Pulse Counting Photometer for Microspectroscopy

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Abstract
A photometer for recording the absorption spectra of microscopic samples in the visible and ultraviolet regions is described. Light intensities are measured by counting the pulse output of a cooled photomultiplier tube. Double‐beam operation is achieved through the use of a rotary shutter and a gating circuit which sorts the beams into separate scaling channels. Optimum signal‐to‐noise levels are obtained by pulse‐height discrimination. In typical operation the noise level is approximately 20 photoelectrons per minute, the signal level at 100% transmission is approximately 20 000 photoelectrons per minute, and approximately 50 absorption points are recorded per hour at an accuracy of ±1%. The system minimizes the errors which result from fluctuations in light source intensity, amplifier drift, and variations in photomultiplier plate voltage so that accuracies of ±0.1% can be achieved with the photometer (but not with the microspectrometer). With appropriate correction for noise and resolving time the system can be operated over four decades of light intensity. Application of this type of photometer to other spectrographic problems is suggested and representative data are presented.

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