A fairly balanced article in USAToday on June 7, 2001 - the NHTSA will study them - see http://www.usatoday.com/money/autos/2001-06-07-xenon.htm
Flash (March 2001): there are legal bluer (whiter, not really blue) replacement bulbs available now from Sylvania.
And now, the oiriginal posting:
In a few words, there are two types of "blue" headlights:
1) The real thing - High Intensity Discharge (HID) arc lamps found on a few luxury cars
2) Fake, dangerous, and illegal painted, dyed or coated replacement bulbs
High Intensity Discharge lamps are a refined version of the bright white outdoor lamps often used at sports stadiums and used-car lots. The lamp has two electrodes sealed in a quartz bulb. The bulb contains an inert gas, a small amount of mercury to help initiate the discharge, and a mixture of rare earth elements that provide a complex spectrum that approximates a white light. This spectrum is typically weak in the red, giving the lamp a blue-white color. The composition of the contents and the electrodes is crucial to bulb life. The composition of the contents affects the fatness, straightness, and stability of the arc and its tendency to erode the electrodes. Because some lamps have a tendency for the arc to bend upward, they can only be used in a vertical position; otherwise the arc touches the bulb and degrades it. HIDís do not burn out the same way as a tungsten/halogen bulb (which fails when the filament fuses). HIDís typically erode the electrodes gradually until the lamp cannot be started.
Automotive HIDís have several differences from outdoor lights: they are smaller, have smaller electrodes, and the electrodes are more closely spaced.
These features determine several key characteristics:
Automotive HIDís can be made to start very quickly, typically reaching 75% brightness in 4 seconds. This depends on a ballast that knows when the bulb is cool and dumps more power into it to start quickly. Large lamps take several minutes to reach full brightness. This depends on the buildup of temperature and pressure in the bulb.
Automotive HIDís can be restarted (with a high voltage pulse) even when hot. The larger outdoor lamps must cool before re-starting. (Some types have an auxiliary starting electrode, because starting an arc across their large gap is very difficult.)
Automotive HIDís have a very small arc which can be focused precisely to put light in the driving lane and not into the oncoming lane.
Automotive HIDís have a shorter life than large lamps. This is due to the smaller bulb and electrodes, both of which can be eroded by the discharge.
The automotive ballast is not a simple current limiter like that of a fluorescent lamp. It contains a high voltage starting circuit, and a power-stabilizing circuit that includes an analog computer to compute the inner temperature of the bulb so that the proper current and voltage can be applied.
Small HIDís are also used in LCD computer projection monitors and LCD projection TVís. In these applications, the quick start feature is not needed, and the unit typically takes a minute or more to warm up.
The color of HIDís is adequately white for headlamps. The lack of red is more than compensated by the opportunity to control the beam better. However, the ideal color for headlamps would be a neutral or yellowish white (like halogens), because of improved visibility in fog, and less glare effect on oncoming drivers.
The link below explains that the U.S. governmental regulations have not caught up with the degree of beam control yet. As a result, the European headlamps (which are also legal in Canada, but not in the U.S.) give better results in fog, because they throw less light upward into the driverís line of sight.
Why are colored incandescent/halogen bulbs such a bad idea? Because they donít naturally put out much blue light, and the blue tinting reduces the visibility ahead drastically. People (mostly guys, Iíd guess) looking for a ďcoolĒ effect, however, like the bulbs, and may even not realize how drastically they are cutting their visibility Ė looking different is enough to convince them that the blue bulbs are better. But watch out! Not only are tinted bulbs dangerous, they are illegal everywhere (just as illegal as smoked glass headlamp covers), and you have no defense if you are ticketed (and the police DO ticket for this). Do not believe any packaging or sales pitch that says they are approved by the government of the U.S. or your state Ė THEY ARE NOT. If you watch for blue bulbs on the street, you will soon be able to tell the difference. Real HIDís have an intense blue-white light from any angle. Blue halogens often have a dichroic coating that changes color at different angles. While dichroic coatings are more efficient than plain blue tint, they still reduce the light output. Tinted bulbs seldom have the same color as HIDs and never have the intensity. You can bet the police can tell which are which.
See more at:
http://www.howstuffworks.com/framed.htm?parent=question387.htm&url=http://www.misty.com/~don/d2.html (18 Mar 2001)
http://home.earthlink.net/~strider2/300zx/blue.html (31 Oct 1999)
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