Evaluation of recent U.S. tungsten-halogen and HID headlamps using CHESS

Abstract

Headlamps with high-intensity discharge (HID) sources produce more light than lamps with tungsten-halogen (TH) sources. The extra light offers the possibility of improving a driver’s ability to see at night, which in turn may lead to better safety. Because low-beam headlighting necessarily involves a tradeoff between seeing and glare control, the extra light has also raised questions about the glare from HID headlamps. However, photometric analyses of the possible benefits of HID headlamps suggest that, in terms of the test points in U.S. Federal Motor Vehicle Safety Standard 108 (FMVSS 108), HID headlamps achieve both greater seeing light and lower glare levels. The fact that HID lamps produce less glare in formal terms is at odds with the popular impression that they are more glaring than TH headlamps. One possible reason for this is that the test points in FMVSS 108 represent headlamp performance in static terms, without explicitly considering vehicle movement and road geometry. In contrast, the Comprehensive Headlamp Environment Systems Simulation (CHESS) model that was developed in the 1970s by Ford Motor Company is an attempt to evaluate headlighting performance under a wide range of real-world conditions. In this study, we applied the CHESS model to headlamps from surveys of U.S. headlighting for the 2004 model year that included TH and HID lamps. The results indicate that: (1) HID headlamps as a group achieve substantially higher scores than TH lamps in terms of the CHESS overall figure of merit, which is intended to balance seeing and glare effects, (2) TH lamps produce a large range of discomfort glare effects and the HID lamps are within that range, although at the high end, and (3) differences among lamps, and the advantage of HID lamps over TH lamps, are more pronounced in the CHESS subscales that are intended to measure pedestrian visibility than in the CHESS overall figure of merit.