Re: what's the difference

["Nicholas J. Yasillo" <n-yasillo@uchicago.edu>]

I don't have the reference handy, but I recall article a few years back in
an electronics throw-away journal (written by a Philips engineer) about the
relative merits of xenon arc lamps and conventional tungsten/halogen
filament lamps for automotive applications.

Claims were that the small arc (as opposed to a relatively large filament)
allowed a more efficient reflector and projector lens to be used with the
xenon bulb, hence more light on the road where it's needed.  As I recall,
the article only claimed a slightly extended life span for the arc lamps
(1500 to 2000 hrs. max.) compared with 750 to1200 hrs. for a typical H-4
tungsten bulb.  Even though there is no filament to burn out, there is
evaporation of the metal electrodes that slowly darkens the lamp by being
deposited on the inner surface.  Eventually, the lamp won't start due to
electrode erosion (esp. at low temperatures).

I've heard that the replacement cost of the xenon lamp elements is over USD
100 each, so on a per mile or per hour basis they are probably more
expensive than plain, old filament lamps (but they certainly deliver better
lighting over their life).

One other minor point, although "tens of thousands of volts" are used to
initially fire the lamp (start the arc), once fired they operate at a much
more reasonable 75 to 150 volts at approximately 0.5 Amp (derived from a
small current regulated power inverter).

An aside: Once fired and up to temperature, the plasma pressure inside
these lamps can reach 50 to 100 Atm., which make all the talk about 1.8 Bar
turbo boost levels pale in comparison.

--->Nick
----------------------------------------
>Aleksander Mierzwa wrote:

>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
------------------------------------------
>>At 23:51 99-10-06 -0400, merlin wrote:
>>
>>What's the difference between projector headlights and xenon's?
>>

Nicholas J. Yasillo
n-yasillo@uchicago.edu