Ranges of stop sign chromaticity under tungsten-halogen and high-intensity discharge illumination

Abstract

The development of high-intensity discharge (HID) sources for automotive headlighting has raised questions about the rendering of colors by those sources, relative to the rendering of colors by tungsten-halogen sources. The rendering of sign colors has been of concern because color is used as an indicator of sign functions. The rendering of the red of stop signs has been particularly of concern because consistent recognition of stop signs is critical for safety. This study was designed to compare the magnitudes of the shirts in chromaticity of stop signs illuminated by HID and tungsten-halogen headlamps to the range of chromaticities of stop signs under tungsten-halogen. A stratified sample of 25 stop signs was selected in Ann Arbor, Michigan. Stratification was by material (enclosed, encapsulated, microprism), age(more than five years in service, less than one year in service), and compass direction faced by the legend side of the sign (north, south). The chromaticities of these signs were measured in the field under both tungsten-halogen and HID (D1) illumination. The shifts between the chromaticities of the signs when they were illuminated with the tungsten-halogen source and whey they were illuminated with the HID light source were moderate relative to the range of chromaticities under tungsten-halogen. We argue that, although the sizes of the shifts should not be interpreted as indicating a problem with color rendering by HID headlamps, they are not small enough to allow us to dismiss the possibility of a problem.