Rear Projection

a special effects technique in which real objects, such as actors playing a scene, are filmed against a background that was shot earlier on motion picture or still film and that is projected onto a screen behind the objects being filmed. The rear-projection screen is a translucent screen on which an image is created by rays passing through it. A special projector, the rear projector, is placed behind the rear-projection screen, where it does not hinder the actors from performing in immediate proximity to the screen. This permits a sufficiently sharp composite picture to be obtained despite the shallow depth of field of the camera lens. Rear projection makes possible combinations of action and objects separated in space and time and, unlike certain other special effects techniques, permits the actor to see the background image during the filming, which is very important for coordinating the actor’s speech and actions with the events projected on the rear screen.

The rear-projection screen may fill the entire frame or only part of the frame, as when a landscape is shown outside a window or open door. The background image on the rear screen may be still or moving. For still backgrounds, dirsquoitive projectors with a powerful, water-cooled light source are used. Moving images are projected using low-noise projectors with a high light output. The projection is usually run at standard speed. In order for the timing of the exposure of the composite image to coincide with the timing of projection of the background image, the camera’s mechanism must operate synchronously and in phase with the rear projector’s mechanism.

Rear projection may be used in stop-motion photography, with irregular time intervals between the exposure of consecutive frames, as in the making of animated films. It is also used in films with a second subsequent projection (seePROCESS PHOTOGRAPHY), as well as in combination with other special effects techniques.