An "HID kit" or "LED conversion" consists of HID ballasts and bulbs (or "LED bulbs") for retrofitting into a halogen headlamp. Kits for replacement of standard round or rectangular sealed-beam headlamps usually include a poor-quality replaceable-bulb headlight lens-reflector unit that's not safe or legal even when equipped with the intended (usually H4) halogen bulb. Often, these products are advertised using the name of a reputable lighting company ("Real Philips kit! Real Osram kit! Real Hella kit! Real Cree LEDs!", etc) to try to give the potential buyer the illusion of legitimacy. On rare occasion, some of the components in these kits did start out as legitimate HID headlight bulbs made by reputable companies, but they are modified (hacked) by the "HID kit" suppliers, and they aren't being put to their designed or intended use. Reputable companies like Philips, Osram, Hella, etc. never endorse this kind of hacked usage of their products. Nevertheless, it's easy to get "HID kits" from China bearing the (unauthorised, counterfeit) brands of major, reputable companies. See this page for just a few examples of the many packaging options offered by just one Chinese maker of "HID kits".

Halogen headlamps and HID headlamps require very different optics to produce a safe and effective—not to mention legal—beam pattern. How come? Because of the very different characteristics of the two kinds of light source.

A halogen bulb has a cylindrical light source: the glowing filament. The space immediately surrounding the cylinder of light is completely dark, and so the sharpest contrast between bright and dark is along the edges of the cylinder of light. The ends of the filament cylinder fade from bright to dark. An HID bulb, on the other hand, has a crescent-shaped light source -- the arc. It's crescent-shaped because as it passes through the space between the two electrodes, its heat causes it to try to rise. The space immediately surrounding the crescent of light glows in layers...the closer to the crescent of light, the brighter the glow. The ends of the arc crescent are the brightest points, and immediately beyond these points is completely dark, so the sharpest contrast between bright and dark is at the ends of the crescent of light.

This diagram shows the very different characteristics of the filament vs. the arc:

When designing the optics (lens and/or reflector) for a lamp, the characteristics of the light source are the driving factor around which everything else must be engineered. If you go and change the light source, you've done the equivalent of putting on somebody else's eyeglasses: You can probably make them fit on your face OK, but you won't see properly.

Here are some downloadable PDF tests done by DOT and CalCoast Labs on halogen headlamps equipped with "HID kits":

And here is a documentary done by Auto Express showing the results of installing "HID kits" in UN ("ECE", "E-code", "European") headlamps, which are designed for notably tight control of glare on low beam:

You can read some of the United States Department of Transportation's statements on the subject here, here, here, and here — all links will open in new windows. And you can read the German perspective here, and the same from Hong Kong here, and the same from New Zealand here (NZ vehicle owner information page) and here (full NZ lighting regulation & inspection manual; see page 1—5). Some "HID kit" marketeers will try to tell you that the kits are technically illegal only because the US headlamp laws are stuck in the past. That's wrong; the world's experts and regulators all say the same thing: Don't!

Now, what about those "retrofit" jobs in which the beam cutoff still appears sharp? Don't be fooled; it's an error to judge a beam pattern solely by its cutoff. In many lamps, especially the projector types, the cutoff will remain the same regardless of what light source is behind it. Halogen bulb, HID capsule, cigarette lighter, firefly, hold it up to the sun—whatever. That's because of the way a projector lamp works. The cutoff is simply the projected image of a piece of metal running side-to-side behind the lens. Where the optics come in is in distributing the light under the cutoff. And, as with all other automotive lamps (and, in fact, all optical instruments), the optics are calculated based not just on where the light source is within the lamp (focal length) but also the specific photometric characteristics of the light source...which parts of it are brighter, which parts of it are darker, where the boundaries of the light source are, whether the boundaries are sharp or fuzzy, the shape of the light source, and so forth.

As if the optical mismatch weren't reason enough to drop the idea of "retrofitting" an HID bulb where a halogen one belongs—and it is!—there are even more reasons why not to do it. Here are some of them:

The only available arc capsules have a longitudinal arc (arc path runs front to back) on the axis of the bulb, but many popular halogen headlamp bulbs, such as 9004, 9007, H3 and H12, use a filament that is transverse (side-to-side) and/or offset (not on the axis of the bulb) central axis of the headlamp reflector). In this case, it is impossible even to roughly approximate the position and orientation of the filament with a "retrofit" HID capsule. Just because your headlamp might use an axial-filament bulb, though, doesn't mean you've jumped the hurdles—the laws of optical physics don't bend even for the cleverest marketing department, nor for the catchiest HID "retrofit" kit box.

A relatively new gimmick is HID arc capsules set in an electromagnetic base so that they shift up and down or back and forth. These are being marketed as "dual beam" kits that claim to address the loss of high beam with fixed-base "retrofits" in place of dual-filament halogen bulbs like 9004, 9007, H4, and H13. A cheaper variant of this is one that uses a fixed HID bulb with a halogen bulb strapped or glued to the side of it...yikes! What you wind up with is two poorly-formed beams, at best. The reason the original equipment market has not adopted the movable-capsule designs they've been playing with since the mid 1990s is because it is impossible to control the arc position accurately so it winds up in the same position each and every time.

In the original-equipment field, there are single-capsule dual-beam systems appearing ("BiXenon", etc.), but these all rely on a movable optical shield, or movable reflector—the arc capsule stays in one place. The Original Equipment engineers have a great deal of money and resources at their disposal, and if a movable capsule were a practical way to do the job, they'd do it. The "retrofit" kits certainly don't address this problem anywhere near satisfaction. And even if they did, remember: Whether a fixed or moving-capsule "retrofit" is contemplated, solving the arc-position problem and calling it good is like going to a hospital with two broken ribs, a sprained ankle and a crushed toe and having the nurse say "Well, you're free to go home now, we've put your ankle in a sling!" Focal length (arc/filament positioning) is only just ONE issue out of several.

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.

Electronic driver assistance and safety systems are being factory-installed on vehicles at an unprecedented and accelerating rate. These systems may malfunction in unpredictable and dangerous ways in the presence of electromagnatic and/or radio-frequency interference (EMI/RFI) from "HID kits". The linked examples suggest the effect is not limited to systems involving wireless data transmission. As the prevalence and variety of electronic safety systems increases, the random danger posed by interference from "HID kits" increases in frequency and severity.

Please note: From time to time, I am asked to comment on what are marketed as "new developments" in HID kits, and those asking sometimes point out to me that these "new developments" might render this article out-of-date, since the copyright date on the article is older than the date of these "new developments". Please understand, marketeers will always be coming up with dazzling new pseudoscience, tempting new hype and sneaky new ways of trying to convince you to buy their stuff. It's what they do. This article will never go out of date, because the problems with HID kits are conceptual problems, not problems of implementation. Therefore, they cannot be overcome by additional research and development, any more than someone could develop a way for you to put on somebody else's eyeglasses and see correctly.

Daniel Stern Lighting (Daniel J. Stern, Proprietor)

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Copyright ©2011 Daniel J. Stern. Latest revisions 08/17. No part of this text may be reproduced in any form without express permission of author. Permission to quote is granted for the purposes of communication with the author.