Resolution with interference filters

Due to the rather stringent requirements placed on the monochromator, a double or triple monocln-omator is typically employed. Because the vibrational frequencies are only several hundred to several thousand cm and the linewidths are only tens of cm it is necessary to use a monochromator with reasonably high resolution. In addition to linewidth issues, it is necessary to suppress the very intense Rayleigh scattering. If a high resolution spectrum is not needed, however, then it is possible to use narrow-band interference filters to block the excitation line, and a low resolution monocln-omator to collect the spectrum. In fact, this is the approach taken with Fourier transfonn Raman spectrometers. 

Any recording spectrophotometer or spectralline photometer is suitable for oxygen consumption measurements if meets three important requirements (1) high monochromator resolution (2) sensitivity and (3) stability. Since hemoglobin derivatives have rather narrow absorption bands in the visible spectrum, a perfect calibration of the monochromator is necessary. Spectral-line photometers using low-pressure mercury lamps in conjunction with appropriate interference filters are well suited for utilization of the Hb02 method, as there are three different filters for the... 

Spectral line interferences also can occur when spectral lines of two or more elements are close but do not produce an actual overlap of their energy envelopes. This type of interference is especially troublesome when the spectral isolation device is a filter. With a filter, lines separated by as much as 50-100 A may be passed through the filter to the detecting circuit, thus producing an incorrect read-out signal. As the resolution of the spectral isolation system increases, such interference possibilities decrease. They cannot be eliminated entirely, however, because of the finite width of the spectral isolation system and the finite slit widths required in such systems. 

In many combustion processes OH radicals are produced as an intermediate product. These radicals can be excited by a XeCl laser at 308 nm. The UV fluorescence of OH can be discriminated against the bright background of the flame by interference filters. A possible experimental setup is depicted in Fig. 10.29. The beam of the XeCl laser is imaged into the combustion chamber in such a way that it forms a cross section of 0.15 x 25 mm. A CCD camera (Vol. 1, Sect. 4.5) with UV optics and a gate time of 25 ns monitors the OH fluorescence with spatial and time resolution [1488]. 

To increase the sensitivity a long cavity length is used, sometimes with multiple passes, achieved by reflection from a corner cube or concave mirrors (25). Mechanical design of a stable cavity is critical for multiple reflections and to refocus the light back into the small core of a fibre. The optical measurement system generally includes an IR LED, interference filter and dual photodetectors (25) and a differential absorption technique for signal and reference channels (24). Further developments need to be made to provide a stable high-resolution optical detection system at low cost. 

The range of applications of Raman spectroscopy has also been extended by several important recent developments, such as Raman microscopy, which makes it possible to study extremely small samples. One can also analyze the surface of an extended inhomogeneous sample to obtain very high spatial resolution, or scan across a surface using fiber optics. It is also possible to use specially developed interference filters or holographic notch filters in certain applications as an alternative to a dispersing spectrometer, provided that one suppresses the fluorescence that would otherwise interfere with the measurements. 

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