Dispersive slit illumination

Figure 3.11 Schematic setup of the ARES spectrograph (1) entrance slit, (2-5) dispersive slit illumination (DSI), (6, 9) spherical mirrors, (7) prism, (8) echelle grating, (10) CCD array detector, (R1 -R5) piezo-eleclrically controlled rotation units...

The fluorescence image of the line illumination on the sample was relayed to the entrance slit of a polychromator. The slit width was set at 70 pm, corresponding to 7 pm on the sample in this configuration. Fluorescence passing through the entrance slit was spectrally dispersed by the grating and detected by a CCD (480 x 640 pixels)... 

The spectral resolution of a detector is defined here as equal to its spatial resolution (in urn ) times the reciprocal linear dispersion of the spectrometer (in nm/um ). It was measured to be 1.5 - 2.5 and 2-4 times poorer for the SPD and SIT, respectively, compared to that of a PMT. All measurements were performed with the same spectometer, utilizing 20 urn slit widths. Because, the proximity focused, microchannel plate (MCP) intensi-fier broadens the line images, the spectral resolution of the ISPD was found to be significantly worse than that of the SPD. Peak widths measured at half maximum intensity were four diodes wide even when only a single diode width was illuminated. 

Figure 13.5 The diverse components of a single beam atomic absorption apparatus. Model IL 157(Thermo JarreU Ash) constructed during the 1980s. 1, source (spectral lamp) 2, flame hurner which provides the atomic aerosol 3, monochromator grating and 4, detector (photomultiplier). The source illuminates a sht situated at the entrance the dispersive system. The exit sht, is close to the detector window. It determines a narrow bandwidth of the spectrum, (AA of 0.2 to 1 nm), which must not he confused with either the width of the exit slit or with the image of the entrance sht.

To locate the specific radiations of an element to be quantified (the different analytical lines) an optical bench of very high quality is required. These apparatuses can be divided into spectrographs and spectrometers (Figure 14.8 and see also Figure 9.12). As above pointed out, the source illuminates a slit which becomes a luminous object, which will be analysed by a dispersive system containing a grating (planar, concave or echelle). 

Fig. 25. Schematic diagrams of (A) conventional and (B) diode array scanning spec-trophometers operated in the dispersion mode. (A) The spectrum is dispersed by a grating or prism (the dispersing element) and scanned across an exit slit. A single-element detector (usually a photomultiplier tube) is used to measure the intensity of a monochromatic beam after it passes through the sample. (B) The sample is illuminated with white light prior to dispersion. The dispersed spectrum is imaged on a linear array detector, and the signals from individual elements provide the information necessary to generate spectral information. [Redrawn from Santini et al. (134) with permission.]...

The dispersive spectrometers suffer from greater wavenumber errors, of a less predictable form, owing to their general mechanical and thermal instability and can also be affected by non-uniform illumination across the monochromator entrance slit [26]. FT-spectrometers typically use a He-Ne laser as a reference beam to monitor the displacement of the moving optical element, so providing an active internal absolute wavelength calibration... 

Optimum resolution of the spectrometer is obtained when the dispersing element is filled with light. Maximum intensity of the spectrum occurs when the entrance slit is uniformly illuminated for the full height of the dispersing element. Often it is not possible to place the source at the optimum position in front of the entrance slit. In this case a lens or combination of lenses can be used to accomplish the same effect. 

The samples of lipid dispersion for Raman spectroscopy were pelleted in hematocrit capillaries and maintained in a temperature-controlled block during laser illumination. Above the phase transition temperature over a hundred milliwatts of light could be used9 but with the partially polymerized samples powers were reduced to less than 10 mW in most cases. Slit widths used on the monochromators were such that resolution was between 5 and 7 cm l depending on the wavelength of excitation and scatterings and differences between the alignment of the two monochromators used limits the accuracy of stated band locations to +/- 3 cm". ... 

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