What are HID headlights?

Check the sales brochure of a high-end new car and chances are you’ll find that the headlights are HID units. But let’s start at the start: What are HID headlights?

HID actually stands for High Intensity Discharge. Unlike old-school automotive lighting (Halogens and sealed-beam lights) which are incandescent (where a filament is heated until it glows) High Intensity Discharge lights are arc lights. That is, an electric current arcs between two electrodes, in an atmosphere of inert gas (typically Xenon) to create a bright, white light that is, as the name suggests, high intensity. For background, a Neon shop signs uses the same arc-lighting principles.

It’s not that simple, though, and creating the arc requires a lot more than a car’s 12 volts. That’s where the ballast comes in, which works like a transformer to step the voltage up to around 20,000 volts to initiate the arc and then a steady 85 volts or so to keep the arc stable.

The big advantage with HIDs is that they are super bright – as much as three times as bright as a conventional halogen globe. You’ll spot them in traffic by their clear, white light, often with a blue edge where the beam is cut off by the lens. And although they require plenty of volts, they don’t draw as much current and typically require just 35 Watts to run them, compared with, say, 55 Watts for a halogen, reducing the load on the car’s battery and charging system.

The downside is that they’re often dazzling to oncoming traffic. That’s why there’s an Australian Design Rule for them mandating that they feature a self-levelling system to keep them pointed where they should be, as well as a washing system to avoid the build-up of grime on the lens which can cause diffraction and dazzling. For this reason, some manufacturers use HIDs for the low-beam lighting and a conventional light for the high-beam system. 

A common question is whether aftermarket kits to convert a car to HID lighting are illegal or not.  It’s not a simple question to answer because it will largely depend on the mood of the police officer and the attitude of the driver when the confrontation eventuates. Provided you comply with all ADRs on the subject (including the washing system) don’t exceed 2000 lumens of brightness and stay within the warm to cool-white colour spectrum (4300 Kelvin to 6500 Kelvin) you should technically be legal. You also need to make sure the HID headlight kits you choose feature ECE certification (A European standard that applies here to HID lighting). And the self-levelling ADR? A classic grey area as many European cars sold here with HID lighting don’t have this feature. Nobody is sure how they slipped through the net, but they did. Again, your roadside attitude will play a huge part in how things pan out.

The big comparison most consumers make is the HID vs LED headlights one. Many specialists now believe that an LED light upgrade represents a better, cheaper, simpler alternative. LED lighting is so good these days, that a legal HID conversion simply doesn’t offer any real advantage.