My HID Xenon installation (low and high beam)

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Like moped said, it takes 5 to 10 seconds for max brightness.

At first it flashes (ignition of gas), then it turns on to about the brightness of halogen while warming up to about three times the brightness of halogen. I had HID on both low and high beam, the HID on high beam does make the "pass" flash effect slightly less. So if I wanted to get noticed, I would just switch high beam on. Within a few seconds the lighthouse would light up their whole interior, so you definitely get noticed. Once the bulb has been warmed up (it's still about three times cooler than halogen in that case), you can use it for flashing off and on to max brightness instantly. I did not see any significant disadvantage of having both bulbs fitted, not significant enough at least to outweigh the huge advantage (a LOT of light all the way to the horizon).

The Pan has two H4 bulbs, so now I have two times low and instant high HID. The high beam in that case works by means of a magnetic switch that uncovers the bottom part of the bulb for high beam. You can see the bottom part of the reflector (for high beam) is blocked in the next pic, as well as the higher aim to the right side of the road: