Rear Projection, Electronic

a technique of using electronic apparatus to obtain a composite television image composed of two or more separate images that combine to form a logical and complete scene. One image—usually an illustration or image from a slide or motion-picture still—serves as the background to the scene; the other image usually consists of the actors and other objects placed against a homogeneous contrasting field provided by a smooth monotone surface, and this image serves as the foreground of the final composite. Maximum differentiation of the reflecting properties of object and background field in the second image is achieved when a colored background is used.

Electronic rear projection is carried out by high-speed electronic switching of the video signals of both images in response to a “silhouette” signal formed when a sufficient change in the video signal level is reached during the transition from background to object. The switching corresponds with the object’s silhouette in such a way that the image of the object is seen within the silhouette, and the background of the scene appears outside the silhouette.

Electronic rear projection is used in all types of broadcasts. When backgrounds must be changed often, the method makes preparations for a telecast faster and less expensive. Modern equipment for electronic rear projection produces high-quality televised images.