BACK PROJECTION [Film Technology]

BACK PROJECTION
[Film Technology]

A cheating method. When the action has to be seen against the same background, this can also be managed without visiting the actual location. One of the oldest method is known as BP. Still plates or motion pictures are back projected on to a translucent screen behind the performer. The actor come in this way be filmed apparently visiting New York, London and Rome alternatively falling in front of a train. A modern version of this employs a beaded screen behind the performer on to which the required background is projected, from the camera lens position. Shortly known as B P. Other name is rear projection. See cheating. In conditions where there is insufficient throw from the projector to the screen or in self contained desk-top-projector, the picture is projected from behind a translucent screen via  surface silvered mirrors. An adaptor of this system is used for TV where film is projected on a telecine machine, which converts the image to television signal. A still or motion picture image is projected. Simultaneously with the action so that foreground action and background scenes are combined photographically. As its name suggests back projection involves images from behind the subject on to a translucent screen placed between the foreground and the projection. As with front projection, when using film plates, a special process projector must be used for back projection which incorporates a register pin movement, as accurate as that of a camera, and which will run interlocked with the camera, ensuring not only that both run at an identical speed but also that the two films remain stationary  and are advanced simultaneously. To achieve the necessary accuracy of registration, print for process projection (front and back) are usually made on negative perforated film. An inherent problem with back projection is the effect of a central `hot-spot’ of light. This may be reduced by the use of a very long focus lenses. It is necessary therefore, to place colour correction filters in the path of the projector light to make the necessary correction. A special register pin projector must be used for front projection composite cinematography or back projection composite cinematography. Shortly known as BP.

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