Projectors
To get the movie on the screen, you use a projector. The projector has six jobs:
- It uses a motor to keep the film moving through the projector at 24 frames per second.
- It uses a sprocket system to stop each frame and hold it still long enough for light to shine through it.
- It uses a shutter to block the light when the film is moving between frames.
- It uses a very bright xenon light bulb to create the light.
- It uses a lens to spread the light and focus it on the screen.
- It uses sound sensors to pull the sound signals off the film.
Photo ©Hemera Technologies/Ablestock
Projector mechanism
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Beside the projector is a big platter system. Remember that it takes miles of film to hold a two-hour show. Two miles of film makes a reel that is maybe four feet in diameter and might weigh one hundred pounds. The film spools off the platter and into the projector. Sprockets grab the film and an electric motor pulls it through the projector.
The film needs to advance one frame, pause for a fraction of a second and then advance to the next frame. Here is one way to do it: The projector uses a small lever known as a claw, which is mounted on a bar next to the film's path. The claw connects to the outer edge of a wheel that acts as a crank. The circular motion of the crank makes the claw lift up and out to come out of a sprocket hole and then down and in to catch onto another sprocket hole. The claw pushes the film ahead exactly one frame. The speed of the sprockets is synchronized with the lever action of the claw to make sure that the claw advances the film at 24 frames per second.
The film stretches over a couple of bars as it passes in front of the lens. The bars keep the film tight and properly aligned. As the film leaves the projector, it is carried on a series of rollers back to the platter's payout assembly and spooled to a take-up platter.
Photo Credit: Distributed under the GNU Free Documentation License
http://www.gnu.org/copyleft/fdl.html
Platter projector
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The light inside a projector uses a xenon bulb that costs about $700 every time it burns out. The lamp consumes 3,000 or 4,000 watts of electricity. Imagine all of the heat that one 60 watt light bulb makes. Now imagine 50 or 60 of them packed together into one small space. That's what is happening inside a movie projector.
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