A SYSTEM OF MESOPIC PHOTOMETRY

This paper describes a system of photometry designed to make no unjustifiable assumptions, and to have a strong visual linkage. In spans the photopic, mesopic, and scotopic regions and is based on brightness matching a 10-degree field, centrally fixated. An intensive investigation was made on the trichromator at the National Physical Laboratory, Great Britain, and an extensive one, using a greater number of observers, on the trichromator at the National Research Council, Canada. A bipartite field allowed equal brightness settings of the aperture colour by each of nine observers. The stimuli tested were both spectral and nonspectral and the data were used to derive formulae which a user could employ. From inserting conventional photometric quantities of a stimulus, which might for example be a light source, the user can obtain its corresponding photometric value whether it be photopic, mesopic, or scotopic.