The 3D projection formats introduced in the 1950s were cumbersome, labour-intensive and required two cameras projecting onto the same screen, making synchronisation difficult. Today, there are three far more sophisticated 3D digital projection technologies in the theatrical market: RealD, Dolby Digital Cinema and Imax 3D.

All of them exploit the way the eye receives light, and the way the brain processes that information.

By simultaneously selecting two viewpoints, or a left eye and right eye, the different technologies enable the brain to converge or fuse those images. By wearing special glasses, the watcher prevents the image from bleeding over from the left to right, or vice versa.

There are substantial differences in how the formats achieve those means. The RealD is a single-projector system that alternately projects the right-eye and left-eye frame. The push-pull system places a filter in front of the projection lens that polarises light differently for each eye, with the circular polarised glasses ensuring each eye receives the correct signal. The frame rate is 144 frames per second, or six times that of a standard 35mm image, and achieves a continuous flow of motion.

The Dolby Digital system uses a colour filter technology licensed from the German firm Infitec. A spinning wheel the size of a CD inserted between the lamp and digital projector separates light into spectral ranges of slightly different iterations of blue, green and red. The wheel spins six times for each movie frame, achieving a 144-frame rate.

The Imax format is a two-projector system that places a polarising filter over each lens. "The projector has an image enhancer that creates more horsepower than any other projection system," says Greg Foster, president of Imax Filmed Entertainment. "With more light, more information can be seen."

The new technology has produced a more stable image that has reduced, if not eliminated, the consumer complaints of eye strain. However, criticisms remain.

The Real D circular polarisation technology demands a special silver screen, the costs of which have been marked at $5,500.

The special glasses, originally awkward and made from cardboard, have become considerably more sophisticated and easier to use. But Dolby's glasses are difficult to manufacture and cost $50 per pair.