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Measurements in such fields as photometry, spectrometry, and astronomy, dealing with very low light levels, may be assisted by the use of photo-multiplier tubes. Spectral characteristics of visible (S-8), blue (S-4), and ultraviolet (S-5) sensitive multiplier tubes are presented. Data are shown illustrating the wide range of linearity of photo-multiplier types 931-A, 1P21, 1P22, and 1P28. The tubes are linear at currents up to the point where space charge in the output stages reduces the ratio of anode current to cathode light flux. Dark currents and the limitations which they introduce in practice are discussed. Fatigue effects are discussed and data shown which indicate good stability at low current levels. The ultimate sensitivity of these tubes when refrigerated to various degrees has been investigated; with suitable precautions, pulses attributable to individual photoelectrons can be counted.
R. W. Engstrom (Sun,) studied this question.