Elimination of stray light

An important aspect of FT-Raman instrumentation is the necessity for optical filtering. The first task is to eliminate the stray light caused by the laser excitation because it will saturate the detector and electronics. The filtering must be capable of reducing the Rayleigh line, which is 106 stronger than the Stokes-shifted lines in the Raman spectrum. In order to be sufficiently... 

Apart from the fact that some of the analyte elements were not detected at their normal concentration levels in urine, the main feature of the analytical results was their overall consistency. The results from the method of additions agreed well with those from the calibration approach and, in either case, the results obtained for the dilute, normal, and concentrated sample solutions also were in good agreement with each other. With the exception of arsenic and titanium, the results for the two methods and for the three urine concentrations were within one detection-limit concentration of each other for all of the elements. Although it has been noted (25) that detection-limit values are "inherently imprecise numbers and that detection-limit concentrations "can only be detected,. . . , and not measured quantitatively, the consistency of the analytical results indicates that the backgroimd correction scheme was effective for elimination of the eflFects of stray light and recombination radiation. As noted earlier, the ratios of net analyte line to net internal reference line intensities were used to decrease the eflFects of sample-to-sample variations in total dissolved solids content. 

The major advantages of FTIR instruments over dispersive spectrometers include better speed and sensitivity, better light-gathering power, more accurate wavelength calibration, simpler mechanical design, and the virtual elimination of the problems of stray light and IR emission. Because of these advantages, nearly all the new IR instruments are FTIR systems. 

Prolonged photobleaching in the infrared is carried out through the monochromator plus a silicon filter which eliminates any stray light of wavenumber greater than about 9000 cm."1. 

Figure 2. Optical schematic of the chamber for grazing angle measurements. Heaters and one thermocouple are located directly behind the sample (A). Gas inlet and outlet are near the IR transmitting windows (B). The mirrors (C) can be rotated and tilted to maximize signal and eliminate stray light. The entire cell (D) can be translated to change the angle of incidence.

Simultaneous with the publication of Hocker et al., there appeared the results of Yardley and Moore [142] on laser-excited vibrational fluorescence in CH4. A mechanically chopped He-Ne 3.39-micron laser [143, 144] was used to excite the asymmetric stretching [/ = 2948 cm-1 (36.55 X 10-2 eV)] vibration, i>3 (see Figure 3.17). The optical arrangement is shown in Figure 3.18. The He-Ne laser tube, 220 cm in length, is shown on the left. Mx, M2, and Ms are mirrors Bx and B2 are baffles to eliminate stray light Lx and L2 are lenses which focus the laser output into a collimated beam having a diameter of 2 mm, and thence, into a Pyrex fluorescence cell. At the focal point between Li and L2 is a chopper wheel, to produce a nearly perfect square wave modulated at frequencies between 600 and 10,000 Hz. An audio oscillator and a 60-W amplifier are used to drive the synchronous chopper motor. An InSb infrared detector (response time of about 4 nsec) is used to... 

Time-resolved fluorimetry is also useful for the elimination of interferences from stray light caused by Rayleigh and Raman scatter. The latter phenomena occur on a time scale of because they have... 

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