Dispersive Monochromators

Two main categories of monochromators can be distinguished one is the dispersive monochromator where an energy spectrum dispersed in space is obtained with a reflection or a transmission diffraction grating (more rarely now with a prism). The other is the Fourier transform spectrometer (FTS). The principles of these two types of spectrometers are described below. 

2 In spectrographs, the dispersing element is immobile and the spatially dispersed spectrum is recorded on a photographic plate or on a linear array charge-coupled device (CCD) detector. 

In the above Littrow mounting, internal modulation of the dispersed beam and an appropriate optical mounting allowed a second dispersion of the beam 

Presently, grating monochromators are used every time a sample must be illuminated with quasi-monochromatic radiations that are tunable in a broad spectral range or for experiments in the visible-UV range. Another interest of dispersive monochromators is the possibility of wavelength modulation of the output of these monochromators in order to get the first derivative of the transmission spectrum. This has the advantage of increasing the sensitivity, and this technique is also used in laser spectroscopy. 

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Although in some cases monochromators are classified as dispersive or nondis-persive monochromators, in this book we are only dealing with dispersive monochromators, since they are most commonly used in optical spectroscopy. In dispersive monochromators, a spatial separation is obtained for the different spectral components of the input beam. As shown in Figure 3.1, the simplest monochromators consist of the following elements ... 

A nondispersive system of monochromation filters either the radiation or the electrical signal generated in the detector. In a dispersive monochromator, radiation is separated according to wavelength. The resolution of a monochromator is defined as where 8X is the minimum distance between the centers of two spectral lines which can just be dis-... 

As the light collection capacity of a dispersive monochromator is frequently low, the use of filters can lead to more precise measurements of emission signals if the bandwidth is suflBciently narrow to avoid spectral interference. Interference filters with a bandwidth of 5 nm are available, and for maximum selectivity these should be used with near parallel light (L7). In atomic absorption the light collection capacity of the monochromator is frequently unimportant as the source intensity is high and the cross section of the optimum absorption zone of the flame is small. 

For the oscillation camera placed on a horizontally dispersing monochromator Kahn et al (1982a), working from Azaroff (1955), derived the polarisation correction P as... 

Simple, low-dispersion monochromators or even interference filters are used for most flame emission applications since few atomic line spectral interferences are expected as a result of the limited population of the higher-lying excited states. For high-temperature sources such as ICPs, higher-dispersion spectrometers are typically used. Instruments set up to do simultaneous multielemental analysis can use direct readers with PMT detection. However, most modern detections systems for this type of source for simultaneous multielemental analysis employ a high-dispersion eschelle grating spectrometer and an array detector such as a CCD or CID. 

The general principle of operation of the FTIR method is similar to the DOAS method, because they both measure and analyze over a broad region of the spectrum. The principal difference is that an FTIR spectrometer is used instead of a conventional dispersive monochromator. The benefits of using an FTIR in place of a monochromator can be summarized in terms of two types of advantage ... 

A much larger optical throughput is possible than with a dispersive monochromator with the same resolution, hence the signal-to-noise ratio is larger. 

Filter spectrophotometer A spectrophotometer that uses filters of fixed, narrow band-pass transmissions of discrete wavelengths spaced across the spectrum, to measure the transmittance or reflectance of materials at these discrete wavelengths. The resulting special data arranged in order constitute an abridged spectrophotometric curve. Thus, the series of filters replaces the dispersion monochromator used in a continuous spectrophotometer. Willard HH,... 

The spectrum of a 45- tm-diameter polystyrene microsphere, shown in Fig. 7C, demonstrates the narrow spectral resolution of the LCTF and was extracted from the image dataset by plotting the intensity of a pixel as a function of image wave number. A reference spectrum of polystyrene microspheres, collected through a conventional 0.5-m dispersive monochromator having 7-cm resolution, is shown in Fig. 7D for comparison. The LCTF Raman spectrum is comparable to the reference spectrum and demonstrates the suitability of LCTFs for Raman spectroscopy. The ability to combine high spectral performance and excellent image fidelity with the LCTF is unmatched by alternative approaches. 

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