The primary purpose of vehicle headlamps is to provide forward visibility for the driver; this is especially important considering that most roadways are unlighted, so that headlamps are often the only source of illumination present. Despite the inherent limitations of low beam headlamp patterns (Andre and Owens, 2001) caused by the necessity for glare control, high beam headlamp patterns are not frequently used (Sullivan et al., 2004). In recent years, headlamp technologies have evolved quite rapidly to include high-intensity discharge (HID) and light- emitting diode (LED) sources. HID headlamps, for example, produce about twice the light output of halogen headlamps presently most common on vehicles (Jost, 1995); this has implications for the light distribution from HID headlamps compa- red to halogen ones. In general, luminous intensities in the central portion of the low beam pattern do not differ greatly between halogen and HID types (although HID intensity values are somewhat higher [Akashi et al., 2008]). However, in the peripheral portion of the beam pattern, HID headlamps tend to have significantly higher light output with the result that they produce longer detection distances (Hamm and Steinhart, 1999) and shorter response times (RTs) (Van Derlofske et al., 2001, 2002) especially to peripheral targets.
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