Chopped light system

With gas-filled detectors, a chopped light system is normally used in which one side of the detector sees the source through the analyzing beam and the other side the reference beam, alternating at a frequency of a few hertz. 

Most experiments were performed with n-type semiconductors because it was easier to detect small ring currents upon addition of a redox system to the solution by using a modulation technique (see Section 4.2.3). A typical result, as obtained with chopped light, is shown in Figure 8.17. A selection of data evaluated... 

Fig. 2.17. Schematic layout of a microscope spectrophotometer system used to measure polarized absorption spectra of very small mineral crystals. The computer-operated, single-beam instrument shown here comprises a polarizing microscope equipped with a stabilized light source (xenon arc lamp or tungsten lamp cover the range 250-2000 nm), a modulator that chops the light beam with a frequency of 50 Hz (the amplifier for the photodetector signals is modulated with the same phase and frequency), and a Zeiss prism double monochromator. Single crystals as small as 10 ji.m diameter may be measured with this system. A diamond-windowed high-pressure cell can be readily mounted on the microscope scanning table for spectral measurements at very high pressures (after Burns, 1985, reproduced with the publisher s permission).

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