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Xenon headlights first illuminated in 1991 on a BMW. And since then, the controversy has been raging ever since: some praise the new best d3s bulb, while others curse it. And both sides are right. First, let's hear the arguments for. So, the xenon light is more than twice as bright as the halogen light: 3200 lm of luminous flux versus 1500 lm for the best H7 specimens. Xenon lamps are also much more economical: they provide 91 lm per watt versus 26 lm/W of halogen. This allows you to spend 35 watts per headlight instead of 55 watts. In addition the new xenon lamps have a life of 2000 hours compared with 450-500 hours of halogen lamps. They are not afraid of vibration as the plasma arc cannot be shaken off like a filament hair.

Xenon lamp does not have enough on-board voltage of 14 V to burn. You have to add to the circuit so called inverters, which increase its level. They used to be 300 volt, but today they can make do with combination of 85V and 400 Hz and even 42 volt! By the way, this leads to the following conclusion: a lamp is not the same as a lamp and you have to be careful when replacing it. For example, D1, D2, D3 and D4 series lamps are not interchangeable! And on the back there is also a letter R or S which you also have to take into account.
You can't do without one inverter though: neither 42V, 85V or even 300V will punch the gap between the electrodes when the lamp is switched on. Moreover, the gap here is larger than in a spark plug, and gas pressure in the bulb is higher than in the combustion chamber. Therefore, a high-voltage (25,000 V) ignition pulse is also needed. The device that generates it can either be external (e.g. D1 lamps) or integrated (D1S lamps). Note that the breakdown voltage threshold is one of the reasons why xenon was chosen: it is the lowest among the inert gases. If there was air in the bulb, you would need much higher kilovolts at such pressure and gap! Other reasons lie in the cost and in the technology of precise gas dosing. In general, an optimal ratio was found. It is not surprising that these lights are called "xenon". Although, jumping ahead, we should say that their construction is somewhat different than that of the halogen ones.

The brightness of headlights is certainly good. But when they shine in the eyes of a passing driver - it is not only dangerous, but also harmful. Have you ever cast a curious glance where a sheaf of sparks is shooting out during arc welding? The effect here is about the same. Certainly, the glass of the lamp and headlights reduces the share of aggressive ultraviolet, but nevertheless... To avoid troubles, you usually use automatic level correcting system and headlight washer together with xenon. Add the cost of lamps and electronics and you understand why real xenon costs so much. I should say that the manufacturers of cheap "Chinese" xenon are not too much concerned with a stable position of the arc between the electrodes. The discharge dances, jumps out of focus, and the beam begins to flutter, negating all measures against blinding. It should also be noted that in fog the xenon light is strongly scattered and creates a solid shroud in front of the driver. At last, as time xenon lamps dim much more than halogen ones (to the end of lifetime H7 lamp loses only 20% of initial brightness and xenon one - all 45%). True, it will happen for a longer period, but who will replace not "burned out", but just faded expensive lamp?

Full, bright luminous plasma xenon gives only at high pressure. While the lamp is off (or in a box), it is 20 bar (beware of crashing!). At work the pressure rises up to 100 bar, but here the headlight protects. Some lamps contain mercury, which is also not very good for health. This is why you should not throw such items in the trash, but recycle them. However, the latest models (D3 and D4) do without mercury, which has been replaced by sodium. At the same time, it gives the light a warm hue.

No, these bulbs don't have twice the amount of xenon pumped into them. It is simply an analogue of the double H4 halogen used in the combined headlight of the high beam and dipped beam. Since it is impossible to make two discharging gaps, the lamp socket has a mechanism (solenoid or electric motor) which moves it back and forth a few millimeters. This is how you get the desired light distribution in each mode.