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By John Bullough and John Van Derlofske, Rensselaer Polytechnic Institute shedding light on GLAREGLARE

Analysis he prevalence of so-called blue headlamps is increasing on North reveals the T American highways. These head- advantages lamps typically use high-intensity discharge (HID) or blue-filtered halogen and sources, which in comparison to conven- looking forward drawbacks of tional halogen headlamps have a higher Regulations for low-beam headlamp luminous intensity have correlated color temperature resulting in a fairly strict requirements on the necessary shape of the headlamp high-intensity cooler color appearance. HID sources, or beam pattern in the central part of the beam. The requirements blue metal halide sources, have higher lumi- are less stringent in the periphery of the beam, where such nous efficacy, longer life, and greater light illumination is most likely to cause glare to oncoming drivers. headlamps. output than conventional headlamps.1 Because of these regulations, the luminous intensity of HID The blue-filtered halogens have about the headlamps is usually similar to that of conventional halogen same light output as halogen headlamps headlamps near the center of the beam, even though HID and slightly lower luminous efficacy headlamps typically produce twice the light output as halogens. because the filters absorb some light, and The remaining light output is generally directed to the beam the lamp wattage is generally increased to periphery (see figure 1 on page 18). compensate for losses by the filter. Given the distributions of light shown in figure 1, it should not The U.S. National Highway Traffic be surprising that HID headlamps tend to result in faster and Safety Administration (NHTSA) more accurate detection of peripheral targets found beyond 5° to published a request for public comments the left or beyond 10° to the right of the driver’s line of sight.2 on headlamp visibility and glare (log on to But is the improved peripheral detection performance with HID dms.dot.gov, click on “simple search,” and headlamps caused solely by their greater peripheral light output? enter 8885 for the docket number), and The spectral sensitivity of the peripheral retina at light levels received more than 4000 responses, most typically experienced during nighttime driving is not of them complaining about increased glare characterized solely by the photopic luminous efficiency function apparently caused by these new V(λ), which represents the combined spectral sensitivity of the headlamps. At the same time, a small but cone photoreceptors in the retina (and has peak sensitivity near vocal minority—generally people who 550 nm), nor by the scotopic luminous efficiency function V'(λ), have used the new lamps while driving at which represents the spectral sensitivity of the rod photoreceptors night—are in favor of the new in the human eye (with peak sensitivity near 500 nm; see figure 2 technologies. The Lighting Research on page 18). Rather, the peak sensitivity of the peripheral retina Center (LRC) at Rensselaer Polytechnic at typical nighttime light levels is between 500 and 550 nm, Institute (Troy, NY) has been conducting representing the combined spectral sensitivity of the rods and research to help find the proper balance cones working together. between the potential visual benefits of Light sources with higher rod-stimulating (scotopic) spectral these lamps and possible drawbacks. content have been shown to result in improved peripheral

june 2003 | spie’s oe magazine 17 with blue-coated halogen headlamps, developments, including HID and blue- compared to conventional headlamps filtered halogen headlamps, history seems producing the same amount of light and to be repeating itself. But at this point, no having nearly identical beam patterns.4 one can really say whether the controversy and excitement surrounding these glaring problems technologies is much ado about nothing. Glare is categorized into two types, Future research will assist in the depending upon its effects.5 Disability optimization of the headlamp system to glare refers to a luminous veil over the maximize forward visibility while field of view caused by scattered light in minimizing glare. oe the eye that reduces contrasts in the visual scene and degrades visual performance. John Bullough is a SPIE Figure 1 Plots of illuminance as a Discomfort glare refers to the painful or lighting scientist member function of angle from headlamps on annoying sensations a bright light can and John Van vertical targets positioned at ground level, cause. Generally, these two phenomena Derlofske is a senior research scientist at the 60 m ahead, demonstrate that significantly are often coincident, especially for the case Lighting Research Center at Rensselaer more illuminance is directed into the beam of headlight glare, but this is not always Polytechnic Institute, Troy, NY. For periphery for HID sources compared to the case. A window in an office can create questions about this article, contact the conventional halogen lamps. Negative disability glare without obvious authors by phone: 518-687-7100; fax: angles refer to the driver’s side, positive to discomfort; conversely, a bright light 518-687-7120; or e-mail: [email protected]. the passenger’s side, and 0° is directly could be uncomfortable without ahead of the vehicle. necessarily impairing visibility. Acknowledgments Recent data collected by the LRC The LRC research studies described in this during field studies conducted for article were supported by NHTSA under NHTSA has shown that the spectral the direction of Michael Perel, by Philips power distribution of oncoming Automotive Lighting under the direction of headlamps makes no significant difference Josef Schug, and by OSRAM SYLVANIA in terms of their effect on the detection of under the direction of Susan Callahan. targets, whether the targets are near or far from the line of sight. The same is clearly References not true, though, for discomfort glare. 1. Jost, K. Automotive Engineering, 103 Using a subjective rating scale to quantify (11), p. 38 (1995). discomfort from oncoming headlamps 2. J. Van Derlofske, J. Bullough, et al., SAE during the same field studies, observers paper 2001-01-0298, Lighting Technology consistently rated HID headlamps as Developments for Automobiles, pp. 1-7, SP- more uncomfortable than halogen or 1595. Warrendale, PA: Society of Figure 2 A comparison of the photopic blue-filtered halogen headlamps. These Automotive Engineers (2001). luminous efficiency function V(λ) and the results are consistent with many other 3. Y. He, M. Rea, Journal of the scotopic luminous efficiency function studies of discomfort glare and headlamp Illuminating Engineering Society 26(1), pp. V'(λ) shows that these mechanisms spectral power distribution.6 The visual 125-138 (1997). have differing spectral sensitivities. mechanism for discomfort is still not well 4. J. Van Derlofske, D. Dyer, et al., SAE understood. One hypothesis is that short- paper 2003-01-0930, Lighting Technology, wavelength cones (which have peak pp. 117-124, SP-1787 (2003). detection at these so-called mesopic light spectral sensitivity at 440 nm) play some 5. M. Rea (ed.), IESNA Lighting levels.3 From a practical point of view, role in discomfort glare, since ratings of Handbook: Reference and Application, 9th however, the difference between current discomfort are highly correlated with the ed. New York, NY: Illuminating HID headlamps and conventional energy from a glare source near 440 nm.7 Engineering Society of North America halogen headlamps is so small as to be It also remains to be seen whether the (2000). unimportant—HIDs have about 5% increased discomfort actually has any 6. M. Sivak, M. Flannagan, et al., SAE higher scotopic content than halogens for indirect negative effects on driving paper 2003-01-0295, Lighting Technology, equal light levels, although it is worth performance. pp. 15-20, SP-1787 (2003). noting that HID lamp chemistry could For decades, incandescent headlamps 7. J. Bullough, Z. Fu, et al., SAE paper possibly be altered to increase scotopic were the only types available. In the 1960s 2002-01-0010, Advanced Lighting content. This effect might not be and 1970s, as halogen headlamps with a Technology for Vehicles, pp. 1-5, SP-1668 unimportant for blue-filtered halogen slightly “whiter” appearance became (2002). headlamps, however, which can have 25% available, complaints about increased glare higher scotopic content than unfiltered as well as reports of improved visibility Have a comment on this article? Go to halogen lamps at equal light levels. abounded. As halogen headlamps became the oemagazine discussion forum at Indeed, the LRC has demonstrated more common, the hubbub surrounding spie.org/app/forums/oemagazine. improvements in peripheral detection their use died down. With more recent

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