The Imax projector uses what's called a "rolling loop" mechanism. The
film is stationary during projection, but it doesn't use a conventional
intermittent-motion sprocket to move the film, nor a claw mechanism
like a camera. Instead, there is a large rotating drum with a series
of "pockets" in its circumference. The film travels in a track between
this rotating drum and stationary guides. During most of the film
projection time, the frame of film in the projection gate is stationary,
but film is fed continuously into the gap between the rotor and the guides.
The "excess" film forms a loop in one of the "pockets" - there is just
one pocket between the input sprocket and the film gate at any one time.
The "pocket" pushes a "loop", really a "wave" of film along with it.
The rotor and its pocket actually move considerably faster than the
linear speed of the film.

Anyway, eventually the pocket reaches the film gate area. The "rolling
loop" or "wave" continues right through the film gate, causing the
frame that was being projected to just "roll" off the gate, and the
next frame to be "rolled" into place. Take a piece of film, spread it
out on a table, and play with it yourself to see how you can form a
loop and push it around. The loop pocket also has a metal shield
around it that acts as a shutter, so no light reaches the screen where
the film is moving - but the shutter is actually narrower than the
film frame, so the screen never goes entirely dark. Frame N+1 has
appeared at the left edge of the screen before frame N is completely
gone from the right edge of the screen.

Once the "loop" passes the gate area, the loop gradually decreases in
size and disappears before the output sprocket. Meanwhile, a new loop
has formed on the input side, and the process repeats again.

The good things about the rolling loop are:

- gentle film handling - you don't have to accellerate and decelerate
a foot or two of film by pulling on the sprocket holes, it's just
pushed along. Film lasts many times longer than 35mm projectors.

- the film gate has 4 steel registration pins to hold each frame in
place during projection (side-to-side and up-and-down directions).
Although Imax images are about 3 times larger than 35mm in linear
measure, the projector is about 10 times steadier - that's a lot
of the reason why IMAX looks so good.

35mm projectors just hold the film in place with friction. That's
why 35mm is so unsteady.

- the film is actually held against the rear element of the projection
lens (which is cylindrical) by a vacuum during projection. So focus
is stable.

The negative side of this is that the mechanism is large, noisy, heavy,
and expensive to build.

(quote shamelessly swipped from Dave Martindale - davem@cs.ubc.ca)

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