Offering more versatility in styling  and better flexibility in packaging, manufacturers have a wider range of options than the old round sealed beam or the rectangular halogen bulbs could give them. In fact, don't think of HIDs as bulbs at all because technically they are not. They are "arc tubes" without any filament. In the tube are two electrodes and a xenon gas that, when energized, creates an arc that lights the tube. The tubes will have a ballast (one for each tube) that begins the lighting process. 

How it works. First the driver has to request the headlights come on, whether by traditional pulling a switch or by using the auto headlights feature which usually involves the BCM. At the input connector of the ballast the supply  voltage (that would normally turn on a traditional sealed beam) arrives. Now (depending on model) with HIDs, the ballast can draw up to about 20 amps from the battery for a period of 5 to 10 seconds. This is known as the "strike state" or "start state". The ballast will provide multiple  high voltage pulse to the arc tube to start the lighting. During this "start state" the ballast can provide anywhere from -600 volts to +600 volts (again depending on the vehicle application). Once the arc is established the "run up state" will begin. In this state the ballast will provide higher than steady state voltage to the arc tube to keep it illuminated. The time the system spends in the run up state depends on the temperature of the tube. Usually in the neighborhood of 15 to 40 seconds. Finally, when the "steady state" is achieved, there is about a 35 to 45 watt power level and a minimum amount of voltage is provided by the ballast to keep the arc tube. 

(Figure above with HID hardware courtesy General Motors Corporation)

As you can see, we are talking some serious voltages here. Some systems can achieve 25,000 volts within the ballast. Therefore extreme caution should be followed here. Make the cautionary notes in SI your guideline for servicing these systems. HIDs work a lot like conventional ignition systems do - take in  a low voltage signal, send out a high voltage to do the illuminating in the tube, so treat it with same type of respect and caution that you would use when checking the secondary side of the ignition system. NEVER probe or test the output connector of the ballast as it could SHOCK or BURN you as well as ruin testing equipment. It would be the equivalent of holding the end of a plug wire, while checking for spark, while someone cranks the engine over. NOT a good idea!

(Figure above of the arc tube courtesy General Motors Corporation)

The glaring issue...
has been a subject of debate for a while now and although there have been numerous complaints about the "blue light", High Intensity Discharge Lighting continues to gain acceptance. A big factor in these concerns is aiming. The U.S. seems to be lagging behind their European counterparts who have passed regulations to reduce glare with load leveling and other techniques such as headlight cleaning. The current glare issue may be related to the same phenomenon that occurred when Halogen bulbs first appeared in the early '80s. Like then, oncoming drivers may have been attracted to the unique look of the lighting system and noticed their increased brightness, according to the National Highway Safety Administration (NHTSA). Initial halogen headlamp introduction elicited some complaints, even though the early halogens used were very similar in performance to the standard sealed beam of the time; the only marked difference was the color. If this is now the case, the NHTSA would expect the number of complaints to decrease as the motoring public gets more used to the color. 

Mik Stubing-Lead Instructor Northeast Region