Over the last 15 years High Intensity Discharge (HID) Bulbs have become popular in luxury and sports cars – even now being fitted to more family focused vehicles, but where did they come from?
HIDs can be found all over the place – not just on the fronts of cars. They were first used to throw huge amounts of light over large areas like football stadiums and indoor venues. They work by passing an electric arc between two tungsten electrodes. Between the electrodes is a mix of metal salts and gas which makes a high intensity light from the plasma formed when the arc passes through. This structure produces much more light than fluorescent or incandescent bulbs.
There are a number of different types: Mercury Vapour, Metal Halide, Ceramic, Sodium Vapour and Xenon Short Arc. Although car headlights are called Xenon, they are not the Short Arc type, but rather the Metal Halide with Xenon gas.
The Xenon bulbs produce a lot more light than a standard bulb with the same power consumption and they give off a very sharp beam, so sharp that you can see a clear flat line on a wall at the top of the beam. Because they are so sharp, each car has to have a self levelling device and a cleaner unit attached – this is a European regulation that all manufacturers have adopted. It is illegal to retrofit HIDs into the shell of an existing bulb as they will dazzle oncoming drivers and temporarily blind them.
HIDs come with different filters due to the large amounts of ultraviolet produced. Filters and glass compounds have been designed to retain the ultraviolet inside the bulb and if you look at the back of an HID bulb you will see a dark shield that is designed to reflect the light forward and into a suitable pattern for driving use. It is this shield that produces the sharp line at the top of the beam.
When you switch them on, they light quickly and then the light output increases until they are at their maximum. This is because when switched on, they are “ignited” by a high voltage pulse and then they are controlled to maximum light because of the intense heat. This is done using a ballast to control the voltage and power consumption.
Another factor in the use of HIDs is that they last up to 4 times as long as normal headlight bulbs and this helps to offset the extra cost in the manufacturing process.
In my view, Xenon HIDs will be replaced by Light Emitting Diodes as seen on recent Audi’s like the R8, A4 and A5. LEDs have come a long way since they were first seen on mass produced wristwatches! It was BMW who started the HID trend in the late 1990s, and we see Audi leading the LED charge as they have clustered them to provide more light, prompting many manufacturers to follow suit. Interestingly, I believe LEDs will spread far quicker than most technologies and we will see them in low cost cars very soon.