A description is given of a photoelectric recording spectrometer for Raman spectroscopy incorporating a high-intensity excitation system and a novel amplification device. Operation of the exciting mercury arc lamps from 50 c/s. mains imparts to the light signal, and consequently to the current from the photo-multiplier tube, a 100 c/s. periodicity. After amplification by a narrow band-pass amplifier, rectification is achieved in a push-pull phase-sensitive (homodyne) rectifier, by "mixing" the signal with a fixed voltage of identical periodicity. The rectified signal is fed through a filter to a cathode follower valve which provides the power to operate a recording milliammeter. This method of rectification, in conjunction with appropriate filter, effectively reduces the bandwidth of the amplifier, with consequent great improvement in the signal/noise ratio. With this arrangement cooling of the photo-multiplier is unnecessary for normal work. Using a spectrometer of small aperture and resolution and a Raman tube of 35 ml. capacity, traces of the Raman spectrum of carbon tetrachloride show no appreciable background noise.
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Miller et al. (1949) studied this question.
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