Front Projection

a special-effects method of motion-picture photography in which the images of actors and objects located in the foreground are superposed in the frame on the image of a background produced by a motion-picture or slide projector on a light-reflecting screen. In the simplest case (Figure 1, a), the motion-picture or slide projector and the motion-picture camera are positioned in front of the reflecting screen at a specified distance from each other, so that the optical axes of their objective lenses are directed toward the center of the screen. In order to avoid spurious shadows on the screen, the area in which the objects being photographed are positioned and moved around is restricted to the portion of the space not covered by the beam from the projector’s lens but within the field of view of the camera.

In front projection in which the projector and camera lenses are brought into optical coincidence (Figure 1, b), the drawback mentioned above is eliminated. The projector and camera are mounted on a common support in such a way that their optical axes are in a single plane (horizontal or vertical) at an angle of 90° and intersecting at a point equidistant from both lenses. A beam splitter or a mirror with a hole in it is placed at the point of intersection at an angle of 45° to the direction of the light flux from the projector. At the same point the optical axis of the camera and

Figure 1. Diagrams of front projection techniques: (a) without optical coincidence of projector and camera, (b) with optical coincidence of projector and camera; (1) motion-picture camera, (2) projector, (3) object in foreground, (4) screen, (5) beam splitter, (6) light-absorbing trap (a combination of a gray filter and black velvet), (7) luminaires

the axis of the light flux reflected from the mirror will coincide; thus, both lenses view the objects being photographed as if from a single point, and the shadows on the screen do not fall within the field of view of the camera.

When a motion-picture projector is used to produce the background image, its operation is synchronized with the operation of the camera so that the camera shutter is open when the projector shutter is open. Front projection can also be carried out in a frame-by-frame mode and in combination with other special-effects techniques. Modern arrangements for this method include raster screens that have a narrowly directed reflection, which substantially reduces light losses.

Front projection has all the merits of rear projection as well as a number of additional advantages: the studio area required during photographing can be from two to three times smaller, the intensity of the light flux from the projector is reduced by a factor of 10–20, and the screen receives more uniform illumination.