A machine used for projecting film images onto a screen.
The basis of the cinematic process is a rapid sequence of still images, photographed and later projected at the same rate, which merge into a moving picture. This effect works because of a psychological effect known as persistence of vision.
The brain holds an image for a fraction of a second after it has been removed from sight: long enough, in the cinema, for the next frame to replace it. The effect of motion is apparent at speeds as low as 12 frames per second (fps) but below 48 fps the dark periods between frames produce an obvious flicker. To avoid this flicker, projectors run at 24 fps, and then display each frame twice (48 pictures per second). 1
1 Case, D., 1985, Motion Picture Film Processing, Media Manuals, Focal Press, London, Boston