are used to measure many X-ray Unes/elements simultaneously. 

Figure 1 Optical path of the Madrid dual Raman spectrometer, (a) PMT mode with a single-channel subtractive dispersion configuration (b) CCD mode with a multichannel configuration. (From Ref. 53, with permission.)...

Modern conventional laser-Raman systems for recording spontaneous Raman spectra comprise essentially an appropriate source, some dispersing and collection optics and a sensitive detector. The two most common approaches are either single-channel detection using a photomultiplier in combination with a multi (double or triple) monochromator set-up, or a multi channel option with an array detector coupled to a spectrograph [12, 20, 21]. The recent commercial development of FT-Raman systems, either as standalone spectrometers or as FTIR adjuncts, has added to the armoury of techniques, and without doubt offers a cost-effective, rapid, readily applicable technique for a wide range of polymer studies [22-24]. 

At present, FT spectrometers are almost exclusively used for mid-IR spectroscopy, except for some special purposes. On the other hand, dispersive spectrometers (two types exist as described below), spectrometers with optical filters, AOTF (acousto-optic tunable filter) spectrometers, inter alia, are used in NIR spectroscopy in addition to FT spectrometers. There are two types of dispersive spectrometers the scanning type with a single-channel detector and the non-scanning type with a multichannel detector. The reason for the existence of such a variety of spectrometers in the NIR region is as follows. 

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