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Movie projector
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Movie projector

This article is concerned with technical aspects of moving film projection. For historical aspects see the article history of cinema

A movie projector is an opto-mechanical device for displaying moving pictures. Most of the optical and mechanical elements, except for the illumination, are present in movie cameras.

Table of contents
1 Physiology
2 Principles of operation
3 Types of projectors
4 Sound
5 Types of lenses and screens
6 Three dimensional
7 See also
8 External links

Physiology

The viewing of motion pictures appears continuous only because of the slow response of the eye, optic nerves, and brain to changes (called persistence of vision). If your persistence was very short you would perceive a movie as a single frame picture, followed by black, followed by the next frame, etc.

It is possible to view the black space between frames and the passing of the shutter by the following technique:

Close your eyelids, then periodically rapidly blink open and closed. If done fast enough you will be able to randomly "trap" the image between frames, or during shutter motion. This will not work with television due to the persistence of the phosphors nor with LCD or DLP light projectors due to the continuity of image, although certain color artifacts may appear with some digital projection technologies.

Principles of operation

Projection elements

As in a
slide projector there are essential optical elements:

Light source

An incandescent bulb or an electric arc light produces illuminating
photons. The traditional carbon arc or modern xenon arc light source produces sufficient heat to burn the film should the film remain stationary for more than a fraction of a second

Reflector and condenser lens

A curved reflector redirects light that would otherwise be wasted toward the condensing lens.

A positive curvature lens concentrates the reflected and direct light toward the film gate.

Film gate and single image

A single image of the series of images comprising the movie is positioned and held flat within an aperature called the gate. The gate also provides a slight amount of friction so that the film does not advance or retreat except when driven to advance the film to the next image.

Shutter

A rotating petal or gated cylindrical shutter interrupts the emitted light during the time the film is advanced to the next frame.

Imaging lens

A lens system with multiple optical elements directs the image of the film to a viewing screen.

Viewing screen

In most cases this is a reflective surface which may be either aluminized (for high contrast in moderate ambient light) or a white surface with small glass beads (for high brilliance under dark conditions).

Film transport elements

Film supply and takeup

Two reel system
In a two reel system the feed reel has a slight drag to maintain tensioning in the film, while the takeup reel is driven with a constant tension by a mechanism that is allowed to slip.

Before the advent of the single reel system for movie theaters, the projector operator would operate two projectors, preloading one with a reel while the other played and switching when cueing marks were displayed on the picture.

Single reel system
In a single reel system (used in modern movie theaters) the film is spliced into a single loop wound on a large single sided reel. The feed is effected by extracting film stock from the center of the reel. This has the effect of drawing film throughout the wound stock and so drawing the projected film onto the outside of the reel. A similar mechanism is used in the now obsolete eight-track audio cassette players.

The single reel system alows for the complete automation of the projection booth operations. The single reel system also eleminates the need to rewind between showings. Since films are still transported in multiple reels they must be joined together when placed on the projector reel and taken apart when the film is to be returned to the distributor.

Feed and extraction sprockets

Smoth wheels with triangular pins engage sprocket holes punched into one or both edges of the film stock. These serve to set the pace of film movement through the projector and any associated sound playback system.

Film loop

Between the feed and extraction sprockets the film runs free in a loose loop, except where it passes through the film gate. This serves as a buffer between the constant speed enforced by the sprockets and the periodic motion through the gate.

Film gate pressure plate

A spring loaded pressure plate functions to align the film in a consistant image plane, both flat and perpendicular to the optical axis. It also provides sufficient drag to prevent film motion during the frame display, while still allowing free motion under control of the advancement pawl.

Frame advance pawl

A pawl mechanism engages the film's sprocket hole one one side, or holes on each side. This pawl advances only when the film is to be moved to the next image. As the pawl retreats for the next cycle it is drawn back and does not engage the film.

Types of projectors

Projectors are classified by the size of the film used. Typical film sizes:

8mm

Long used for home movies before the video camera, this uses double sprocketed 16mm film, which is run through the camera twice. The 16mm film is then split lengthwise into two 8mm pieces that are sliced to make a single projectable film with sprockets on one side. See the
8mm film article for more information.

Super 8

Developed by Kodak this film stock uses very small sprocket holes close to the edge that allow more of the film stock to be used for the images. This increases the quality of the image. The film is premade in the 8mm width, not split during processing as is the earlier 8mm. See the Super 8mm film article for more information.

16mm

This was a popular format for audio-visual use in schools and as a high-end home entertainment system before the advent of broadcast television. It is also the smallest format that can carry an optically encoded sound track.See the 16mm film article for more information.

35mm

The most common film size for theatrical productions during the first half of the 20th century. In fact, the common 35mm camera, developed by Leica was designed to use this film stock and was originally intended to be used for test shots by movie directors and cinematographers. See the 35mm film article for more information.

70mm

High end movie productions are often shot using this size and some theaters are capable of projecting it. 70mm film is also used in both the flat and domed IMAX projection system. In IMAX the fiilm is oriented for even more effective image area than in other formats.

Some high quality productions intended for 35MM anamorphic release are shot in and the master prints constructed using 70mm film stock. A 35mm print made from a 70mm master print is significantly better in appearance than an all 35mm process.

See the 70mm film article for more information.

Sound

Optical analog

With 16mm and the larger sizes it is practical to add a narrow channel of optically encoded sound track. This is read using an iluminating light and a photo-sensor. The sound track is offset from the corresponding film image as it must be read where the film is moving at a constant speed, near one of the feed sprockets.

Optical digital

Modern theatrical systems use optical representations of digitally encoded multi-channel sound.

See the 35mm film article for more information on both digital and analog methods.

Magnetic recording strip

With Super 8 it became possible to add an iron-oxide sound recording strip after the photographic processing was complete, allowing the addition of sound recording to the processed film. Some wide screen technologies also used magnetic strip recording.

Types of lenses and screens

Orthographic

Before the advent of certain wide screen technologies, lenses always reproduced the exact proportions of the film image onto the screen. Such lenses are relatively simple to design and manufacture. Prior to modern wide screen, the industry standard image ratio of width to height was 4:3.

35mm Vistavision was a wide screen orthographic system. The wide image was obtained by running the film horizontaly across the gate so that the width limitation of the film was transformed to a height limitation. See the Vistavision article for more information.

Anamorphic

The 1950's saw the development of wide screen films using special lenses for filming and projection. The images on these films retained the same proportions as in the earlier films (a 4:3 width to height ratio). The wide image is compressed onto the film in the camera using additional cylindrical elements within the lens, with a corresponding lens used in the projector to expand the image to the wide screen. This technique is called anamorphic projection and various implementations have been marketed under several brand names, including CinemaScope, Panavision and Superscope, with Technirama implementing a slightly different anamorphic technique using vertical expansion to the film rather than horizontal compression. Of the anamorphic methods, arguably the best image was produced by the Todd-AO (for Michael Todd and American Optical using 70mm film and a large, curved screen. Around the World in Eighty Days starring David Niven and Cantinflas was the leading general release production using this process. A popular film recently released (2004) as a remake, the original was also released as a Cinimascope print, by comparison to the Todd-AO original as watching televison is to being in a movie theater. Similar 70mm processes include Super (and Ultra) Panavision and VistaVision.

Fish eye with dome

The IMAX® dome projection method (called "OMNIMAX®") uses 70mm film oriented to maximize the image area and extreme wide angle lenses to obtain an almost hemispherical image. The field of view is tilted, as is the projection hemisphere, so one may view a portion of the ground in the forground. Owing to the great area covered by the picture it is not as bright as seen with flat screen projection, but the imerssive qualities are quite convincing. While there are not many theaters capable of displaying this format there are regular productions in the fields of nature, travel, science, and history, and productions may be viewed in most U.S. large urban regions. These dome theaters are mostly located in large and prosperous science and technology museums.

Wide and deep flat screen

The IMAX® flat screen system uses large format film, a wide and deep screen, and close and quite steep "stadium" seating. The effect is to fill the visual field to a greater degree than is possible with conventional wide screen systems. Like the IMAX® dome, this is found in major urban areas, but unlike the dome system it is practical to reformat existing movie releases to this method. Also, the geometry of the theater and screen are more amenable to inclusion within a newly constructed but otherwise conventional multiple theater complex than is the dome style theater.

Multiple cameras and projectors

One wide screen development during the 1950's used non-anamorphic projection, but used three side by side synchronised projectors. Called Cinerama, the images were projected onto an extremely wide, curved screen. Some seams were said to be visible between the images but the almost complete filling of the visual field made up for this. This showed some commercial success as a limited location (only in major citys) exhibition of the technology in This is Cinerama, but the only memorable story-telling film of two made for this technogy was How the West Was Won, widely seen only in its Cinemascope re-release.

While neither a technical nor a commercial success, the business model survives as implemented by the production and limited (but long run by modern standards) exhibitions of IMAX dome movies.

Three dimensional

For techniques used to display pictures with a three dimensional appearance, see the 3-D film article for some movie history and the Stereoscopy article for technical information.

See also

External links