5.3 What are the differences between xenon, and carbon-arc lamphouses? (Motion Picture Presentation - theatrical projection)
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This article is from the rec.arts.movies.tech FAQ, by Scott E. Norwood snorwood@nyx.nyx.net with numerous contributions by others.
5.3 What are the differences between xenon, and carbon-arc lamphouses? (Motion Picture Presentation - theatrical projection)
Most commercial theaters currently employ xenon bulbs; these are glass
tubes containing a highly pressurized xenon gas through which high
electrical current is passed (usually 220V, 50 amps or higher). They
typically last for several thousand hours prior to needing
replacement. Aside from being rotated and changed at regular intervals
(they start to flicker as they get old), xenon lamphouses need very
little maintenence (unless the bulb explodes due to the high pressure
inside the bulb, in which case the rear reflector in the lamphouse
must be re-silvered). Bulb glass tends to weaken as it ages, and thus
extreme care should be taken when replacing bulbs to ensure that the
bulb does not explode.
Older installations may use or have once used carbon-arc lamphouses;
in these setups, high electrical current is passed between two carbon
rods (one positive and one negative), creating an electrical arc and a
very bright flame in the gap between the two rods. In order to operate
such a lamphouse, the projectionist inserts the rods into their steel
holders, closes the lamphouse, switches on the power, and, watching
through a shielded piece of glass, carefully brings the rods together
(using positioning knobs on the side of the lamphouse), causing them
to touch. At this point, the arc will strike, and he can bring the
rods apart and allow the current to stabilize. As the carbon burns
down during the show, a motor brings the rods together, maintaining a
constant distance between the tips of the rods, which must be tweaked
by the projectionist as the show goes on, in order to maintian
consistant on-screen light. Every 30 minutes to an hour of use, the
rods will burn down and must be replaced.
Separate rods are used for `positive' and `negative' poles; a longer,
thinner one is placed in the positive holder, and a shorter, fatter
one is used for the negative holder. These designations should be
marked on the box of carbon rods. Fumes from carbon-arc lamphouses are
highly noxious, and should be well ventilated.
Note that both xenon and carbon-arc lamphouses require DC power,
provided either by DC mains or by a rectifier circuit (which converts
standard AC power to DC). Older theaters may use motor-generator sets
to generate DC power.
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