Influence of Sign Retro-Reflection with Vehicle Headlights under Mesopic Vision Conditions

Abstract

Coefficient of retro-reflection (COR) is one of the most important parameters in evaluating visual performance of highway signs. The value of this parameter is influenced by geometric condition, material structure, and so on. For the various retro-reflective materials characteristic different reflectance spectra, these materials show colors as white, red, etc., with the lighting from headlights. In recent years, Xenon and white light emitting diodes (LEDs) were used as vehicle headlights, compared with Halogen lamp (light source A), the reflectance spectra on the same retro-reflection sign are not the same. For the region of reflective brightness, the mesopic vision theory was used to analysis the COR influenced by headlights. Based on existing test conditions the mesopic spectral luminous efficiency and equivalent brightness were calculated. The results show that the gain of equivalent visual brightness depend on surface brightness, overlay of sources, and reflectance spectra; light sources with wide-band spectra show higher gain level than traditional light sources.