How do movie projectors work?

There are several different formats of movie projectors: 8mm and Super-8 (usually for home movies), 16mm (typically used for industrial and corporate presentation) and 35mm and 70mm for professional use and feature motion pictures. There are also a handful of other pro formats including IMAX. For this discussion, we'll concentrate on the 16mm format. 16mm film has an overall width of 16 millimeters--the actual size of the image on the film is close to 10mm wide by 8mm high. On one edige of the film are the sprocket holes that allow the projector to pull the film through the mechanism. There is one sprocket hole each at the break between each frame. On the other edge of the film is the sound track, which is read optically by a separate lamp inside the projector. No matter which format, all movie projectors have several basic elements: a supply arm on which the film to be shown is placed; a take-up arm, on which an empty spool is placed to collect the film already passed through the projector; sprocket wheels that grap the film's sprocket holes and pull the flim through the projector; a film gate, which is the hole between the projection lamp and the lens where each frame of film is actually projected; and the exciter lamp and sound drum, where the film's optical sound track are converted into electrical signals that drive the audio system and speaker. Older projectors had to be threaded manually, with the user placing the film around the sprocket wheels, through the film gate and around the various spindles and guides. More recent machines have a feature called auto-load, which takes care of threading the film for you. Using this feature, you simply insert the loose end of the film (called the leader) into a specific slot on the unit. A spinning sprocket wheel grabs the film and pulls it in and forces it through the rest of the mechanism. Once the end of the film has passed through the projector, you can stop the machine and attach the film to the take-up reel. The auto-load function must be disengaged before you show the movie--if it isn't, all you'll see is a blur of images. This is because the auto-load system pulls the film through the machine in one continuous motion without stopping each frame of film in front of the film gate. 16mm film is projected at a rate of 24 frames per second (fps). Each frame of film is positioned very briefly in front of the film gate. The projector mechanism then closes a shutter in front of the film gate and advances the movie to the next frame, at which point the shutter opens again, projecting the frame onto the screen. The reason this rapid series of still images appears as fluid motion on the screen is due to a phenomenon of human eyesight called "persistance of vision". The eye cannot distinguish still image that flash at more than 1/17th of a second--when the image disappears, the image on the retina remains for a split second. What the eye can detect is an apparent flutter of the projected image near 1/17th of a second. To eliminate the effect, the projector also closes the shutter in the middle of each frame, so each frame of film is really shown twice at 1/48th of a second. Old movies were originally shot at 18 fps, but for some reason are usually projected at 24 fps, noticeably speeding up the action on screen. 8mm and Super-8 project at 18 fps. Because of the stop-and-go motion of the film through the mechanism, a little extra space for the film is required on either side of the film gate to prevent the film from jamming the system. This area is called a loop. The film's soundtrack is read by an exiciter lamp located further down the mechanism from the film gate. Because of this, the sound that corresponds to each frame isn't directly alongside it, but about 6 inches ahead of it. If a section of the film is spliced out, the sound will be chopped off after the picture is cut. The optical track works by the amount of light allowed to pass from the exciter lamp to the sound drum. The more transparent the track, the louder the sound output will be. This process is called amplitude modulation. Once the signal is read and the light converted to an electrical equivalent, it is amplified like any other sound source. 8mm, Super-8 and 16mm film have a picture size that's 4 units wide by 3 units high (4:3)--this is called the aspect ratio. In fact, television adopted the same 4:3 ratio so it would conform to the same standard. What the TV engineers didn't emulate, though, was the frame rate of film. TV's frame rate is 30 fps, so if you try to tape a movie directly off a screen, you'll have a major flickering problem. There are professional projectors available that use a special shutter that closes 5 times for each frame of exposed film, matching it to video's 30 fps standard. On the flip side, when movie scenes require video and computer monitors in the shot, those video sources are converted to 24 fps to match the film's standards to avoid the same problem with flicker. Not too long ago, TV stations used to run film directly to air using a device called a telecine--which meant basically that the special 5-bladed projector would project directly into the lens of a video camera. A more complex version of the basic telecine was the multiplexer, or film chain, that had several movie projectors and/or slide projectors projecting into the video camera through a series of 50/50 mirrors. These days, there are flim projectors that have a video tap--no lens, but an internal video camera that connects directly to other video equipment. For high-conversion of film-to-video, machines called film scanners are used to produce extremely high-resolution images required by TV and cable networks and production houses

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