Daylight factor meter

Abstract

A brief survey and criticism of existing methods of measuring daylight factors is followed by a description of an improved daylight factor meter, in which the current outputs of two photoelectric cells are compared by a null-balance method. When one cell is exposed inside the building and the other outside, the ratio of the illuminations at the two positions is indicated directly on a calibrated scale by the balance position of the potentiometer. The circuit is so arranged that the load resistances of the cells are unchanged by changes in the balance position and, for a constant daylight factor, the balance position is not affected by changes in the average luminance of the sky. The instrument can be calibrated directly on an optical bench, either for use with the outside cell exposed to a full hemisphere of sky or for use with the outside cell so masked that light is only received from a part of the sky.