Measurement of Low Light Levels by the Photon Counting Method

Experiments are described using electron counting techniques for measuring low light levels with photomultipliers. An accuracy of 1 per cent has been achieved in measuring cathode currents of the order of 10 −15 amp. or less with the photomultiplier cooled to — 183°C. The luminous flux falling on the cathode of area 2.4 sq. in. was c.10 −11 lumen. At higher levels, the limitations in resolution of the counting equipment available caused 20 per cent loss in counts at cathode currents of 10 −13 to 10 −14 amp. The limitations of the technique for photometric applications are discussed, together with the statistical considerations governing the resolution times necessary for the amplifier and counting circuits in order to increase the range over which the counts are proportional to the luminous flux incident on the cathode.