December 20, 2004
What are the black dots you see every 20 min at the cinema?

When going to the cinema you'll sometimes notice a black dot flashing in the top right hand corner of the picture every 15-20 minutes. You'll find that this is almost always just before a complete change of scenes.

Full lenght movies are generally distributed on multiple reels of film. The Reel change markers, simply reel markers, cues or "cigarette burns" are used to indicate to the cinema's projectionist that it is time to change the reel of film. More information about the workings of a film cinema film projector can be found here:

The two reel system was almost universally used before the advent of the single reel system for movie theaters in order to be able to show feature-length films. Although one reel long-play systems tend to be more popular with the newer multiplexes, the two reel system is still in significant use to this day. The projector operator operates two projectors, threading one with the next reel while the other projector plays the current reel. As the current reel approaches its end, the projectionist looks for cues, also known as cigarette burns, at the upper right corner of the picture. Usually these are dots or circles, although they can also be slashes. (Some older films have occasionally been known to have used squares or triangles, and even positioned the cues in the middle of the right edge of the picture.) The first cue appears twelve feet (3.7 m) or eight seconds at 24 frame/s before the end of the reel, and signals the projectionist to start the motor of the projector containing the next reel. After another ten and a half feet (3.2 m) or seven seconds at 24 frame/s, the changeover cue should appear, which signals the projectionist to actually make the changeover. When this second cue appears, the projectionist has one and a half feet (457 mm) or one second at 24 frame/s to make the changeover - if it doesn't occur within one second, the tail black leader of the exhausted reel will be projected on the screen.
Fun trivia: it appears that in the recent Pixar production "The Incredibles" the reel markers are featuring the Incredibles logo instead of the usual black dot. I saw the film in a digital projection cinema and hence there were no I-logos...

See also Jim's comments on HouseOfFusion.

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