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How Does an 8 mm Film Projector Work?

Film is an expensive medium, with its cost a barrier to use in consumer markets. The introduction of 8 mm film, which was 16 mm film cut in half, reduced the cost to a level that made it popular for home movies in the 1960s and 1970s. The introduction of this film format went together with the design of film projectors that consumers could easily use to play their movies. These film projectors reduced component parts to a minimum to simplify operation, while preserving essential functions.

The Film
- A typical 8 mm film projector has two arms that hold the film reels. The arms may be fixed or swing back into the body of the projector for storage. The reel toward the front of the projector holds the film initially. The film itself has perforations along one side. Sprockets within the projector engage the perforations, and pull film from the front reel through the projector. From there, the rear reel takes up the film as it comes out of the projector, and winds it up. After the movie is finished, the projector re-winds the film directly from the rear reel back onto the front reel.
The Bulb
- A key element of the film projector is the bulb that shines light through the film to project the movie on the viewing screen. The bulb for 8 mm projectors can be less powerful and smaller than that for larger sizes, but it must have the same intensity to produce a bright image from the small film frame. It is located toward the rear of the projector, usually with a concave mirror behind it to focus the light, and a cooling fan to dissipate the heat. The film passes in front of the bulb, and behind the lenses used to focus the image on the screen.
The Shutter
- To play an 8 mm movie, the projector must show 24 frames of film per second. Each frame displays for a fraction of a second before the next frame appears. The sprockets of the projector move the film one frame at a time. When a frame arrives in front of the bulb, the shutter opens, and lights up the frame. It then closes again to let the next frame move into place. For 8 mm projectors, the shutter is typically a propeller-shaped element that rotates to make the blades interrupt the light when required.
The Film Path
- The film runs from the front reel through the projector to the rear reel. It starts on a sprocket at the front of the projector. This first sprocket pulls the film off the front reel. The film then passes over several pulleys before forming a loop below the lens. Above and below the lens are sprockets that pull the film, one frame at a time, through the opening between the bulb and the lens. The loop below the lens, and a corresponding loop above it, cushion the jerky motion of the film so it can still wind and unwind smoothly at the reels. After leaving the lens, several more pulleys guide the film to the rear and top of the projector, where the rear reel takes up any slack, and winds up the film.

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