Front and Rear Projection in Filmmaking: Techniques and History
Front and Rear Projection in Filmmaking
Digital Light Processing (DLP) Projectors
They are made with single-chip projectors (using a rotating wheel that divides the colors) and three-chip DMD projectors. The operation of a 3-chip projector is basically the following:
a) A digital projector based on DLP technology transfers the digitized image file to three optical semiconductors known as Digital Micromirror Devices, or DMDs. Each of these chips is dedicated to one of the primary colors: red, green, or blue. A DMD chip contains a rectangular array of a million microscopic mirrors.
b) Light from the projector lamp is reflected in the mirrors and combined in different proportions of red, green, and blue, corresponding to the scanned image to create an array of different colored pixels that make up the projected image.
c) The DMD mirrors tilt toward the light source or out of it a thousand times per second to reflect the movie on the screen sequentially, with clarity and a range of over 35 trillion colors. Digital projectors being installed today are 2K (two million pixels), although 4K projectors are also available. Although it is assumed that the quality of analog 35mm film would be approximately 8K, we must take into account that the copy that goes to the cinema has deteriorated since the original negative and also continues spoiling in successive projections. Possibly, 4K projectors provide greater image quality to the viewer. It also has other advantages, such as the possibility of projecting content in other formats (such as live TV broadcasts, for example, and other multimedia content).
Projection Screens
Projection screens are made of two parts: the base (which may be of various different materials, such as cloth, plastic material, polyvinyl chloride, etc.), and the reflecting surface that is attached to the first. The latter should provide uniform brightness and have a high reflective power, so they are of different types:
- Pearly (bearing a layer of spherical glass beads)
- Lenticular (based on spherical contours)
- Translucent (for rear projection)
- Scotchlite (built with highly reflective material)
Front Projection and Rear Projection
Front projection and rear projection are two techniques used in special effects in film to achieve what is called a composite film, i.e., combining several images into one. They’ve been largely replaced by the technique of chroma key and the valuable assistance of the computer.
Rear Projection
Rear projection began to be used in early films. This is done with a projector equipped with a powerful light source (as it must use a lens of long focal length to avoid creating a bright area in the middle of it) and a translucent screen. The projector is placed behind the screen and in line with the camera, and the object or subject with which the composite image was formed is placed in front of the screen. It started being used in the first films to recreate scenes that were complicated or expensive to shoot in a real setting. For example, in almost all Hitchcock films, you can see scenes in which rear projection has been used.
Front Projection
Front projection began to be used frequently in the 1970s, after its use in the movie 2001: A Space Odyssey. There are variations (such as that used in Superman with the Zoptic system), but all are based on the use of a special high-gain Scotchlite screen by 3M, which has the property of reflecting light directly back toward its source. In contrast to rear projection, images are projected onto the front of the screen instead of behind it. The camera and projector are placed next to each other at right angles, with a half-silvered mirror placed between them at a 45-degree angle. Projector images affect the reflective surface of the mirror, which bounces them back onto the screen, constituting the substance of the composite image. The objects and actors in the foreground receive light from the projector, but this is weak compared to the reflecting screen, and their shadows are hidden by themselves (all with the aid of illumination of the studio itself).