Description of the Lamp / Bulb
Safety and Reliability Requirements
Legality of Alternative Headlights
Links to Related Sites
Other, more experimenter-friendly low wattage arc lamps
The main part numbers are:
D2S - plain
D2R - like D2S but with heat-resistant black paint on spots to control the light output pattern
D1S - like D2S, but with integral ignitor
D1R - like D2R but with integral ignitor
The above are 35 watt lamps. D2S and D1S types nominally produce 3200 lumens of light and the D2R and D1R types nominally produce 2800 lumens of light.
The arc tube or inner bulb is made of plain fused quartz and has tungsten electrodes with the distance between the tips approx. 4.2, maybe 5 millimeters (approx. or slightly under .2 inch). Its construction resembles that of a miniaturized short arc lamp, but true short arc lamps have a much more concentrated arc.
The arc tube has xenon gas in it at a couple of atmospheres to maybe a few atmospheres when cold and a few to maybe several atmospheres when hot. There is also mercury in the bulb, and when it is vaporized the mercury adds at least 20 atmospheres of pressure for a total pressure of around or maybe even over 30 atmospheres.
Metal halides - salts - are also in the arc tube. The formulation in automotive HID lamps includes sodium and scandium halides (probably iodides) and maybe traces of others such as lithium and thallium halides.
More ordinary metal halide lamps do not have high pressure xenon but have low pressure argon instead. The high pressure xenon is used to obtain some usable light output during warmup before the other ingredients have vaporized.
The D2 types require a starting pulse. 7 kilovolts may on an average spark through these bulbs, but for reliability you need more, maybe 10 or possibly 12 kilovolts. Automotive use requires ability to restart a hot bulb with the mercury vapor pressure high, and this requires even more voltage - 12 to 15 kilovolts and maybe even more for good reliability. The usual ballasts supposedly produce starting pulse voltages like 18 kilovolts minimum, 20 kilovolts typical.
D1 types have an integral ignitor which the ballast has to work with.
Starting pulses must be repeated frequently until the arc is established.
The ballast must supply an open circuit output voltage - other than the starting pulses - of over 300 volts, preferably 400 or maybe preferably 450 volts - to force the arc to establish.
D1 and D2 type lamps are 35 watt lamps. Once the arc is established, the ballast must supply limited current or else the arc will draw extreme current and this will be bad for the bulb and/or other parts. The voltage across the lamp is normally around 80-90 volts when it is warmed up, but will be less during warmup. The ballast must handle a lamp voltage possibly as low as 16 volts early in warmup, although this voltage usually bottoms out higher - probably at least in the 20s of volts.
The ballast must deliver 35 watts to the lamp when the voltage across the lamp is between 70 and 110 volts. When this voltage is lower, the ballast must deliver at least .5 amp but generally no more than 2 amps and preferably as close to 35 watts as possible. Higher currents are preferred - a partially warmed up metal halide lamp sometimes has an unstable arc at lower current.
An automotive grade ballast often delivers boosted power (above 35 watts) at some times during warmup to give near-full light output. Note that a xenon arc or a mercury vapor arc does not produce visible light as efficiently as a metal halide arc does. Automotive grade ballasts with boosted power at some points of warmup have circuitry that models the thermal characteristics of the bulb. (Update 11/27/2021: Or, the ballast monitors warmup progress of the bulb, such as by monitoring voltage across the bulb, and I suspect some ballasts did that at least as far back as in 2008.) The maximum safe current for the bulb's electrodes must not be exceeded during a power boost during warmup.
A voltage across the bulb higher than 110 volts only occurs in the early stage of establishing the arc or if the bulb is failing. The ballast should deliver enough power to heat up the electrode tips enough for the arc to establish - more is better and over 35 watts is OK as long as the current is not excessive. But excessive power delivered to a bulb that is fully warmed up or in poor condition from age can cause the bulb to explode.
D1 and D2 lamps and most other metal halide lamps require AC. DC is tolerable briefly, and then preferably only if the bulb is cold. A DC electric field, hot quartz or hot glass, and salts or alkalis is not a good combination - electrolysis effects can occur which can create weak spots or cracks in the arc tube.
The AC delivered to a D1 or D2 type bulb usually has a frequency of a couple hundred to a few hundred Hz. Higher frequencies are probably OK with D2 types but the ignitors in D1 types may only work correctly or even be adequately conductive in a certain range of frequencies.
The AC current waveform in a D1 or D2 type lamp is traditionally a squarewave or close to a squarewave. Other waveforms have higher peak current for a given average current or RMS current, and the higher peak current is harder on the electrodes and may shorten the life or cause problems with the use of higher currents during warmup.
Metal halide lamps should not be overpowered, except where permissible for
accelerated warmup and near-full light output during warmup. Overpowering
one will shorten its life and increase the risk of the lamp exploding.
Underpowering a metal halide lamp is also bad. If the electrodes are not hot enough, they do not do a good job of conducting electrons into the arc and voltage drop in this process (known as the "cathode fall") is excessive. Excessive cathode fall causes positive ions in the arc to hit the electrode at excessive speed which "sputters" electrode material onto the inner surface of the arc tube. For more info on discharge lamp mechanics, look in my Discharge Lamp Mechanics File.
It is not recommended to experimentally operate metal halide lamps at reduced power. Besides the bad effects of high cathode fall on hot electrodes, an unusual temperature pattern can have the chemicals in the arc tube condense in locations that can block some of the light. And if the electrode cathode falls are excessive and unequally so, a DC electric field can result, which can cause destructive electrolysis effects on hot salts on hot quartz. This can cause the arc tube to crack.
Metal halide lamps should have power input within 10 percent of their rated wattage.
The bulb must be clean and free of dirt, grease, organic matter, ash, salt, or alkali. Salts, ash, and alkalis have a tendency to slowly leach into red-hot and nearly red hot quartz which will result in strains, weak spots, and maybe cracks.
A metal halide lamp does not like frequent starting. D1 and D2 types can be blinked, but this should only be done for a limited amount of time. Starting causes wear on the electrodes. Excessive evaporation of electrode material will deposit it onto the inner surface of the arc tube which results in darkening and overheating of the arc tube. In D1 and D2 and some other metal halide lamps, there is a halogen cycle which cleans deposited tungsten electrode material from the inner surface of the arc tube. Prolonged continuous operation at proper internal temperatures is required for the halogen cycle to work.
It is illegal to use on public roads homebrew headlights or headlights using a bulb other than what they were DOT-approved to use. For example, a headlight that is DOT approved and normally uses a 9005 halogen bulb is almost certainly not DOT approved for anything else - especially not a D2S for example.
Many HID conversion kits come with disclaimers to the effect of "off road use only". Such disclaimers may appear in the kit seller's ads, web site, or on the kit packaging. Less honest retrofit outfits may merely fail to let you know that such a retrofit is not road-legal. More dishonest retrofitting outfits may even falsely claim that their headlights or ones modified with their product/service are road legal.
DOT requirements have lower and/or upper limits (sometimes both) on candela ("beam candlepower") into many different directions, as in various angles above, below, and to each side of straight ahead. In the unlikely event your headlight meets all of these and other technical requirements, it is still illegal unless it is submitted for testing and certification.
As for what can happen if you use illegal headlights?
1. Often enough, nothing. This depends on location. In some USA cities, law enforcement of traffic regualtions in general is lax. Police are generally not equipped to do headlight photometry anyway.
2. Some unlawful HID retrofit headlights as well as many fake HID headlights that neither meet complex photometric and colorimetric standards nor have valid legal certification are obvious to a few cops.
If the cop believes you have unlawful headlights, you can be stopped and ticketed. Depending on your state, the violation may be having an invalid inspection sticker or whatever violation of headlight law. Depending on your state and the mood of the cop, you may in extreme cases be barred from driving the car at night (or at all) until it has legal headlights and it has passed inspection again with the legal headlights.
3. Excessive light in some directions can dazzle other drivers. It is possible for you to be legally liable if this causes or contributes to an accident. Modified headlights might have insufficient light in some directions, and you could be held legally liable if that causes or contributes to an accident.
4. You may have trouble with your insurance company if you have an accident while driving a car that cannot legally pass inspection or has fraudulently passed inspection, even if the inability to legally pass inspection did not contribute to the accident. You might also have trouble with your insurance company if you are cited for driving without valid current inspection stickers or are cited for having fraudulently passed inspection.
Note that at least in some states, "off road" lights must be inoperative when driving on a public road. This may mean having opaque covers on the lights and/or having wiring to the lights disconnected.
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