A surface, as that afforded by a curtain, sheet, wall, etc., upon which an image, as a picture, is thrown by a magic lantern, solar microscope, etc.
In a camera. the surface upon which the lens projects an image for viewfinding and, usually, focusing purposes. In SLR cameras. almost universally a fresnel screen with a fine-ground surface. Often incorporates a microprism or split-image rangefinder.