Determination of Time and Illuminance for the Beginning of Mesopic Vision under the Daylight

Abstract

Change of spectral luminous efficiencies for brightness was investigated under the daylight where the illuminance varied from about 3, 000 lx down to 0.1 lx in the time course of evening of February. The experimental method was to match the brightness of test stimuli of blue and orange colored papers with a gray scale and to express the brightness by the equivalent value of the gray scale, V_B or V_OR, respectively. The lightness remained about the same until 17: 25 hrs, when a change started such that V_B increased and V_OR decreased to indicate the beginning of the mesopic vision. The change ceased at around 17: 55 hrs by giving the increased amount of V_B of about 1 and the decreased amount of V_OR of about 2.5. The illuminance was about 10 lx when the mesopic vision started. Such mesopic vision never started at the down area where artificial illuminations were abundant.