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Figure 1—The percentage of missed target presentations as a function of anglefrom the line of sight. Targets were positioned at 60 m away and illuminated bytypical halogen and HID headlamps.angles from the drivers’ line ofsight. 3As far as peripheral visual performanceis concerned HID lamps differfrom conventional halogen headlampsin two important aspects:total light output and spectral composition.It is the first of these, moretotal light output (HID lamps produce2-3 times more light than traditionalhalogen sources), that isprobably the primary factor in theincrease in performance. Greatertarget illuminance results in increasedperformance.Why would this be particularlytrue at the edge of the beam?Wouldn’t the extra light be distributedevenly through the beam pattern?Regulations for headlampshave strict requirements for distributionin the central part of the headlampbeam, which tends to makethese central portions very similar,regardless of source type. The lightdistribution at the edge of the headlampbeam pattern, however, is notas well regulated. Therefore theextra light produced by HID sourcesoften ends up in the periphery, resultingin a wider beam and improvedperipheral visual performance.Although target illuminance andtherefore beam pattern is arguablythe most important factor whenconsidering headlamp visual performancethe role of light spectralcomposition cannot be entirelyoverlooked. Another recent studywas performed to determine therole that HID lamp spectrum playson visual performance. 5 This studyoffers evidence that HID lamp spectracan produce small but measurableimprovements over halogensystems for off-axis visual performance.The exact magnitude ofthese differences depends on theoff-axis angle, the light level and thetarget contrast.Do HID headlamp systems producemore glare?It depends on how glare isdefined. Generally, two types of glareare recognized: discomfort glareand disability glare. Disability glareis defined by the direct reduction ofvisual performance. Light from glaresources get scattered in the eye andis perceived as a luminous veil overthe scene. This veil reduces the contrastof objects and hence their visibility.Discomfort glare is a sensationof annoyance or distraction thatis often measured by means of asubjective rating scale. Discomfortglare does not necessarily impairthe visibility of objects.Recent studies have shown whileHID lamps do not result in any increaseddisability glare they do resultin a significant increase in discomfortglare. 6,7 That is, for thesame illuminance at the eye, HIDheadlamps do not directly reducevisual performance any more thantraditional halogen sources, butthey do produce a greater feeling ofdiscomfort. Disability glare is generallymeasured by foveal or on-axisperformance, which follows thestandard photopic (cone) spectralresponse. Therefore, for equal illuminanceat the eye you wouldexpect equal disability glare.Discomfort glare, however, doesnot seem to follow a photopicspectral response or even a scotopic(rod) response. In fact, littleis known about the basic mechanismsbehind discomfort glare orits spectral response. It does seemthe short wavelength or “blue”content of a light source affectsthe discomfort glare it produces,with greater short wavelengthenergy resulting in more discomfortglare. HID headlamp sourceshave more energy than halogenlamps in the short wavelengthregion of the visible spectrum.Current research is trying to determinethe exact spectral responsefunction for discomfort glare. It isinteresting that even thoughimproved peripheral vision and discomfortglare both depend onspectral content, and both appearto increase with more energy inthe short visible wavelengths, thetwo responses are not the same.That is, increased visual performanceat low light levels does nothave to mean more glare.Note, however, that even thoughdiscomfort glare does not directlyimpact visual performance it doesnot mean that it is not important.Discomfort glare can indirectlyaffect performance by causing a driverto look away from an area of theroadway that should be paid attentionto or by otherwise adverselyaffecting behavior. Prolonged exposureto high levels of discomfortglare may also have effects overtime and could possibly lead togreater fatigue.Can visibility and glare be balancedfor HID headlamps?Yes. However, there is no singlesolution to this challenge. Automotivelighting designers will continueto struggle to balance the visualbenefits and potential drawbacks ofHID headlamps, at least in the nearfuture. This was the case in the pastfor sealed beam halogen lamps andwill be in the future for other newheadlamp sources, such as whiteLEDs. Advances will take place insource design, optical design, beamshaping and system intelligencethat will all contribute to optimizedsystems. However, research will bethe key in this process to definingwhere the balance point should beand how to strike it. For example,determining the exact spectral12 LD+A/May 2003 www.iesna.org