Re: what's the difference

[Aleksander Mierzwa <alexaudi@kki.net.pl>]

At 23:51 99-10-06 -0400, merlin wrote:

>What's the difference between projector headlights and xenon's?

The term "projector headlight" defines the headlamp optics, while the term
"xenon" defines the light source. Two different things.

Ordinary headlamp has a reflector, which directs the light towards the lens
consisting of a set of little prisms which are responsible for creating the
desired light pattern. In more modern headlamps these are absent and the
desired light pattern is created already by the properly shaped reflector.
In other words, this kind of headlamp works much like your pocket flashlight.
Projector headlamp uses a different concept: it uses a small optic lens
which "projects" the light onto the road, just like your old diascope did.
The desired light pattern is achieved by placing a correctly shaped
diaphragm between the bulb and the lens. 

Now to the so much hyped "xenon" lights. In the ordinary automotive
headlamp bulb (whether it's the clumsy DOT 9004, or the H4, or the newest
H7 bulb) the light is produced by passing a current through a metal
filament. This way the filament is heated to a point where it starts to
glow. Unfortunately, this technology, althought greatly improved through
the years, has its drawbacks: the efficiency is poor (big part of the
energy is wasted on producing heat rather than light), the lifespan of the
filament is limited and it's difficult to produce light with spectrum
similar to that of the daylight.
The "xenon" bulbs use a far different concept, called High Intensity
Discharge. The bulb itself is just a little glass bubble containing Xenon
gas and two electrodes. Very high voltage (tens thousand volts) is applied
to the electrodes, which causes the gas itself to glow. The efficiency is
much better, lifespan much, much longer and the light produced is very
white as opposed to the yellowish glow of the filament bulbs.

--
Aleksander Mierzwa
Warsaw, Poland
87 5KT