Photoelectric Photometers: Their Properties, Use and Maintenance

Abstract

This Paper examines the problems arising in the application of emission photocells and selenium rectifier photocells to photometry. It deals with the difficulties of securing a close approximation to a linear scale, and to the spectral sensitivity of the standard eye; and also with the life, stability, and fatigue of photocells, temperature coefficient, dark-current, non-uniformity over the area of the sensitive surface, stray radiation, and the use of photoelectric instruments for measuring fluctuating light sources. In the case of rectifier cells the questions of the response of the cell to oblique illumination and of the size of cell best suited to any given purpose are also discussed. A number of colour correction filters proposed for use with both types of photocell, are cited.The principles and methods of use of standard types of photometer circuits are described and critically discussed. The circuits included are the direct-reading and the integrating circuits for emission cells, and the light-meter, Campbell-Freeth, and balanced-cell circuits for rectifier cells. As an example of a complete instrument a new portable emission-cell photometer is described in which the amplifier circuit, comprising two stages of D.C. amplification, includes an electrometer tetrode, and gives a current gain of 2.107 with stability adequate for measuring photocurrents down to 2.10−14 ampere to within ±5%.The Paper is an attempt to collate the information necessary to give the user a clear understanding of the possibilities and limitations of photoelectric photometers, and of the precautions necessary to ensure results of the highest possible accuracy. Selection of this information is based on experience of the last ten years at the National Physical Laboratory. Little that is novel is described, except the circuit for use with the electrometer tetrode, and work, as yet incomplete, on the fatigue of rectifier cells under monochromatic light of different wavelengths.