Gas discharge lamps generate light according to the physical principle of electrical discharge. Through the application of an ignition voltage from the ballast (up to 23 KV in 3rd generation HELLA ballasts), the gas between the lamp electrodes (filled with the inert gas xenon and a mixture of metals and metal halides) is ionized and made to glow with the aid of a light arc.
During the controlled feeding of alternating current (at approx. 400 Hz) the liquid and solid substances evaporate due to the high temperatures. The lamp only achieves its full brightness after a few seconds when all the components have been ionized.
To prevent destruction of the lamp through uncontrolled increases in current, the current is limited by a ballast. Once the full light output has been reached, an operating voltage (not the ignition voltage) of only 85 V is necessary to keep up the physical process. Luminous flux, luminous efficiency, luminance, and service life are significantly better than with halogen bulbs.
Gas discharge lamps are categorized according to their respective development version: D1, D2, D3, and D4. The "D" stands for "discharge". There are some major differences between the generations. The D1 lamps, for instance – the original xenon bulbs – have an integrated ignition section. D2 lamps, on the other hand, only consist of the socketed burner itself and, unlike all other development versions of automotive gas discharge lamps, have no exterior protective glass bulb around the discharge tube. All further developments have a UV protection bulb and are much more robust in their design.
The old D1 is often mistaken for the current D1 S/R lamp with integrated ignition module. Further developments of the D1 and D2 lamps, the D3 and D4 lamps, are more environmentally compatible, as they use no mercury. Due to different electrical parameters (42 V instead of 85 V arc voltage, with identical output), the D3 and D4 lamps cannot be used with the control units for D1 or D2 lamps.