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How do HID Xenon Bulbs Work and Are They Road Legal?

HID xenon bulbs are quite a recent development in automotive lighting technology. The HID stands for High Intensity Discharge, which refers to the unique way in which the bulbs produce their light. Unlike ordinary halogen headlight bulbs, HID xenon bulbs do not contain a conventional metal filament, so in order to ignite the xenon gas and metal halide within the glass bulb a very high voltage spark is needed which is delivered by two electrodes.

The high voltage that is used to generate the spark is around 20,000 volts, which is considerably higher than a car's normal 12 volt supply and so a metal box, called a ballast unit, is fitted to help boost the voltage. Once the spark has been generated and the bulb is illuminated the voltage drops down to a steady 85 volts.

The xenon gas belongs to the group of elements known as the noble gases, and is held within a single sealed glass tube above the base of the bulb. When ignited the xenon helps to produce a light that is 300% brighter than halogen, and because the light is also much whiter it more closely resembles natural daylight.

The bulbs used in HID xenon headlights typically last around 3000 to 4000 hours and when they do finally fail they normally give a warning by flickering intermittently before eventually failing to light up at all. It is possible to change these xenon bulbs yourself, however because of the high voltages associated with HID systems always make sure the electrical power is off, and if you are at all unsure arrange to have the work carried out by a qualified mechanic or auto technician.

When buying replacement HID bulbs you may be surprised at their high price, especially when compared to standard halogen headlight bulbs, however they do not have to be replaced in pairs and because of their exceptional service life they do still offer great value for money. There is also now a growing range of upgrade HID xenon bulbs that allow you to personalise the night time appearance of your car. Whilst the standard HID bulbs produce a white light with a yellow tint, upgrade xenon bulbs are available in blue, purple or pure white tints.

Because they produce such a bright light many drivers wonder whether HID Xenon bulbs are actually road legal. Well the simple answer is that they are completely road legal if they are part of a manufacturer's factory fitted HID lighting system. It is possible to buy aftermarket kits that upgrade standard halogen headlights to HID xenon bulbs, however at present the law does state that these type of HID conversion kits are not legal.

About the Author

Jo Alexander is an online, freelance journalist and keen windsurfer. Jo lives by the sea in Essex.

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