According to the way HID's work, there isn't a filament like a regular bulb. An arc is zapped between two electrodes exciting xenon gas. The gas then vaporizes metallic salts which produce the light.

Now to the question.
This past weekend I took a trip to some high elevations up in the mountains (+6000ft.). There were several tunnels that went through the mountains you had to go through. None of the tunnels were lit so you had to use your headlights.
The HID's would never "light up" all the way. They just stayed a very dim blue. Would the thinner air have anything to do with the gas in the headlights?
Once I was off the mountain, I checked the lights and they were working fine again. Wierd!!

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'01 CLS #4055 Red/Parchment-all the goodies

 

The thinner air would have nothing to do with the inert gas in your lights. It's probably just a simple matter of light acclimation of your eyes, especially if the situation was that the exit of the tunnel was visible. Typical sunlight is something like 1000 candela (not sure about the correct magnitude), which is rarely approached by anything you'd normally encounter in the night.

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051/LP/SR/LD/HH

 

i live in albuquerque which is damn near 6000 ft and i never have a problem, even if i go up the mountains, like 10000 plus feet.

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The tubes should NOT be affected by altitude. Look up EG&G or Harold Edgerton. Harold Edgerton was an MIT professor who was a pioneer in high speed stroboscopic photography.

In fact, here is a link to a Bio. page, that mentions the use of Xenon lamps used from planes over Axis territory during WWII. All's I can say, is that Xenon flash tubes are on commercial and private jets flying well above 6000 feet.

Here is an except and link:


"During World War II, Edgerton served as technical representative for the Army Air Forces and devised a strobe system for nighttime aerial photography of ground targets and operations. Planes equipped with his apparatus photographed the coast of Normandy immediately before D-Day and were also used in Italy and the Far East..."

http://www-tech.mit.edu/V109/N58/edge.58n.html

Electrical breakdown voltage will lessen with altitude, but the lamps themselves are unaffected.



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Silver 2001 CL-S with NAVI

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Sounds to me that you turned your headlights on as you saw the tunnel approaching and then turned them off after passing thru. Is it possible that you never gave them a chance to warm up?

 

didn't somebody say that we have auto dimming headlights? It calucates how much light is needed and adjust the headlights accordingly. Maybe the car saw the light at the end of tunnel and adjusted for it? Or was that a feature on the new TL, i forget.

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