HID (high-intensity-discharge) lights are often installed as original equipment on highend cars. But there are plenty of aftermarket conversions to be found (and even some ordinary quartz-iodine bulbs incorrectly labelled as HID; caveat emptor). These aftermarket conversions use an HID arc lamp and a ballast, just like the OEM kind, but are designed to retrofit into a conventional socket and reflector designed for quartz bulbs. They can be had for substantially less than the OEM prices for factory-installed HIDs. Unfortunately, the shape of the tungsten filament (of the original bulb) is different from the shape of the arc in the HID envelope, which makes the HID beam pattern of the assembly unpredictable. While you may be able to see fine, it will throw glare in odd directions, which will make it difficult for you to see in any kind of precipitation. It will also shine into oncoming drivers’ eyes. Plus, these HID conversions are illegal, but purveyors get around that by labelling them “For off-road use only”. Not recommended.

The tungsten filament in a conventional QI bulb has a square bright spot that cools off slightly at the ends, whereas the arc in an HID is an elliptical shape of nearconstant brightness. To achieve the same beam pattern, they require different reflectors.

Adjusting your beams

Proper headlight aim is critical. Check aim with a driver in the car and a full tank of fuel. The low-beam cut-off (a) should be slightly below the centre of the lens (b) to keep glare out of oncoming traffic and reduce the light reflection that occurs during wet weather. The bend in the cut-off should be slightly to the right of the centre of the lane, for the same purpose. This will illuminate the road surface in front of the car, as well as the kerb area to the left.