I have always been curious but have forgotten to ask this...how in the world are automotive HID bulbs instantly bright, I would think HID would be unsuitable for auto use due to the haracteristic HID ramp up time and color shifting, and the inability to restrike instantly when hot...but obviously this has all been overcome somehow...and I'm puzzled LoL!  Thanks guys!

- Mike

Automotive MH lamps in HID headlights (35W, can be operated also with regular 35W pulse start MH equipment) that comes original with the manufacturer, opeartes with a 12V DC operated electronic ballast that overdrives the lamps X2 during run-up, resulting in an instant full pressure mercury and an overall run-up time of 20 secs. The lamps have high pressure xenon buffer, to allow instant light after turning on. The 12V DC operated ballast can instant hot restrike the lamps with 60KV pulses.
Sometimes, if you turn the HID headlights off and back on, and the computer of the vehicle, have the ability to control the ballasts of the lamps, it may intentionally delay the operation of the headlights, and after some time, automatically turn on the headlights (And so simulating a hot restrike of penning start and internal starter HPS lamps)(Media projectors may do this with their short-arc MH/MV lamps as well).

Regarding to glaring from HID lamps: These are comes in Israel only from 3rd party HID kits that installed inside the existing halogen headlights, or from 3rd party HID headlights. Both are illegal in Israel, and may result in a disqualification of the vehicle at the annual licensing test, and taking down of it from the road.
Also, Ash thinks that glaring headlights aren't limited to HID, and that halogen can also cause glare if the filament of the lamps isn't aligned well. From my experience, glaring from regular halogen headlights, is only caused if they are operated at a high lights mode. Glaring headlights at normal lights mode, I observed ONLY from MH headlights, and only from the ones that the Transportation Minister of Israel, banned (Ie. HID kits on existing halogen headlights and 3rd party HID headlights).

The auto HID ballasts have arctube temperature compensation, so when the cold lamp have lower efficacy, they boost the power to get the rated output.
Inside is a simulator of the arctube temperature, usually in a form of a large capacitor, where the voltage correspond to the arctube temperature. And this voltagethen steer the converter's output power.

After ignition the mercury pressure is not full instantly (it take ~10 seconds for the mercury and about 20secondsfor the rest to fully evaporate and stabilize), but that is exactly the reason for using the Xenon as a buffer gas: The Xennon produce usable visible light even when cold (and it is gaseous at virtually any temperature), so the lamp can produce enough light even when completely cold.

The hot restrike is allowed by two factors: Really high voltage ignition pulses (~30..60kV) and high ballast OCV (~500V rectangular) capable to feed the lamp with enough current even in the cold cathode mode, so the discharge is capable to rather quickly heat up the electrodes.

But even when cold, the Xenon buffer ask for rather higher ignition voltage, so it need ~3..5kV ignitors when (normal 35W MH suffice with ~1.5..2.5kV)

I think that I've seen a spectrum of HID headlights turning on. The mercury lines, were at full intensity as soon as they were turned on, and additional several seconds for the sodium and scandium to appear.

There is low pressure of the mercury always present and so these become the source of the mercury discharge. Moreover the heating electrodes "burst" a cloud of mercury as they heat up (within 1 second upon ignition), so it become visible as well.

But the mercury pressure grow significantly in the first ~10 second after ignition and as such take over the most lumens generated in that stage - what mean the feed power could decrease from ~80..90W (just upon ignition, when practically only the Xenon is able to radiate any useful lumen output) down to about 50W (when the mercury radiate most of the lumens).

The Hg lines spike above the Xenon continuum, because with Hg all the energy is concentrated to few narrow lines. But as they are only narrow spikes, their lumen output remain small compare to the broad, rich Xenon spectrum.

This is VERY impressive information and truly appreciated!  Thanks so much guys.  I would imagine under such conditions they wouldn't last as long as a standard HID lamp...but I will venture to guess the engineers somehow covered that too LoL, maybe by building with extra robust electrodes and envelope material.

Well, they don't last as long, about 1000..2000hours is their usual rated life.
But that mean about 50000..100000km (30000..60000miles) of nighttime driving, about the double or triple of total driving time.
"Regular" halogens are rated only for about 250..500 hours, so there is an improvement in lifetime...

The complete passenger cars are designed for ~10khours of operation, what mean about half million km, longer lifetimes are expected only from some commercial vehicles...

I did not know that!  Thanks man :-). I always wondered about the hrs lifespan of ordinary halogen headlights as well!