Color Separation near the Cutoff of Low-Beam Headlamps

Abstract

Projector low beams are subject to color separation near the vertical cutoff caused by dispersion of light when passing through the lens. Color separation is especially of concern with high-intensity discharge (HID) lamps, because the resulting changes in color appearance to oncoming drivers likely contribute to the discomfort-glare complaints for HID lamps.The purpose of this study was to evaluate the extent and magnitude of color separation in HID and tungsten-halogen projector low beams. Specifically, we made colorimetric measurements near the cutoff of 8 projector HID and 4 projector tungsten-halogen lamps. For experimental control, we also evaluated 9 nonprojector HID and 5 nonprojector tungsten-halogen lamps.Major color changes were present for 38 percent of the HID projector lamps, 50 percent of the tungsten-halogen projector lamps, and (as expected) for 0 percent of the HID or tungsten-halogen nonprojector lamps. Transformation of the chromaticity data into a perceptually uniform color space indicated that the color changes caused by separation near the cutoff were often considerably greater than the typical color differences between tungsten-halogen and HID sources. This suggests that some of the recent concern about headlamp color and glare on the part of the driving public may be caused by color separation—in either tungsten-halogen or HID lamps—rather than by HID lamps themselves. To minimize discomfort glare (especially for HIDs), it is important to avoid color separation in the regions of the beam pattern with substantial luminous intensity levels that may be directed toward oncoming drivers.