Veiling luminance and visual adaptation field in mesopic photometry

Abstract

In mesopic photometry, adaptation luminance is needed to derive the mesopic luminances for the measurement field. The average luminance of the visual adaptation field is considered as the adaptation luminance. The visual adaptation field has yet to be defined in terms of the size, shape, or location within the visual field. A study in three road lighting situations was conducted, in order to determine the feasibility of using the road surface as the adaptation field compared to circular or elliptical adaptation fields. Currently, the road surface is used as the measurement field for calculating road lighting. Using the road surface as the adaptation field resulted in 76–113%, higher average luminance than obtained using circular or elliptical adaptation fields when the road was bordered by a park. High-luminance sources outside of the visual adaptation field cause veiling luminance. Veiling luminance increases the adaptation state, but not the luminance within the measurement field. The bias veiling luminance can cause on mesopic luminance calculations was estimated to be less than 2%. The estimated bias can be considered trivial in practical road lighting measurements.