A method of correcting the cosine error of selenium rectifier photocells

This paper describes a simple and effective method for correcting the deviation from the theoretical value of the response of selenium rectifier photocells to obliquely incident light. The technique has been applied successfully to the measurement of daylight illumination in model rooms, at the Royal Institute of Technology, Stockholm, and more recently at the Building Research Station, Watford. The construction of the "cosine-correcting" unit is described, and the principle of operation whereby the degree of correction can be controlled to suit different types of cells, is briefly outlined. The unit has been designed primarily for use in model rooms, and for this reason it has been made as compact as possible. It has applications in all branches of photometry and illuminating engineering where the accurate measurement of light is of fundamental importance.