This article is from the rec.arts.movies.tech FAQ, by Scott E. Norwood snorwood@nyx.nyx.net with numerous contributions by others.
The color balance. Tungsten lamps have a 3300 degree Kelvin `color
temperature,' whereas xenon lamps have a 5500 degree Kelvin color
temperature. Basically, xenon lamps give a `bluer' light than tungsten
lamps (carbon-arcs fall somewhere in between). To compensate for this,
a small filter is changed in the printer to make prints for both types
of lamps. This change is independent of the print timing, and so can
be made well after the timer is completed with his job. In practice,
however, all theatrical prints are balanced for xenon, as no
commercial theater commonly uses tungsten lamps.
 
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