Xenon headlamps, sometimes known as highintensity discharge (HID) lamps, are becoming popular despite their initial cost because of increased light output and reduced power consumption. They use a gaseous-discharge operating principle. A xenon light works a little like a sodium streetlight that produces light using an arc between electrodes through a gas. Sodium streetlights have the disadvantage of taking up to 5 minutes before producing normal light levels—not an acceptable shortcoming in a vehicle headlamp.
Vehicle HID lamps use the light emitted by ionized gas instead of a glowing filament. They consist of an arc source, electrodes, and a ballast module. The ballast module produces a high-voltage ignition pulse in the region of 20,000 volts to ignite an arc in the xenon gas then maintains the arc at a voltage of around 85 volts. In this way, normal light output is achieved within 4 seconds of arc ignition, which is satisfactory for highway operation. The efficiency with which HID headlamps convert electrical energy into light energy exceeds 80 percent compared to around 20 percent for a halogen headlamp. Table 10–1 shows some American Trucking Association (ATA) Technical and Maintenance Council (TMC) data on the relative luminosity (brightness) and efficiency of halogen versus HID low-beam headlamps.