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The CitroŽn XM headlamp problem - a solution 

At a recent visit to PTS Developments in Wokingham, I mentioned the problem and Pete Sparrow recommended fitting HID gas discharge (Xenon) lights.  These are not to be confused with regular halogen bulbs with a blue tint; these are the same lights that are fitted to many top-of-the-range cars and have a light output tilted towards the blue end of the visible light spectrum.
Among the claimed advantages of HID headlights are higher intensity, longer life, superior colour, and better directivity.
HID lamps are about 3 times as efficient as halogen lamps. Thus, even when the efficiency of the DC-DC converter is taken into consideration, the lower power input can actually result in much brighter headlights than are possible with halogen bulbs. This reduced power also leads to cooler operation and less drain on the battery and alternator.
Lifespan - an HID lamp can be expected to last 2,700 hours or more. Essentially, the HID lamp may outlast the car.
Spectral output - the light from the HID lamp is richer in blue (and more like daylight) than halogen bulbs, thus enhancing reflectivity of signs and road markings.  Furthermore, clothing that contains white flecks (undissolved washing powder) reflects – as anyone who has been to a disco with UV lamps can confirm.
Beam pattern - the small arc size of the HID lamp permits the optical system to be optimised to direct light more effectively to where it is needed and prevent it from spilling over to where it is not wanted.  Couple this with the complex surface reflectors and you have an extremely well demarcated beam pattern.
It is necessary to equip the car with a number of electronic components.  The HID bulb itself is similar in basic design to traditional HID lamps (a common example being the fluorescent tube used in interior lighting). Two electrodes are sealed in a quartz envelope along with a mix of solids, liquids, and gasses. When cold, these materials are in their native state (at room temperature) but are mostly gases when the lamp is hot. Starting of these lamps may require up to 20 KV to strike an arc but only 50 to 150 V to maintain it. Lamps may be designed to operate on either AC or DC current depending on various factors including the size and shape of the electrodes. A unique set of ballast operating parameters must be matched to each HID bulb.
Of all the problems that had to be addressed for HID headlights to become practical (aside from the cost), the most significant was the warm-up time which was solved by programming the controller to deliver constant power to the lamp rather than the more common nearly constant current that would be provided by a traditional ballast. With this twist along with a special lamp design, the lamp comes up to at least 75% of full intensity in under 2 seconds.
As hinted earlier, this is not a cheap solution but is money well spent; not least because the kit can be removed and fitted to another car at minimal cost.  The standard headlights may be refitted when it is time to sell the car.
This really is money well spent – especially if you live in a rural area.  My XM is utterly transformed.  Driving at night is much easier.  The diffuser was left in situ and the beam pattern is unchanged.  Judging by the failure of oncoming drivers to flash their lights, they cause minimal annoyance to other road users. 
Clearly, these lights can be fitted to any car but the law requires any such car to be fitted with either a headlamp beam adjuster or self-levelling suspension and headlamp washers.

© 2006 Julian Marsh