Alliance for Lighting Information

The XENON Headlight Issue

by Jefferey F. Knox

In recent years the use of xenon headlights has increased significantly. Many people, myself included, find them to be very uncomfortable and terrible glare sources. Many people have argued vehemently that the reason for this is that they produce two to three times the light of a halogen headlight and that increased lumen output is the cause of the problem. Well, as with many lighting ordinances, this headlight glare discussion tries to assign a component attribute (lamp lumen output) to a systems (glare) problem.

First, let me address some of the misleading numbers. The lamps with color temperatures of 5400 Kelvin and 6000 Kelvin (no degrees - Kelvin is an absolute scale) are for lamps categorized and labeled as DR lamps. The DR stands for Daytime Running lamps - these lamps are not approved for nighttime usage. The xenon lamps that are approved for nighttime use all have a color temperature of 4100 Kelvin - not too far from metal halide sources. These lamps are rated at 3200 lumens. While some older halogen headlights produce around 1000 lumens most of the newer sealed beam lamps have a significantly higher lumen output. Both of my cars use H7 lamps that produce 1500+ lumens.

Next, lets address the glare issue, but before we can understand glare we must first understand a few technical properties of light.

The purpose of a lamp is to convert electrical energy into luminous flux (light). Luminous flux is quantified by the unit known as a lumen. We do not see raw lumens, in fact we cannot see light until it interacts with a surface or material and then what we see is the light reflected, transmitted or generated by the surface. In the case of a bare lamp what we see is the filament, plasma, or the lamp envelope. Luminous intensity is defined as the amount of luminous flux (lumens) produced in a given direction - the unit for luminous intensity is the candela (lumens per steradian, sometimes called candlepower). Luminous intensity indicates the ability of a light source to produce illumination in a given direction. The light we actually see is known as luminance (brightness) which is the luminous intensity per projected area off a surface - this is quantified with the unit of the footlambert which is a directionally dependent unit. As the direction of view changes so can / does the luminance of a surface. Everyone has experienced this with a glossy magazine and a downlight. There is an angle between the light source and the magazine that makes it very difficult to see the page because it is too luminous (bright), but if you tilt the magazine, effectively changing the viewing angle, the page becomes visible.

The technical definition of glare implies very high contrast. Contrast is defined as the (luminance of the task (source) minus the luminance of the background) divided by the luminance of the background. This is basically a ratio of how bright the source or surface is when compared to the background it is viewed against. There are two ways to control glare. One is to decrease the luminance (brightness) of the source. The second is to increase the luminance (brightness) of the background. As an example consider the glare from headlights during the daytime versus the glare from the same headlights on an unlighted street at night.

Now to the headlight issue itself. The Federal Motor Vehicle Safety Standard 108 and the Society of Automotive Engineers Specification J2009 regulate the design of automobile lighting systems. These documents state a maximum candlepower that a headlight may produce above horizontal. This maximum candlepower is regardless of the source. This means that a xenon headlight can produce no more luminous intensity above horizontal than can a halogen headlight. If we assume the area of a headlight reflector system is equal regardless of the source then the luminance of a headlight viewed from above horizontal can be no higher with a xenon source than a halogen source. Thus the 'glare' from a xenon system cannot be higher than the glare from a halogen system. 'Where are all those extra lumens produced by a xenon lamp' you might ask? The headlight reflector system directs them below horizontal where they will be useful in lighting the roadway surface. A xenon headlight is much more luminous than a halogen system when viewed from below horizontal. This explains why truck/SUV headlights appear much brighter when viewed from a passenger vehicle regardless of source - the horizontal plane passing through the truck headlights is higher than the your eyes when seated in a car, thus you are viewing the unrestricted luminous intensity zone below horizontal. I find trucks or SUV's with xenon headlights are almost blinding. Trucks and SUV's with halogens are bad enough.

I don't believe that the discomfort associated with xenon headlights has anything to do with them being more glary than halogen headlights because mathematically they aren't. Why do we feel discomfort from xenon headlights - deductive reasoning indicates that it must be the spectral power distribution of the source - what else is left? Short wavelengths (blue) of light are more energetic than longer wavelengths (red). Ultraviolet light is much more damaging to pigment and tissue than infrared light.

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