The definition of a standard observer for mesopic photometry.

Abstract Eighteen persons have matched test lights of various spectral distributions to a standard white by the method of direct comparison at levels throughout the mesopic range. The results support the system of photometry suggested by Palmer (1966) and could be used as the basis of a standard mesopic observer at least as satisfactory as the existing scotopic and photopic observers. A method of allowing for the effect of field size is described.