Chromatic aberrations near the cutoff of low-beam headlamps

Abstract

Projector low beams are subject to color aberrations near the vertical cutoff caused by dispersion of light when passing through the lens. Color aberrations are especially of concern with high-intensity discharge (HID) lamps, because these color changes likely contribute to the discomfort-glare complaints for HIDs. The purpose of this study was to evaluate the extent and magnitude of color aberrations in HID and tungsten-halogen projector low beams. Specifically, we made colorimetric measurements near the cutoffs of 8 projector HIDs and 4 projector tungsten-halogens. For experimental control, we also evaluated 9 nonprojector HIDs and 5 nonprojector tungsten-halogens. Major color changes were present for 38% of the HID projector lamps, 50% of the tungstenhalogen projector lamps, and (as expected) for 0% 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 aberrations 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 aberrations—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 aberrations in the regions of the beam pattern with substantial luminous intensity levels that may be directed toward oncoming drivers.