The objective of the following is meant as a one stop source of information regarding the use, technology, and purpose to the use of high intensity discharge (HID) xenon lighting systems. There is a LOT of misinformation and myth mixed in with the true information on the forums. The following information is meant to quell the myths and half-truths as much as possible as well as provide a basis for the uninformed to learn about HID. It is meant specifically to be a very easy read, in lay language. If some parts seem vague, it is to avoid confusion. Any diagrams here are just representations that are not necessarily to scale.

PART 1: The Basics (“Knowing is half the battle”)

What is it and why is it better?

I think just about everyone knows HIDs were first used by car manufacturers as OEM and that is where their increasing aftermarket interest comes from. But what is the drive for a manufacturer to use them as OEM? Quite simply they realize the benefits of using the new technology to create better vehicle lighting systems with a much higher level of equipment longevity. Be aware that 90-99% of the aftermarket “kits” out there do not fall into the same qualifications as the OEM equipment, so comments about quality and specification may not apply.

OEM HID arose out of the need for better lighting and alleviation from the limitations of older incandescent halogen bulb technology. Halogen technology has its limitations in achieving increases in output. Minor increases in luminous output (brightness) require a decrease in longevity of the bulbs and the byproduct of heat. Anyone that has used aftermarket higher wattage bulbs knows that they typically do not last very long. However, today’s newer cars that are still equipped with halogen systems the apparent focus has been on improving the optics rather than increasing the output of individual bulbs. Halogen’s major drawback is the fact that it requires a thin filament to produce light to convert electrical current to heat and light. Cars take some abuse in performing their task and bulb filaments eventually fail in part due to vibration.

HID alleviates the filament issue, because, as I will cover later, they do not use filaments at all. The HID bulb (appropriately called a capsule or a burner) has the benefit of producing approximately three times more lumens without the drawbacks of using a filament. The issue of vibration is therefore removed from the equation allowing the bulbs to be incredibly hardy as a light source. OEM bulbs and ballasts are meant to perform as “life of car” parts, meaning they should not experience failure under normal use for the natural life of your car. In most cases they will last until after you trade, sell, or junk your car.

There is also a benefit in the color of the light produced by HID lamps over that of conventional halogen bulbs. A couple things need to be said about correlated color temperature first, as it is rather difficult to explain in lay language. The degree Kelvin that color hue is correlated with does not indicate in any way to the operating temperature of our source. The correlated color temperature scale simply relates the color hue of light produced from different sources to one benchmark, a super heated block of carbon. What is important to know is that we can relate the scale to daylight and familiar color hues. Typical daylight is about 5000k, pure yellow is around 3000k, white is around 4000k, and blues become apparent around 5500-6000k. I say apparent because most people think of natural daylight as white, where it really has some blue hue to it. Most unfiltered/uncolored halogen bulbs put out around 3500-3700k light, and as they age they go down the color scale becoming more yellow. All OEM HID comes with 4100k bulbs (there are also OEM 5000k and 6000k) and as they age they go up the color scale becoming bluer.

Those values only indicate where most of the light produced by the source falls in the color spectrum. In actuality, there is light all over the scale being produced. Without getting too much into Human physiology and the realm of sensation and perception, our physiology dictates that our eyes see better in natural daylight. Millions of years of evolution have adapted them to be that way. The closer to natural daylight your output, the more “useable” light there is for you to see. Each color is made up of a combination of intensity from each wavelength, and the 4100k color hue has spectra most heavily weighted in wavelengths around 505nanometers, where our eyes are uniquely most sensitive. A value too low or too high on the scale skews everything to that one side and decreases the intensity in our sensitive range. Therefore the 4100k produced by OEM HID bulbs is beneficial over the typical halogen output.

There is a gimmick among aftermarket kit producers to advertise 8000,10000, 12000, and yes even 30,000k kits, which completely negates all the benefits of using HID in the first place. The fact is these are cheaply made bulbs and the manufacturer does not even know what color temperature the output really is. The equipment to measure color hue is very, very expensive and not something a manufacturer of illegal kits would want to waste money on. In addition, anything that high on the color scale would produce almost no spectra in the sensitive range of visible light, 10000k is better suited for insects that see in the ultraviolet spectrum.

The color output is also subject to scattering of light; certain wavelengths will scatter more from interference than others. Diffraction occurs through interference with the water vapor in air, more so when there is a lot of it such as in inclement weather. It is the same effect that causes the sky to be blue! We perceive different wavelengths of light as different colors of the visual spectrum (rainbow). In order of large to small wavelength the major colors are red, orange, yellow, green, blue, indigo, violet. Smaller wavelengths are subject to diffraction more than larger ones because of simple physics. As you go towards blue hues, the more subject it is to scattering due to interference, meaning it creates what we would call glare and difficult visibility in bad weather. Yellow or orange-yellow has the combination of the least scattering and a majority of output still inside of the sensitive visible range, making it optimal for inclement weather. This is why you see yellow fog lights and honking big yellow lights on some rally cars.

The diffraction of blue light also lends to the phenomenon of “eye fatigue.” Eye fatigue refers to the phenomenon in which shortest wavelengths of blue actually are incorrectly focused by the eye simply because of physical design of our anatomy. Briefly, very short wavelengths in the blue spectrum cause some blurring of our field of vision, which can become worse over the time of exposure. Maybe you won’t notice when you get in your car, but an hour down the road you may start having difficulty with visual acuity.

Now you know why it is better, but how does it work?

HID bulbs produce light by using an electrical arc between two electrodes across a void rather than resistance across a filament as with halogen bulbs. The void is filled with a mixture of mercury vapor, sodium salt, scandium salt, and xenon gas. Just a note, OEM bulbs no longer use mercury for reactions, they use something that is hopefully more environmentally friendly and less dangerous should a bulb break. The high luminous output is due to the mixture being heated to a gaseous phase and ionizing, and a continuous reaction ensues until the source arc is turned off. The exact ratio of elements within the void also determines the correlated color temperature of the output. Let’s just say the formulas of OEM bulbs are closely guarded secrets of Osram and Philips. Aftermarket kit bulbs are generally cheap knock-offs, I wouldn’t be surprised to find that they still use mercury as part of their formulations. Poor quality control over the ratio of elements is what leads to premature and drastic color changes, decrease in luminous output, and failure.

HID bulbs age, and over time, they gradually lose luminous intensity and climb up the color temperature scale. However, this occurs in a logarithmic fashion. Within the first several hundred hours of use the bulbs will undergo a quick color change; this is commonly called “burn-in” or “color shifting.” The end result is that a yellow or very white looking 4100k bulb will probably end up being in the proper 4100-5000k range most of its useful life. Here is a not to scale representation of that process:



Actual 5000k OEM bulbs are meant as replacements for older 4100k bulbs should one fail or fall victim to a read bumper of another car. That way the newer bulb will more closely match the older bulb in the other headlamp.

HID also requires the use of a ballast and igniter to power the bulbs. Some ballasts are physically packaged as an all inone unit with an integrated igniter, others have separate packaging of the ballasts and igniters. The igniter is responsible for converting the 12volt DC supply from your battery to the 20,000-24,000 volts AC required to initially light the bulb by creating a sufficient arc. Once the bulb is lit, the ballast is responsible for supplying the appropriate 85volts AC, and phasing to maintain the electric arc. More importantly, it controls the amount of current flow to the bulb, much like a resistor. Without the current limiting control of the ballast, the reaction within the bulb would burn out within a second.

An important thing to note here is that the distance between the igniter/ballast and the bulb should never be altered. Wire, just like a filament, has resistance, and this is taken into account with the design of a ballast. Change it, and you change the operating parameters. Should you have a ballast with a separately packaged igniter, it is possible to lengthen the distance between the two if required.

Using HID on a halogen equipped vehicle requires the use of a relayed wiring harness to bypass the stock wiring. The stock wiring was chosen based on the electrical requirements of halogen system, not HID. HID will draw more current at start up than a halogen system will, but once lit the HID does use less power. Changing a fuse to meet the current draw of ballast is a very bad idea, which will tax the capacity of the wiring and cause it to excessively heat up. That, of course, is very rare, but other problems can occur as well. The most common being that only one ballast will be able to draw enough power to produce a sufficient arc, leaving the other one flickering in a desperate attempt to light. This is not something you want to happen, as it will decrease the life of your ballast and bulbs by doing so. A relay is a very simple device that closes a separate circuit when power is applied to its coil. The idea here is to use your stock wiring to power the relay’s coil to close a circuit directly connecting the battery to the ballasts.

All ballasts are not created equal. The general consensus is aftermarket ballasts face the same problems as knock-off bulbs; they will not stand the test of time like OEM will. Most aftermarket kit ballasts are based off inferior designs from companies that no longer exist. As a matter of fact one specific design that is common to at least 50% of kits out there is based off a design from a company that went under years ago. The reason the company went under is because the design had so many failure problems that every OEM that used them on their cars dropped them. They sold the design and since then it has been copied to death and is relabeled under every name imaginable. According to one source it took Philips over 5 years to invent the first automotive HID ballast that worked, so skimping on a properly designed ballast is not recommended. An improperly made ballast can prematurely destroy a bulb and it would appear as if it was the bulb itself.

We will only concern ourselves with low beam optics, since that is what we use to drive around with. One thing to note, low and high beam have less to do with intensity than they do with the actual height of the radiated beam. A low beam is designed specifically to slope gently downwards towards the ground from its point of origin avoiding direct radiance into oncoming traffic. This does, of course, limit viewing distance to the total run of the slope. A high beam actually focuses the highest intensity of the beam at a radiance in parallel (or near parallel) to the ground.



There are essentially two very different forms of optics we are concerned about here, reflector based and projection based. Reflector based has several different forms within its own category, but we will only focus on the newer “free form” reflector technology. The goal of both forms of optics is to take light from a source, reflect as much of its radiance forward, while uniformly dispersing it. The goal of dispersion is to have a nice wide beam with the concentration of the light being greatest at the top of the cutoff in the middle of the beam. Having a beam that is not correctly focused means you will have hotspots, which will decrease your total visibility. Having too much light intensity in the foreground limits the distance at which you can see objects.

A reflector is an overall parabolic shape, with multiple facets to direct light forward at a slight downward slope. They more commonly incorporate a bulb shield over the bottom and front of the bulb to block light from radiating up, straight, and down to the bottom of the reflector bowl where it would reflect back up again.

Some halogen reflector designs even incorporate more than one bulb to increase the intensity of the center of the beam pattern. A reflector needs to be manufactured within close tolerances to avoid excessive glare, since surface irregularities will cause some stray light. Every reflector is designed around one specific bulb type, since every bulb has a different filament direction and location; 1mm can make a world of difference. Reflectors are not universal for every bulb type and filament position out there. I’ll get to that again a little later.

Projectors start off with the same concept as reflectors, harness the light, and reflect it forward. But it is more uniformly distributed by the incorporation of a plano-convex lense. The razor sharp beam shape is produced by a cutoff shield that blocks a portion of the reflected light from hitting the lense.

By changing the shape of the lense, depth of the lense, design of the cutoff shield, and focal point of the projector, manufactures are able to make very wide, very uniform, and very precise beam patterns.

Projectors, also by the nature of their design, sometimes create color that may be considered a fringe benefit. Although it provides no useful lighting function, cutoff color, sometimes referred to as “flicker”, can create an amazing show. Because a projector purposely uses a shield to block some light, it causes interference with light at the very edge. The phenomenon of diffraction takes place when light bends around an object causing interference. Smaller wavelengths are subject to more diffraction since they can more easily escape at a steeper angle.

This is why you see blue and purple flickers from some projector HID cars. As the car goes over a bump or other imperfection in the road surface, the suspension hops up and down. This in turn causes the headlight beams to cast up and down. As the thin, blue band of color crosses the plane of your sight, it appears as a quick flicker from white, to blue and back to white.

Notice the thin band of blue on top of the cutoff:



Same projector as seen from the front, these two pictures simulate color flicker:





Most people tend to think that degree Kelvin color temperature is what makes the brilliant blues you see with a projector. In fact, that is only because aftermarket kit manufacturers are trying to simulate the natural byproduct of projectors by creating 6000, 7000, 8000, and 10,000k kits. The fact of the matter is using “bluer” bulbs will not increase the brilliant deep blue flicker created by the projectors; it will only greatly decrease your road visibility. All OEM HID based projectors use 4100k bulbs for the best possible output on the road; the individual differences you see from car to car are the different projectors they use.

What is the deal with aftermarket kits?

There are cars with OEM reflector based HID so I can use HID in my halogen reflectors to get better output, right? It’s not that easy, there are repercussions from this one size fits all approach. Each and every reflector is designed around one specific bulb type, the reason being that every bulb type has a unique position for the source whether it is a filament or an arc. Since the tolerances for the one bulb type have to be small to prevent excessive stray light, using a completely different source position can change the entire output. Using a brighter source like an arc lamp in a halogen application will only compound this particular problem.

A lot of aftermarket kit sellers will say they precisely line up the focal points to match their halogen counterparts, however, the quality control this requires will most likely not appeal to kit sellers. It is just too difficult for the majority of manufacturers out there, and they would need specialized equipment to do so. This little ploy also falls through when you get to applications such as H3 bulbs which have a horizontal filament as opposed to the longitudinal arc of a HID capsule. Once again, here is a not to scale representation of a very, very simplified reflector design:


And an example of that, notice the lens flare:




The other drawback to inadvertently changing the position of the source is beam width can be changed. By moving the source far back, the beam gets increasingly narrow and heavily weights the intensity near the center of the cutoff. This has the very dangerous effect of limiting the visibility of the road shoulder where pedestrians, animals, and disabled vehicles are likely to be located. Just to bring this back to the H3 HID kit example from above, the narrow beam is exactly what happens when using a kit in H3 applications.

Bring the source far forward, and the beam widens and moves the properly weighted intensity from the middle of the beam outward, leaving a dull area directly ahead.

OEM Retrofitting vs. aftermarket kit:

The term “retrofit” lately seems cover a broad spectrum of custom fabrication of optics or aesthetics into a headlamp. It really can cover anything from custom angel eyes to adding LED turn signals. Of course, here we are interested in OEM HID retrofitting, or replacing stock halogen optics with those designed specifically for use with HID. This can be done with either OEM HID reflectors or OEM HID projectors, the demand being for the latter. Reflectors can still be made very well but they are subject to design limitations. Reflectors can also be difficult to retrofit into another housing depending on the application. Projectors are more efficient at uniformly distributing light and have the benefit of creating razor sharp cutoff of the beam pattern. Although DOT specifications require a certain specified slope to all low beams regardless of source optics, having a sharp cutoff means the ability to raise the cutoff slightly. This gives you greater visibility farther down the road. Car manufacturers are required to set low beam heights to these specs, but it is becoming increasingly apparent that they too are aiming a little higher. Normal spec. is 2.1” drop over 25ft., where one can safely achieve a 1-1.5” drop over 25 ft. The other major benefit here is beam width which is generally greater than that achieved with a reflector. Although, some newer projectors incorporate both a projector and a smaller reflector to make them extremely wide:

Width is very important to seeing the sides of the road, more so the right hand side shoulder. Illuminating the shoulder means greater visibility of pedestrians, animals, and disabled vehicles.

Retrofitting an OEM HID based projector has other fringe benefits as well, and many times people seek out a projector retrofit for the sole purpose of color flicker. In this case it is important to choose a more colorful projector as well as one that has good lighting qualities. Almost every HID projector can be modified in some way to create even more color in the cutoff than they have stock. Sometimes this has a trade off in deteriorating the cutoff sharpness or changing the beam pattern by moving the focus. Modifying a projector should only be done in very small increments to prevent deterioration of the beam. The additional color can be achieved by a variety of means through bending the cutoff shields, changing dimensions between the separate parts, and removing stock spacers. Usually only one method works well for each projector, and some projectors respond much better than others.

There is a cost associated with everything, and retrofitting can be difficult. It is something that definitely comes easier to some than others, not something that everyone can do-it-yourself. In most cases, everyone screws up the first time; there is a learning curve, even the experts will tell you that. If you have someone else do the work for you, depending on what you want, it can get expensive. There are times when the aftermarket kit is a more cost friendly solution to better lighting, but it MUST be done responsibly.

Halogen projectors are one instance in which an aftermarket HID kit may be the better option. This is very dependent on what type of application your stock projector uses. There are several things that need to be considered first. Always use a high quality kit, preferably one that uses OEM ballasts, and at least claims to use OEM bulbs, although you’ll have a hard time proving the bulbs are OEM. Try to find a kit seller that has the “rebased” HID bulb dimensions that correspond to the halogen counterpart, if they cannot show you that then don’t bother. Many halogen projectors do not have cutoff shields that are what I call complete. They may have a very thin shield that has open areas underneath or holes in it. It is required that you somehow block those to block light from glaring. Not every bulb application will work; H3 is just one that no matter how good the projector is it will not work correctly. H7, on the other hand, almost always works very well with a high quality kit.

Fog lamps and/or driving lights are often great for aftermarket kits as well. You do need to be responsible enough to only use them when in inclement weather and when there is no opposing traffic. Generally you would want a 3000k yellow bulb kit here, since using anything higher may make your results worse than stock in bad weather. If you can’t find a good quality 3000k bulb kit (there are 3000k OEM bulbs), try using a 4100k kit with optically clear yellow lense protection like lamin-x. Even with a 3000k kit you may want to use yellow lense protection to get the output as close to orange-yellow as possible.



A word about legality:

The National Highway Traffic Safety Administration (NHTSA) is responsible for setting the standards for automotive safety. As of November 1st, 2005 the NHTSA has revised the Federal Motor Vehicle Safety Standard 108 (FMVSS 108) that covers vehicle lighting, due to increasing consumer and manufacturer concerns. The current safety standard has been changed to allow the use of other than OEM source lighting. However, the standard still requires the aftermarket replacement lighting to meet all the same qualifications and photometric requirements as OEM.

The interpretation of the new standard is that anything not stock must meet the same requirements for glare and beam pattern output. Aftermarket HID kits in a stock halogen reflector in probably 90% of cases are going to fail the photometric requirements, and therefore are illegal for road use. 90% of aftermarket replacement lamps (OEM knock-offs, non-projector) are not going to be legal either; as a matter of fact, the manufacturers of such lamps have been under scrutiny for a while. Aftermarket projectors are probably okay with their stock halogen bulbs, some may be okay with HID kits as long as the cutoff shields completely block all stray light. Properly constructed OEM HID Projector and OEM HID reflector retrofits are definitely going to be compliant since the sources already meet the requirements with the vehicles they come on. An OEM HID projector retrofit that has a color modified projector in most cases will also meet the requirements just fine, as long as care is taken not to deteriorate the cutoff and beam pattern.