Filters, light interference

The contribution of spectrometer surface scatter depends upon the design, quality, and alignment of the spectometer components. Surface scatter may originate from the grating, slits, mirrors, refractor plates, photomultiplier(s), light baffles, interference filters, dust, and the spectrometer interior surfaces. Essentially all internal surfaces may contribute to scatter. 

Fig. 10.2 Schematic diagram of a reflected interference microscope 1 - tight source, 2 - heat reflecting filter, 3 - coUimator, 4 - diaphragm, 5 - light filter, 6 - photo film, 7 - projection ocular, 8 - semi-transparent silvered mirror, 9 - aperture diaphragm, 10- auxiliary lens, 11 - immersion lens, 12 - sample under investigation, 13 - substrate...

Unlike colour filters, interference filters and gratings can lead to some optical artefacts of which all workers in this field should be aware. All processes of interference of light waves depend on the difference in optical path between the beams, as illustrated in Figure 7.7. The general law for constructive interference is then... 

Currently, FPDs are made to detect sulfur and phosphorous compounds through specific wavelength filters, and are able to detect respective CWA compounds such as the G-agents, mustard, and certain TICs, including PH3, SO2, and HjS. Since other chemicals that do not contain these two elements do not emit these characteristic wavelengths, they do not interfere with detection. With the recent interest in detection of TICs, FPDs are of potential usefulness because various light filters can be installed to permit detection of other elements as well. 

A monochromator which isolates a narrow band of light. Glass or gelatin filters, interference filters, diffraction gratings or prisms can be used to isolate a narrow wavelength band of light. 

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