A Photoelectric Method of Measuring Directly the Ratio of Two Illuminations

An arrangement is described which permits direct measurement of the ratio of two illuminations. It makes use of the approximately logarithmic voltage-current characteristic for gasfilled photocells. The circuit is independent of large variations in the absolute value of the light intensities used, and the ratio of the illuminations can be varied over a range from 0.25 to 4. This range can be widened by the use of shutters excluding part of the more intense light. The circuit can be used with advantage in measuring, for example, the reflexion or absorption of light independent of the intensity of the light source.