Advantages: They're inexpensive. You can pick them up for between $3 and $9 most anywhere.
Disadvantages: They're extremely dangerous. At 50 mph on a dry day, you are out-driving them. They produce a very dim, unfocussed, tunnel-shaped pattern with very little light to the sides of the roads. Because the filaments are transverse (running side to side), much of the usable light from the filament is wasted on the non-reflecting top and bottom inside surfaces of the rectangular lamps. Backglare in rain, snow and fog is extremely debilitating. Light output specifications for these lamps have not changed since 1956.
Halogen Sealed-Beam headlamps
Disadvantages: All of the disadvantages of the normal sealed beams are still applicable, because these transverse-filament lamps throw the same beam pattern and offer only a slight increase in candlepower over normal sealed beams. Halogen sealed beams also are the shortest-lived of all automotive headlamps, although some longer-life versions have been marketed.
Sylvania, Wagner and others have recently introduced "new" halogen sealed beams they are claiming are three times better than the old versions. In my own on-road tests (As well as several lab and road tests by other lighting authorities, both regulatory and non-regulatory), these lamps were only marginally better at lighting-up the area immediately in front of the vehicle, and were actually worse in terms of evenness of illumination. The typical USA headlamps' "tunnel vision" effect was amplified. They were MUCH worse in bad weather, with lots more backglare reflected off rain, snow and fog.
You have not run out of options for your standard-headlamp vehicle just because you've reached the end of the Sealed Beam display at your local auto parts store, however!
There are replaceable-bulb lamps that can do a much better job than sealed beams and which directly replace sealed beams. These "E-code" headlamps are described later in this article.Replaceable-Bulb Headlamps
Aerodynamic USA-spec ("DOT"; is on the lens) headlamps
Many people incorrectly assume that these aerodynamic lamps are the same as the lamps in use in Europe. This definitely is not the case! The USA-specification aerodynamic lamps can be identified by the word "DOT" on the lens. These headlamps are marked with the type of bulb they take:
HB1: 9004 (high AND low beam, 2 transverse filaments) HB2: 9003 (high AND low beam, 2 axial filaments) HB3: 9005 (high OR low beam, 1 axial filament) HB4: 9006 (high OR low beam, 1 axial filament) HB5: 9007 (high AND low beam, 2 axial filaments) H1 (high OR low beam, 1 axial filament) H7 (high OR low beam, 1 axial filament)
D1, D2, D2S, D2R, 9500: Arc Discharge Tube (high intensity discharge headlamp, high OR low beam)
If there is no "HB" or "H" or "D" class marking on the lens of a DOT replaceable-bulb headlamp, it takes a type HB1 (9004) bulb.
Advantages: When the headlamp burns out, you need only replace the bulb, not the e ntire lamp assembly. Also, automotive stylists like these lamps because they can make them any size and shape they desire.
Disadvantages: These lamps are subject to the same beam pattern- and light output-restrictions as sealed beams (above). In many cases, the light output of these lamps is much worse than even the relatively dim illumination provided by sealed beams. The HB1 (9004) bulb is notoriously inefficient. This, like the transverse-filament sealed beams, wastes a tremendous amount of the usable filament luminance on non-reflecting areas of the headlamp, such as the top and bottom of rectangular headlamps.
Often, the lenses are made of polycarbonate or other plastics which are hyped as a good anti-shatter replacement for glass, but which quickly deteriorates, becoming cloudy and yellowed or scratched. Polishing is not recommended on these lamps, which are coated with anti-UV and hardening coatings that can create unpredictable or short-lived results if they are polished off or heated up (as by the friction of a buffing pad).
It is interesting to note that senior veteran headlamp engineers at Carello, Hella, Valeo-Cibie, and Osram-Sylvania are on record as stating that some of the US aerodynamic headlamps are among the worst, most dangerous developments in worldwide auto headlamp technology in the last two decades. In particular, the author has been told by several lighting engineers that many of the headlamps using 9004 bulbs simply do not provide adequately wide or sufficiently even illumination. The light is, of course, legally wide and legally even, but that's a different question!What about higher-wattage bulbs for my DOT-spec headlamps?
One word: DON'T!
Even if your headlamps use glass lenses and metal reflector housings, you still can't safely use these bulbs because the wiring in DOT-spec headlamp systems is not capable of handling the level of current these bulbs will draw. If you disregard this and do it anyhow, you create several serious fire hazards in the engine compartment and, more worrisome, in the passenger compartment at the headlamp switch. This problem could be circumvented with the installation of an extra-heavy-duty relay and heavy wiring, but the wiring is only as robust as its highest-resistance section; the 9004/HB1, 9005/HB3, 9006/HB4, and 9007/HB5 bulbs used in DOT-spec headlamp systems all have extremely small electrical contacts. Go ahead and have a look; they're really spindly! These contacts get extremely hot even under normal (45 to 65 watt) loads. They get dangerously hot under higher (80w, 100w, etc.) loads. When you go much above 65 watts, these contacts become the point of maximum resistance. Things start melting and burning, and due to the voltage drop involved with such resistance, you will not achieve any improvement in your headlamps' ability to light-up the night. No reputable bulb manufacturer makes these overwattage DOT bulbs for that reason.
The ones you may find are third-rate junk that not only will not last very long, but also are made in factories with little or no quality control and do not tend to be made to the extremely precise filament-placement specifications demanded by today's advanced headlamp designs for proper lamp performance.
Even if you could find (nonexistent) overwattage bulbs in these formats with oversized contacts and ceramic bases, make your wiring adequate to handle the extra load, and be sure your headlamps contain no plastic, you *still* would be wasting your time and money to use the overwattage bulbs, because the optics and beam pattern specified in such headlamps by the DOT does not respond to overwattage bulbs, except to cause massive dazzle to oncoming traffic and drivers in front of you via the rear view mirrors, and to cause extreme backglare in fog, rain or snow. Any way you slice it, overwattage bulbs are a bad deal all around. You'll do well to avoid them.European-compliance ("E-code") replaceable-bulb headlamps
What does "E-code" mean? It's a quick way of referring to a European-specification headlamp. The "E-code", signified by a capital "E" in a circle on the lens of the lamp, signifies that the lamp has passed the ECE (European regulatory) tests for light output, durability, quality control, beam pattern, etc. The United States does not subscribe to ECE auto safety regulations, which is why US-market cars do not come equipped with ECE headlamps.
Although US and ECE headlamps both start with light sources (bulbs) that produce about the same amount of light, the two kinds of lamp handle the light differently. The ECE specifications for the low beam pattern tend to bring about headlamps with a subjectively more useful beam pattern. Lighting to both sides of the road tends to be much more evenvery widely so you can see to the sides of the car, including animals (of the two or four-legged variety) that may be lurking in the shadows, waiting to step into the road. Road signs along the driving side (the right side) of the road tend to be illuminated for a greater distance down the road than with DOT headlamps.
The Low beam pattern is less offensive to oncoming traffic because of a sharp cutoff across the top of the beam pattern. On the left side of the beam pattern, the cutoff is horizontal. On the right half of the beam pattern, the cutoff rises to the right at a 15 degree angle (or, alternately, the right side of the cutoff is also horizontal, but is "stepped" upwards with respect to the left side cutoff). This sharp delineation between illumination areas (areas that need to be illuminated in order for you to see what needs to be seen) and glare areas (areas that are likely to include other road users' eyes) tends to reduce backglare in bad weather. Because there is minimal light above the horizontal (in contrast to DOT lamps, which are required to produce quite a bit of light above the horizontal), light tends to be thrown on the road, the roadsides and the road signs, NOT back in your eyes via reflection from rain, fog or snow. Backglare in bad weather is completely eliminated. The effect, with properly-aimed ECE headlamps in bad weather, is a nighttime driving experience in which you see the *road*, the *obstacles* and the *signs* , not whatever might be falling out of the sky towards the road. The light tends to be more evenly distributed on the road in front of the car, with less of the streaky/blotchy/tunnel-of-light effects that are common to many DOT headlamps.
ALL headlamp beams are a compromise! It is not a simple task to design a good headlamp, because so many of the jobs we need a headlamp to do conflict with one another. Too much light above the horizontal can cause too much glare for other road users and too much backglare for the driver in bad weather, as well as a "veiling glare" effect even in relatively good weather. But insufficient light above the horizontal can make it difficult for the driver to read overhead road signs and can reduce seeing distance in some conditions, such as when going down a "sag" in the road. Insufficient light on the pavement in front of the car tends to make drivers uncomfortable, apparently causing a "black hole" effect in which it is difficult for the driver to judge accurately the position of his car on the road. But too much ligtht on the pavement close to the car can reduce the driver's ability to detect obstacles farther down the road. A super-sharp cutoff makes a lamp very easy to aim visually, using height measurements and a vertical wall, but can create distracting effects where the driver sees the cutoff bouncing up and down as he goes over road irregularities; at the same time, a very vague cutoff (or none at all) makes a lamp difficult to aim visually, but can reduce the on-the-road effects of a very sharp cutoff. And these are just a few examples! Every aspect of a headlamp beam and there are many, many aspects that make up a headlamp beam must be carefully adjusted to create a good overall compromise. The goal is to pick the compromise that works the best in the conditions in which you drive. The author has found that in the vast majority of the driving conditions he encounters, the ECE headlamps do a better job of allowing him to see what must be seen in time to react properly.Sounds great, but my car has DOT headlamps. How can I convert?
Before we start talking about conversions, please note that in the United States, most ECE headlamps do not comply with FMVSS108 and may not legally be sold for use on public highways. However, they can be sold for use off road. E-code headlamps are available to replace all of the standard-format (ex-sealed beam) size shapes, except the miserably miniature sealbeams used on many late-model GM products such as the '93-'97 Camaro and '80s-'90s Chevy and GMC pickups. (Here is an example in which a legal but plainly inadequate compromise was put together in the engineering of this lamp system. Shame on the engineers who perpetrated this sad joke of a headlamp system on the motoring public!)
Many rallyists and performance driving enthusiasts will recognize the Marchal, Cibie, Carello and Hella names...these are a few of the companies who produce E-code headlamps in world-standard ("sealed beam") formats. Their mounting parameters are the same as sealbeams, so they can be installed in cars that use standard-format headlamps with minimal or no modifications required to your vehicle or wiring.
"But my car has DOT replaceable-bulb headlamps, not standard-format sealed beams. Am I out of luck if I want to use ECE headlamps?"
Perhaps, but Not necessarily! E-code headlamps also can be obtained for many later-model cars with the DOT replaceable-bulb headlamps.
It has always been difficult to obtain E-code headlamps for late-model American cars with DOT replaceable-bulb headlamps. Demand is strong for these items. It seems that many US drivers are unhappy with the beam pattern compromise wrought by the makers of their DOT headlamps! Although it doesn't occur to many people that there would be many American cars outside of North America, it's quite true--even non-exotic and comparatively large models like the Chevrolet Caprice, Ford Taurus and Plymouth Acclaim have European model equivalents with E-code headlamps, not to mention internationally-popular American vehicles like the Jeep Grand Cherokee and Chrysler minivans.What about my Japanese car? It has replaceable-bulb DOT headlamps.
There is strong demand for E-code headlamps for late-model Japanese vehicles. Unfortunately, availability of these items in North America is currently nil. There is an element of Japanese car design philosophy that will stand in the way of universal availability of E-code headlamps for these cars: Many times, Japanese automakers apply the same model name (i.e. "Civic" or "Camry") to completely different cars in different markets. The Honda Civics sold in Great Britain, for instance, has headlamps of an entirely different size/shape compared to those sold in the US. And the current Toyota Camry in the US and Canada is completely different from the Camry sold elsewhere in the world. That's bad. But there is also good news: Many Japanese automakers are making increasing use of new US headlamp laws that facilitate the use of headlamps that meet both ECE and DOT specifications; the window of overlap between the two standards has been widened to increase the feasibility of such "harmonized" headlamp beams.Can I put an E-code headlamp in place of the sealed beam on my motorcycle?
YES! Motorcycles with sealed beam headlamps use the same standard formats as cars. ECE headlamps are legal for use on motorcycles. (This raises interesting questions about the logic of permitting ECE headlamps on motorcycles used on the road, but not on cars used on the road. If I use a bandsaw to cut my car down the middle so it's a "single track vehicle"--the US Government's name for a motorcycle--would they have a problem with an ECE headlamp on one half or the other???)
Can't I just replace my DOT bulb with an E-code bulb and keep my same headlamp assemblies?
No. Each individual headlamp assembly is made to accept only one type of bulb. While there are sometimes options to squeeze a bit more performance out of a given type of bulb (HB1/9004, HB3/9005, etc.), there is no mixing and matching. For instance, you cannot install an H7 bulb in a headlamp designed for an HB5 bulb, or a 9007 bulb in place of a 9004. Not only will the bulb not fit, but even if you could make it fit, such a mismatch would result in a dangerously damaged beam pattern; you would only be making a bad headlamp worse.Tell me about the bulbs used in E-code headlamps.
The bulbs used in E-code headlamps tend to last a long time, and can be replaced in a minute. Most ECE headlamps use the H4 bulb, a twin-filament (high beam/low beam) bulb described more fully on the bulb information page in the Tech section of this website. Stock-wattage bulbs from quality manufacturers (Osram, Philips, GE, etc.) can be obtained most anywhere; prevailing price is $4 to $6 for a regular H1 or H4 bulb.
Specialty bulbs in higher wattages and/or with dichroic coatings to improve visibility in poor weather can easily be obtained and, in most cases, used safely. Unlike the DOT beams, E-code headlamps respond quite well to bulb upgrades. The electrical terminals on bulbs used in E-code headlamps are sufficiently large that--with properly upgraded wiring to handle the increased current draw--the burning/melting problems mentioned in reference to DOT headlamps are avoided.What are "city lights" ?
E-code lamps sometimes include a City light.(These are called "side lights" by speakers of British English. This can be a bit confusing, because they face *front*, not to the side.) What is a city light? It's a 5-watt lamp that sticks through the lamp's reflector into the lamp itself. European vehicles are equipped with city lights rather than US-style amber parking lamps. City lights are legal for use as parking lamps in the US and in Canada; amber parking lamps are NOT mandatory.How would I make the city light work on my car? The DOT headlamps I'm replacing didn't have them.
The city light has two wires. Ground one of its wires, and run the other of its wires to the dim filament feed on your front park lamps, preferably instead of (but optionally in addition to) your existing dim amber park lamp filaments. This makes for parking lamps that WORK, and if a headlamp ever malfunctions, oncoming traffic still sees you as a double-track vehicle. In addition, it makes your front turn signals much clearer because they now go "BRIGHT-off-BRIGHT-off" instead of "bright-dim-bright-dim" when the lights are on. Yep, another aspect of lighting that the Europeans got right and we didn't. City lights are especially useful if you have fog lamps. On foggy days, you can put on the city lights which will show other drivers exactly where your car is, and switch on the fog lamps so you can see.
Disadvantages of E-code headlamps
1) Cost. Standard-format E-code headlamps cost significantly more than the sealed beam headlamps they replace. This is explained by the quality of the construction materials (see above) and the fact that they are meant to stay in use for several decades, because you need replace only the bulb, not the whole unit.
The cost of aerodynamic late-model E-code headlamps usually is comparable to the cost of the DOT replaceable-bulb headlamps they replace.
All in all, the increased cost is money very well-spent; you'll be convinced the first time you switch-on your E-code lamps at night! Those of us in the lighting world often joke that selling good automotive lighting is something akin to selling illicit drugs; once somebody buys one set, they come back for more, and more, and more. Switching from a car with E-code headlamps to one with DOT headlamps has been compared to putting on dark sunglasses before driving at night.
What about legality?
In the United States, E-code headlamps may be sold only for off-road use, because they do not conform to US Federal Motor Vehicle Safety Standard 108. It is a matter of State law as to what sort of headlamps you can or cannot install on your own vehicle. The vast majority of US states do not specify any particular type of headlamp, as long as it emits white light, has an upper and lower beam setting, and is aimed in a manner so as not to dazzle other road users. If your state does not have a vehicle headlamp inspection, it would be extremely unlikely that you would experience difficulties if you were to use off-road headlamps on the road.
Some eastern Seaboard states, such as NJ, PA, VA and MD, still specify that E-code headlamps may only be used on motorcycles, which is puzzling indeed--they seem to be saying that if you cut your car in half down the middle, you will be permitted to use lights that will allow you to SEE at night, but NOT if you wish to have all four wheels on the ground at once! This is one of those "olde-tyme" laws that never got wiped from the books by "enlightened" lawmakers. It's interesting to note that while most states don't care what kind of headlamp you install on your own car, Oregon, Washington and a few other states explicitly PERMIT E-code headlamps on all vehicles, because of their superiority in the rainy and foggy weather conditions experienced there.
So what do I do if I'm in one of those Eastern Seaboard states?
If you live in one of those states, it is likely that your vehicle is subject to inspections that involve the use of a mechanical headlamp aimer. Most states dropped this antiquated methodology in the 1960s.
The problem comes when the inspector tries to use the mechanical aimer on your E-code headlamps. E-code headlamps are meant to be aimed by shining them at a wall and setting the beam height and position according to a prescribed set of marks on the wall. They don't have the three little pips on the front of the lens that are for attaching the aimer cup. This is indeed a sticky problem, though there are several workarounds.
The most straightforward method of getting through inspection is to swap back to the DOT headlamps, get your inspection sticker, then go home and reinstall the good headlamps. This can be tedious, but people have been doing it for years. Reports are coming in that some inspectors will see a vehicle with E-code headlamps and give it an inspection sticker anyhow, realizing that if they flunk the car, the owner will simply go change the headlamps to pass inspection and then replace the E-code headlamps.
Recent changes to US headlamp specification regulations have permitted headlamps that are a "harmonized compromise" between European and US beams. It's a "damaged" European beam that meets the European requirements, while meeting the US' inferior and antiquated headlamp beam pattern specifications. Such lamps are often dual approved. (viz. new Honda products, some new Mercedes, etc. for instance--DOT and ECE compliance marks are present.)
The crucial change to the law is the one that (finally!!) allows visual aim of headlamps in the US. Such headlamps use a beam pattern that has some characteristics in common with the European pattern. , and they are almost universally dual-approved units. So what's the difference now that we allow headlamps that are similar to European lamps? Well, the beam pattern specifications are still different, and the lens markings are not the same. DOT headlamps designed for visual aim have special lens markings to tell the inspector that he shouldn't waste his time looking for the three lens pips or the little bubble level, but instead should shine the lamps at an aiming screen. Your E-code headlamps won't pass inspection if they lack the marks to tell the inspector which visual aiming technique to employ, thus cluing him in that he has encountered a set of non-DOT headlamps. A headlamp inspector uses the markings on the headlamp lens to determine that the lamp is to be visually aimed.
If you wish to see these markings for yourself, look at the lenses of your headlamps. These markings are placed near the top or near the bottom of the lens, very conspicuously, in block letters not less than 3mm high.
On a low/high beam visually-aimable headlamp that is similar to a low/high beam H4 European headlamp, markings on the lens read:
USA DOT HB2 VOL
On a high-beam-only headlamp using bulb type H1, H2, H3, HB3, HB4, or H7, similar to a European high beam headlamp using those same bulb types, markings on the lens read:
USA DOT H1 VO
(Of course, "H1" in the above marking string is an example. The correct bulb type is indicated by the marking in that position in the string.)
On a low-beam-only headlamp using bulb type H1, H2, H3, HB3, HB4, or H7, similar to a European headlamp using those same bulb types, markings on the lens read:
USA DOT H1 VOL
(Of course, "H1" in the above marking string is an example. The correct bulb type is indicated by the marking in that position in the string.)
These markings are required to be at least 3mm high so that they are properly legible.
IMPORTANT: This information is provided for EDUCATIONAL PURPOSES ONLY. The author does not suggest or condone falsely marking motor vehicle components.
"I saw some Hella replacement headlamps that say "SAE and DOT approved". What about those?"
For those with large (7") round and large (7"x 6") rectangular sealed beams, This is a replaceable-bulb lamp that does have the provisions for mechanical aiming equipment, enabling even those subject to draconian inspections to get some of the benefits of the E-code lamps without the inspection hassle. These Hella "Vision Plus" (around here we call it the "Vision Minus") come in a box festooned with such meaningless banners as "DOT and SAE LEGAL"! (SAE is not a regulatory body; there is no such thing as "SAE legal".) The performance of these lamps is substandard compared to any good E-code lamp.
And speaking of "any good E-code lamp", leave the $28.99 JC Whitney specials on the shelf.
Nice theory, but will I get pulled over for having E-code lights?
Of course, I cannot guarantee that it won't happen. Have I ever been pulled over for having non-USA headlamps? Absolutely never. I've driven with these lamps in four different cars in 14 states, including the full length of California, Oregon, Washington, Nevada, Utah, Wyoming, Idaho, Colorado, Montana, Illinois, etc. and have absolutely NEVER raised the attention of any cops, even when I drive past them with my lights on. And the car I drive most often is using special dichroic bulbs that cast a very strong yellow light, which is clearly a non-stock light color. I still have never had a problem. The fact of the matter is that back in the '70s when all cars had sealed beams, E-code lamps stuck out like sore thumbs. But today, with the proliferation of so many different headlamp designs, together with the elimination of headlamp inspections in at least 45 states, nobody knows or cares what kind of headlamps you're running, as long as they don't call attention to themselves with odd light colors (i.e. blue bulbs) or gross misalignment that produces undue glare for oncoming traffic.
So as far as on-road legality goes, there are practically no worries, you won't likely get stopped for your lights if they are aimed correctly. Remember, E-code headlamps are less offensive to oncoming traffic, so it's not as though you'll be calling attention to yourself with glaring lamps.
If you live in the US states of Oregon, Washington, Alaska or Massachussets, or in the great nation of Canada, then E-code lamps are 100 percent legal and none of these concerns apply to you even on a theoretical level.
Other Types of Headlamps
Projector ("Ellipsoid", "DE") Headlamps
The projector headlamp functions rather like a slide projector, using a plano-convex lens to distribute the light, rather than a parabolic aluminized reflector. They must produce the same light patterns as every other lamp and conform to the same intensity requirements as every other lamp for whatever compliance system (DOT, E-code) they're built to. They offer new stylistic possibilities, and they can be made to give a sharper cutoff (useful in E-code setups), and they can be field-switched between left-dip and right-dip (useful in Europe). They are more efficient than conventional (reflector) designs, because they are able to use more--and waste less--of the light than reflector-type lamps.
There are no retrofits for most vehicles currently using reflector-type headlamps, with the exception of certain BMW and VW models. In addition, these headlamps are extremely deep, so they cannot be fitted in tight spaces.
High-Intensity Discharge ("HID", "Arc-Discharge", "Xenon") Headlamps
These lamps are coming as standard or optional equipment in some expensive cars. They are not currently available as retrofit items.
The technology at work in an arc-discharge headlamp is exactly the same as the electronic flash on your camera; an electric spark (or arc) jumps a gap inside of a capsule filled with Xenon gas. The arc produces much more light than a glowing filament, and the light is of a higher "color temperature", meaning it more closely approximates a true-white color.
HID headlamps are essentially miniature versions of the lamps under which you watch your favorite football team do battle at night. These lamps would take MINUTES to warm up if it weren't for a sophisticated electronic "Starter" assembly. The latest HID headlamp systems produce halogen-level (i.e. legal) light within 0.3 seconds of switching them on, and full intensity very shortly thereafter. The initial warmup voltage spike runs to tens of thousands of volts. Steady-state system voltage is around 90v.
Because there are not yet any twin-arc discharge tubes (analogous to a dual-filament headlamp bulb), we can't have single lamps that serve for dip and main beams. Initial efforts to do so involved tilting the arc tube capsule up and down. Results were less than optimal.
Valeo Lighting, the people who make Cibie and Marchal headlamps, showed off a NIFTY new innovation at the Society of Automotive Engineers International Congress '98 that is going to fix the problem. Instead of trying to tilt the arc capsule, they use a stepper motor to shift the entire reflector fore and aft along the capsule axis, while keeping the arc capsule stationary. The optical prescription of the reflector they demonstrated (on video) is such that in the one position it produces an excellent European dipped beam AND a knock-your-socks-off European main beam. (see comment above regarding muddling of technical advancements by stupid US laws). Best of all, it is a compact and lightweight system. Its minimum allowable size is well within the size and shape formats of current large rectangular and large round sealbeam headlamps. Perhaps one day Valeo will offer this system as a retrofit for cars that use standard-format headlamps. Although it isn't terribly likely, we can always hope!
Now, what about that special light color?
The marketing types like to brag about how the light color of HID headlamps has been "specially tuned to mimic the sun". In actuality, the light spectrum has *not* been "tuned" to anything, aside from tweaking the gas and evaporated-salts mix to conform to regulatory definitions of "white light". The bluish tint is a side effect of using an electrical arc as the light source. This color is quite good to drive behind, but is vastly more likely to cause glare to oncoming drivers (overstimulation of Retinal Purple in the human eye by this very color range absolutely FOOZLES night vision for a long time after relatively brief exposure). In fact, it was law in France for many years that headlamps had to produce light with substantially LESS output in the blue frequency range where HID lights are particularly strong, specifically to reduce glare to oncoming drivers.
A backlash has already started in Europe, where drivers are complaining about being blinded by BMW's overloaded with a trunk full of beer.
Unlike US and Canadian regulators, European regulators recognize the danger presented by excessive headlamp glare, and so European cars with HID lamps MUST have dynamic headlamp levelling. On-the-fly headlamp vertical aim adjustment has been required by European directives for quite some time now, but dashboard dial control of the vertical aim has been acceptable. No more! Recent European regulations require that the headlamp levelling of HID-equipped cars be linked to the suspension system of the car so the lamps don't glare as much to oncoming traffic when the rear of the car is loaded-down or the car is heading up a small hill.
European light manufacturers also have greatly refined their latest HID systems to provide fewer "hot spots" in the signal image (the image you see when you look at an operating lamp). This helps to reduce glare as well.
The glare problem with HID lamps also is quite present in backglare situations (rain, snow, fog). For this reason, precise control of the beam pattern is mandatory, and cutoff gradients are steeper (sharper cutoff) than we've ever seen in US headlamps. In other words, HID lamps' extreme potential for glare is dragging the US, with regulators kicking and screaming, into the 20th century, where the rest of the world has been for years, in terms of beam pattern light distributions. Just in time for the 21st century :-\
Disadvantages? The usual suspects: Weight (BIG penalty) and Cost (MASSIVE penalty).
We won't see HID lamps trickle down to less expensive cars for another few years at least, especially as there are some brand new developments in very-high-efficiency halogen light sources that cost about the same as previous halogen bulbs. The glowing filament will be with us for quite a while yet.
While the system is designed to last the life of the car, components are *extremely* expensive should anything go wrong. And the controller is a one-use-only item; if you're in a collision, it shuts itself down permanently so as not to shock rescue workers.
"What about those bulbs I've seen advertised for my regular headlamps that will make them look like HID headlamps?"
Various companies and individuals are selling halogen bulbs that have a coating that makes them light up with a bluish color. I receive at *least* four inquiries about these things per week. At least three of those four are from users who are angry because they installed them and not only cannot see properly, but frequently have also been ticketed.
ARE THERE ANY LIGHTING MODIFICATIONS THAT LOOK ODD, BUT ARE ACTUALLY OK?
Yes. There are new headlamp bulbs on the market, meant for use in regular halogen headlamp assemblies. They produce yellowish-white rather than pure-white light. These bulbs do not have the dangerous effects of the blue bulbs discussed above, and have been proven (and approved) to improve bad-weather visibility and reduce glare. They look unusual, but they are actually OK as long as they produce not full-tint selective yellow, but rather selective-yellow tinted white light. Most all "yellow" bulbs from reputable manufacturers (Osram/Sylvania, Philips, Narva, Wagner, GE) are of this "yellowish white" category; you have to put forth some real effort if you wish to find European-type full-yellow bulbs for some special purpose, such as the restoration of a French vehicle or certain types of off-road usage.