HID bulbs have a capsule filled with a mixture of noble gasses and metal halide salts. Unlike a halogen bulb that relies on a glowing tungsten filament to produce light, xenon bulbs work differently. When the ballast fires, it produces around 25K Volts; this is required to start the arc across the two electrodes inside the capsule of the HID bulb. At first, the added intensity and blue color you see is from a type of emission around the two electrodes. In a short time, the temperature in the arc chamber rises, vaporizing the salts. These vaporized salts are then ionized by the arc into a plasma. This plasma is what emits light. It also serves to allow the required arc voltage to drop to around 84V, which is easy for the ballast to maintain - making them very efficient.

Typically, HID bulbs have a very long lifespan of around 2500+ hours, compared that to a standard halogen bulb, which generally only lasts around 300 hours. Some early High Intensity Discharge bulbs ran DC current, but they suffered from short bulb life. This is because in DC, one electrode is constantly bombarded by high speed positive ions. This results in a noticeable shortening and deforming of the electrode. Over time, this erodes away the electrode to the point that it fails. By running AC, the high-speed ions, hit both electrodes, essentially doubling the life of the bulb. Another benefit is that the electrodes, run a little cooler using AC because the anode and cathode change about 400X per second compared to DC where one is always anode and the other always cathode (shared heat over two electrodes versus just one).

Things to Consider

1. Size/Fitment: The first and foremost item of importance is the size of the bulb. It doesn't matter if you spent $250 on the best set of bulbs known to mankind; if they won't properly fit the housing/fixture that they're intended for, your light output will be awful. This goes for any install, whether you're considering an OEM HID projector based retrofit requiring D2S, or an upgrade to the factory halogen projectors designed for H1, H7, etc. When the bulb is fully seated in the back of your projector, the distance and centering of the arc-capsule relative to the bulb base is very important. Projector optics are so sensitive that if the bulb doesn't fit well, these points will be off, and your output will suffer. Find out what size you need first before looking into your options. Size Guide

2. Precision: The alignment of the arc capsule is crucial in achieving good light output. If the capsule is out of place in the reflector, even if it's off to one side or tilted down by a only a millimeter - the imperfection will be magnified and projected on the road through the lens. Buying a set of bulbs built to a set of higher standards will pay off with a tighter beam pattern, reducing otherwise hotspots, and producing a cleaner cutoff line. Make sure the bulb glass that rises from the base is surrounded by some sort of metal framework or tight-fitting ceramic sleeve to ensure the best possible beam pattern. Avoid bulbs constructed with a plastic-only base.

3. Halide salts: The blend of salts in the capsule that compose the precise formula to achieve their Kelvin rating (such as 4300K) is what sets them apart. The main salts in 4300K bulbs are Scandium Iodide and Sodium Iodide, Dysprosium Iodide is also added to some bulbs. While in higher Kelvin bulbs, more Indium Iodide is used. This is what gives the salts in those bulbs their red color. It also is really expensive and hazardous to use, generally increasing their price. Less salts = less light and higher operating temperatures. When looking at the bulb (unlit) you should see some deposits in the capsule/bubble - those are the halide salts and they should be aplenty.

4. UV Output: The outer glass shell on the bulb is there to filter harmful UV rays from the capsule, so it should be produced with that in mind. Materials like quartz are often used since they're more apt at softening UV output relative to other lesser expensive options. Like a person out in the sun, UV can damage/tarnish the interior of your headlight housings and projectors after prolonged exposure. Beware of this and shop for bulbs built with glass that offers some level of filtration.

5. Kelvin: We commonly get customers looking for high kelvin [ie 8000K] bulbs because they want their headlights to be "brighter". There is a huge misconception here. As the Kelvin rating of a bulb increases, its Lumen rating decreases. Simply put: 4300K bulbs will put more usable light on the road compared to 6000K, 8000K, 15,000K bulbs. TRS does not offer bulbs with Kelvins higher than 6000K because A) We don't cater to obnoxious "enthusiasts" who want their headlights to look like a Christmas tree, and B) it starts to defeat one of the main purposes of the headlight upgrade: more light output. Kelvin Reference Guide

6. Wattage: While it's not the end of the day if you run a 55w bulb with a 35w ballast or a 35w bulb with a 55w ballast, it is ideal to couple the two ratings. A 35w bulb ran at 55w will burn at a reduced Kelvin rating relative to its supposed measure. The lifespan may also be reduced by a marginal amount. A 55w bulb on a 35w ballast may last longer because of the beefed up electrodes, but may not always strike on as reliably.

7. Low beam or Bi-xenon Projectors: When choosing a bulb from TRS, it does not matter whether you are using low beam or bi-xenon projectors. Unlike halogen headlights, with bi-xenons, nothing happens to the bulb when the high beams are activated. It does not get any brighter or shift positions. Simply put: you don't need "low beam" or "bi-xenon" specific bulbs. Our S2K-R low beam projectors and FX-R bi-xenons both use "D2S" bulbs.

8. Warranty: Like any other bulbs on your car or in your house, HID bulbs will eventually burn out. When purchasing anything other than a set of $10 cheapo bulbs from eBay, make sure you're buying from a manufacture or distributor that offers a decent warranty and replacement policy so that in case something does happen to the bulb, you'll be covered quickly with a replacement.

9. Replacement policy: Remember that bulbs color shift over time, so if your bulb goes bad a year after you bought it and the seller replaces it, it will not match the other one. Most reputable companies will replace both bulbs with new units should one go bad in a certain period of time. You should also be sure that you won't have to return the defective bulbs before any replacements are issued, since as soon as your headlight burns out, the clock starts ticking before you're issued a citation from the police.

10. Authenticity: If you're considering forking out the big bucks for a set of more expensive OEM bulbs such as Philips 85122's or Osram CBI's - you can never be too careful checking the sellers reputation. Within the last year, we've seen more and more vendors advertising fake Philips bulbs especially. These bulbs are typically sold around the same average price or just under compared to the genuine ones, but don't offer nearly the same level of performance and quality. We estimate that more than half of all OE brand HID bulb sales on sites like eBay or Amazon are not real, so if the price looks too good to be true- it probably is. Buyer Beware!

Our Recommendation for Retrofitters

The Morimoto XB35 bulbs easily offer the most bang for the buck for D2S bulbs. They're available in a variety of K ratings to suit everybody's tastes, they're precisely aligned for a nice crisp beam pattern, and they use a special mixture of halide salts to give them intensity and longevity similar to the specialty Osram bulbs. Philips 85122 is a great middle ground, and if you want the best of the best, go for the Osram CBI. As the gold standard today, they have the most lumens, great reliability, and you can still get a ton of color flicker with them despite their 5000K color temp.

Further Reading

Quick Size Reference Guide:

D2S: Used for projector-based HID headlight setups. The most common type of HID bulb.(S stands for Shielded, as in the "cutoff shield" inside the projector).

D2R: Used for older reflector-based HID headlight setups.(R stands for reflected).

D2C: "Universal" HID bulb that can be used in reflector or projector based HID headlights. Only produced by aftermarket manufactures.

D4S: An entirely new generation of more environmentally safe HID bulbs. These bulbs are 100% mercury free, and are marginally brighter than their predecessors. They require special ballasts designated for use with D4 style bulbs. Designated as "XenEco" bulbs by their manufacture; Philips. According to our customer demand for OE replacements, reliability isn't up to par with D2 systems.

D1S/D1R: Similar to the D2 series bulbs in that "S" denotes for projector use, and "R" denotes for reflector use. The main difference compared to the "D2" series is that the igniter is integrated into the base of the bulb. The igniter is built into the bulb here instead of being part of the ballast system. Newer than D2S/D2R, but not as popular since most higher performance projectors are for D2 bases.

D3S: Similar to the D1 series, but with eco-friendly (mercury free) capsules like D4's.

H1: The most popular non D series bulb, uses a D2 capsule built on a compact base and compatible with the Morimoto Mini H1 Bi-xenon projectors.

All Other Sizes: H1, H3, H4, H7, H8, H9, H10, H11, H13, 9003, 9004, 9006, 9007 etc. These are the most common sizes for original equipment halogen headlight bulb sizes, which often have HID versions available through aftermarket manufactures. You will never find a set of H7, H4, etc HID bulbs that are truly made by Philips, Osram, or any other OEM manufacture. (Despite the fact that they're fairly easy to find, those are fake).

Kelvin Reference Guide:

3000K: Has a golden yellow output perfect for use in fog lights. The yellow light penetrates through snow, rain, and fog more effectively than white or blue light, making it more ideal for bad weather applications.

4300K: Has a slightly off-white output similar to that of sunlight. All cars that come equipped with HID headlights from the factory use 4300K bulbs. If you want the most usable light, 4300K is the kelvin rating for you.

5000K: A pure white output, no tinge of yellow, and no tinge of blue.

6000K+: As you move into the higher kelvin ratings, your light output will appear more bluish-purple in hue, but the bulbs won't be physically as bright.

D1S/D1R: Similar to the D2 series bulbs in that "S" denotes for projector use, and "R" denotes for reflector use. The main difference compared to the "D2" series is that the igniter is integrated into the base of the bulb. The igniter is built into the bulb here instead of being part of the ballast system. Newer than D2S/D2R, but not as popular since most higher performance projectors are for D2 bases.

Color-Shifting: As HID bulbs age, they go through what is known as "color-shifting". When the bulbs are brand new, the output is whitish-yellow and as physically bright as it will ever be. As the bulbs age, their output turns more pure white/bluish and also starts to dim a bit. That's why cars with factory 4300K bulbs that are 5+ years old look like they're running 5000K or higher bulbs. This is due to the deformation of the electrodes that occurs as time goes on. To understand this, one must understand that near the electrodes, a region of plasma glows deep blue. As the electrodes are deformed, this region grows larger and contributes more to the color of the bulb. Bulbs that resist color-shift have modified electrodes to prevent this deformation (e.g. Philips 85122+). Another tactic is to make it so that the blue area around the electrodes is not in an area that the projector can "see". This helps to minimize its effect as well.

Color Flicker: Choosing a high-kelvin bulb doesn't guarantee you'll get the exotic color flicker effect that everybody lusts after. The effect is produced entirely by the projector and lens combination.