Persistence of Vision

• The basis of traditional movie projection is an optical illusion known as persistence of vision. Each frame of film contains an image that is just slightly different than the image which came before it. The differences are so minute that, when projected at a certain speed (24 frames per second), the human eye is no longer able to differentiate between them, and the image appears to move. Projectors are built to facilitate that process.
  
  

Illumination and Projection

• Films are made of celluloid, which is a transparent medium upon which the images are printed. When you shine a light through them, the image is projected onto a wall and, by using lenses to magnify the image, can appear many times larger than the film itself. The projector facilitates this process. Every piece of film has a series of evenly spaced holes along either side. A series of sprockets within the movie projector pull the film across the front of a lamp, pausing and holding it there for a tiny fraction of a second. A shutter is used to block the light while the film advances each frame, which ensures a smooth image. A lens on the other side of the film then magnifies and projects the image out onto the screen.
  
  

Sound

• Sound on film was originally achieved through a record, which was synchronized to the image onscreen. Eventually, better methods were produced. One side of the film contains an optical track, which is essentially a line of transparent film that shrinks and widens according to the specifics of the soundtrack. A light in the projector shines through it and is picked up by an optical sensor, which turns it into an electrical analog signal. The signal is then sent to the speakers and amplified, where it emerges as the sound from the movie.
  
  

Changing Times

• Traditional methods of movie projection are giving way to newer, digital forms of projection. Instead of physically pulling the film through its mechanism, the film is stored digitally on a hard drive--as data, not celluloid--and projected from there. The shift has not been without controversy. Celluloid is an extremely fragile medium, whereas digital information never degrades. Digital projection also saves money on distribution costs, since the film doesn't need to be physically brought to the theater any more. On the other hand, digital projectors are more expensive (especially during installation) and less durable than older projectors. Film scholars also cite a fundamental difference in the way the image is projected and thus in the way the mind processes the images being shown. Regardless of the issues, digital projection seems to be moving forward and may replace traditional projectors completely before too long.