The most dangerous part of the attempt to "retrofit" Xenon headlamps is that sometimes you get a deceptive and illusory "improvement" in the performance of the headlamp. The performance of the headlamp is perceived to be "better" because of the much higher level of foreground lighting (on the road immediately in front of the car). However, the beam patterns produced by this kind of "conversion" virtually always give less distance light, and often an alarming lack of light where there's meant to be a relative maximum in light intensity. The result is the illusion that you can see better than you actually can, and that's not safe.
It's tricky to judge headlamp beam performance without a lot of knowledge, a lot of training and a lot of special equipment, because subjective perceptions are very misleading. Having a lot of strong light in the foreground, that is on the road close to the car and out to the sides, is very comforting and reliably produces a strong impression of "good headlights". The problem is that not only is foreground lighting of decidedly secondary importance when travelling much above 30 mph, but having a very strong pool of light close to the car causes your pupils to close down, worsening your distance vision...all the while giving you this false sense of security. This is to say nothing of the massive amounts of glare to other road users and backdazzle to you, the driver, that results from these "retrofits".
HID headlamps also require careful weatherproofing and electrical shielding because of the high voltages involved. These unsafe "retrofits" make it physically possible to insert an HID bulb where a halogen bulb belongs, but this practice is illegal and dangerous, regardless of claims by these marketers that their systems are "beam pattern corrected" or the fraudulent use of established brand names to try to trick you into thinking the product is legitimate. In order to work correctly and safely, HID headlamps must be designed from the start as HID headlamps.
What about the law, what does it have to say on the matter? In virtually every first-world country, HID "retrofits" into halogen headlamps are illegal. They're illegal clear across Europe and in all of the many countries that use European ECE headlight regulations. They're illegal in the US and Canada. Some people dismiss this because North American regulations, in particular, are written in such a manner as to reject a great many genuinely good headlamps. Nevertheless, on the particular count of HID "retrofits" into halogen headlamps, the world's regulators and engineers all say DON'T!
The only safe and legitimate HID retrofit is one that replaces the entire headlamp—that is lens, reflector, bulb...the whole system—with optics designed for HID usage. In the aftermarket, it is possible to get clever with the growing number of available products, such as Hella's modular projectors available in HID or halogen, and fabricate your own brackets and bezels.
Installing HID optics (such as projectors designed to accept an HID bulb) in halogen headlamp housings can be done, but it is a great deal more complicated and difficult to do correctly than is commonly understood. Typically the process involves baking the headlight assembly to loosen the adhesive, removing the lens, cutting the reflector, mounting the HID projector, and using silicone to reseal the lens. Sounds simple? Sure, but there are significant and substantial issues and challenges. The projector has to be mounted very precisely with respect to its centre of gravity; if not, it will shake out of alignment (and eventually off its mounts). Many sealants, adhesives, and paints produce gases that attack and fog lamp optics. The low beam projector has to be aimed correctly relative to the high beam or else the finished headlamp will be aimable so the lows or the highs are pointed in the correct direction, but not both. Once the headlamp has been opened, it is very challenging to get a good and durable seal against moisture and dirt ingress. None of these challenges is insurmountable, and there are outfits specialising in this kind of optical transplant. Shop very carefully if you are in the market, pay careful attention to the guarantee offered on the work, and be aware that even if the transplanted optics come from a legal headlamp, the end result—the modified headlamp—is no longer compliant with the applicable regulations.
Scrutiny of the effect on safety of "HID kits" has tended to focus on the dangerous damage to headlamp safety performance (seeing and glare alike) caused by the installation of HID bulbs in headlamps meant to use halogen bulbs. But that's not the extent of the problem. There's a new and very serious threat to vehicle safety posed by "HID kits": Drivers are reporting that electromagnetic and radio-frequency interference from "HID kits" is causing malfunctions in their vehicles' electronic systems including tire pressure monitoring, electric power steering, traction control, electronic stability control, windshield wipers, brake system fault warning, FM radio, and others. For examples, please see here, here, here, here, here, here, here, here, here, here, here, here, here, here, and here.
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