Gas discharge and LED lighting

Gas discharge lamps

Gas discharge headlamps (GDL) are now being fitted to vehicles. They have the potential to provide more effective illumination and new design possibilities for the front of a vehicle. The conflict between aerodynamic styling and suitable lighting positions is an economy/safety tradeoff, which is undesirable. The new headlamps make a significant contribution towards improving this situation because they can be relatively small. The GDL system consists of three main components.


This operates in a very different way from conventional incandescent bulbs. A much higher voltage is needed.

Ballast system

This contains an ignition and control unit and converts the electrical system voltage into the operating voltage required by the lamp. It controls the ignition stage and run-up as well as regulating during continuous use and finally monitors operation as a safety aspect.


The design of the headlamp is broadly similar to conventional units. However, in order to meet the limits set for dazzle, a more accurate finish is needed, hence more production costs are involved. The source of light in the gas discharge lamp is an electric arc, and the actual discharge bulb is only about 10 mm across. Two electrodes extend into the bulb, which is made from quartz glass. The gap between these electrodes is 4 mm. The distance between the end of the electrode and the bulb contact surface is 25 mm – this corresponds to the dimensions of the standardized H1 bulb.

At room temperature, the bulb contains a mixture of mercury, various metal salts, and xenon under pressure. When the light is switched on, the xenon illuminates at once and evaporates the mercury and metal salts. The high luminous efficiency is due to the metal vapor mixture.

Ultraviolet headlights

The GDL can be used to produce ultraviolet (UV) lights. Since UV radiation is virtually invisible it will not dazzle oncoming traffic but will illuminate fluorescent objects such as specially treated road markings and clothing. These glow in the dark much like a white shirt under some disco lights. The UV light will also penetrate fog and mist, as the light reflected by water droplets is invisible. It will even pass through a few centimeters of snow.

Cars with UV lights use a four-headlamp system. This consists of two conventional halogen main/dip lights and two UV lights. The UV lights come on at the same time as the dipped beams, effectively doubling their range but without dazzling.

LED lighting

Light emitting diode (LED) displays were first produced commercially in 1968. Almost from this time, there has been speculation as to possible vehicle applications. Such LEDs have certainly found applications in the interior vehicle, particularly in dashboard displays. However, until recently, legislation has prevented the use of LEDs for exterior lighting. A simple change in the legislative language from ‘incandescent lamp’ to ‘light source’, has, at last, made it possible to use lighting devices other than filament bulbs.


Operation of the switch allows the supply on the N or N/S wire (color codes are discussed on page 85) to pass to fuses 7 and 8 on an R wire. The two fuses then supply left sidelights and right sidelights as well as the number plate light.

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