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 by projecting them on a projection screen. Most of the optical and mechanical elements, except for the illumination and sound devices, are present in movie cameras.

Physiology

According to the theory of persistence of vision, the perceptual processes of the brain and the retina of the human eye retains an image for a brief moment of time. This theory is said to account for the illusion of motion which results when a series of film images are displayed in quick succession, rather than the perception of the individual frames in the series.

Persistence of vision should be compared with the related phenomena of beta movement and phi movement. A critical part of understanding these visual perception phenomena is that the eye is not a camera, ie: there is no "frame rate" or "scan rate" in the eye. Instead, the eye/brain system has a combination of motion detectors, detail detectors and pattern detectors, the outputs of all of which are combined to create the visual experience.

The frequency at which flicker becomes invisible is called the flicker fusion threshold, and is dependent on the level of illumination. Generally, the frame rate of 16 frames per second (fps) is regarded as the lowest frequency at which continuous motion is perceived by humans. (Interestingly this threshold varies across different species; a higher proportion of rod cells in the retina will create a higher threshold level.)

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.

Since the birth of sound film, virtually all film projectors in commercial movie theaters project at a constant speed of 24 fps. This speed was chosen for financial and technical reasons - it was the slowest speed (and thus required the least film stock and was cheapest for producers) at which a satisfactory reproduction and amplification of sound could be conducted. There are some specialist formats (eg Showscan and Maxivision) which project at higher rates, often 48 fps.

Silent films usually were not projected at constant speeds ^* but rather were varied throughout the show at the discretion of the projectionist, often with some notes provided by the distributor. Speeds ranged from about 18 fps on up - sometimes even faster than modern sound film speed (24 fps). Contrary to received opinion, 16 fps - though sometimes used as a camera shooting speed - was dangerously inadvisable for projection, due to the high risk of the nitrate-base prints catching fire in the projector. (A dramatic rendition of a nitrate print fire and its potentially devastating effects is famously found in Nuovo Cinema Paradiso, which revolves around the goings-on of a projectionist.)

Principles of operation

Projection elements

As in a slide projector there are essential optical elements:

Light source

An incandescent lamp 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. Xenons were introduced in the 1950s and are now the more common source, being easier and safer to maintain for the most part.

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.

Douser

(Also spelled dowser.)

A metal blade which cuts off light before it can get to the film - usually this is part of the lamphouse. Some projectors have both a manually controlled and electronically one each; the electronic one is used for changeovers. Dousers protect the film when the lamp is on but the film is not moving, preventing the film from melting from prolonged exposure to the direct heat of the lamp.

Film gate and single image

A single image of the series of images comprising the movie is positioned and held flat within an aperture 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. Modern shutters are designed with a flicker-rate of two or even sometimes three times the frame rate of the film, so as to reduce the perception of screen flickering. (See Frame rate and Flicker fusion threshold.)

Imaging lens and aperture plate

A lens system with multiple optical elements directs the image of the film to a viewing screen. Different lenses are used for different aspect ratios. Each of these lenses comes with an aperture plate, a piece of metal with a precisely cut rectangular hole in the middle of equivalent aspect ratio. The aperture plate is placed just behind the gate, and masks off any light from hitting the image outside of the area intended to be shown (most modern films have extra image on the frame that is meant to be masked off in the projector).

Viewing screen

Main article: Projection 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). In a commercial theater, the screen also has hundreds of small, evenly spaced holes in order to allow the passage of sound from the speakers and subwoofer which often are directly behind it.

Film transport elements

Film supply and takeup

=Two reel system

= The two reel system is also known as a changeover system, after the switching mechanism that operates between the end of one reel and the beginning of the next. 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.

The two reel system was almost universally used before the advent of the single reel system for movie theaters in order to be able to show feature-length films. Although one reel long-play systems tend to be more popular with the newer multiplexes, the two reel system is still in significant use to this day. The projector operator operates two projectors, threading one with the next reel while the other projector plays the current reel. As the current reel approaches its end, the projectionist looks for cues, also known as cigarette burns, at the upper right corner of the picture. Usually these are dots or circles, although they can also be slashes. (Some older films have occasionally been known to have used squares or triangles, and even positioned the cues in the middle of the right edge of the picture.) The first cue appears twelve feet (3.7 m) or eight seconds at 24 frame/s before the end of the reel, and signals the projectionist to start the motor of the projector containing the next reel. After another ten and a half feet (3.2 m) or seven seconds at 24 frame/s, the changeover cue should appear, which signals the projectionist to actually make the changeover. When this second cue appears, the projectionist has one and a half feet (457 mm) or one second at 24 frame/s to make the changeover - if it doesn't occur within one second, the tail black leader of the exhausted reel will be projected on the screen. On some projectors, the operator would be alerted to the change by a bell that operated when the feed reel rotation exceeded a certain speed (that reel rotates faster as the film is exhausted), or based on the diameter of the remaining film (Premier Changeover Indicator Pat.411992), although many such projectors do not have such an auditory system.

During the actual operation of a changeover, the two projectors use an interconnected electrical control connected to the changeover button so that as soon as the button is pressed, the douser on the old reel is closed in sync with the douser for the new reel. If done properly, a changeover should be virtually unnoticeable to an audience. In older theaters, there may be manually operated, sliding covers in front of the projection booth's windows. A changeover with this system is often clearly visible as a wipe on the screen.

The size of the reels can vary based on the projectors, but generally films are divided and distributed in reels of roughly 2000 feet (610 m) about 22 minutes at 24 frame/s. Some projectors can even accommodate up to 6000 feet (1,830 m), which minimizes the number of changeovers in a showing. Certain countries also divide their film reels up differently; Russian films, for example, often come on 1000 foot (305 m) reels, although it's likely that most projectionists working with changeovers would combine them into longer reels of at least 2000 feet (610 m), to minimize changeovers and also give sufficient time for threading and any possibly needed troubleshooting time.

=Single reel system

= There are two largely used single reel systems (also known as long-play systems) today: the tower system (vertical feed and takeup) and the platter system (horizontal feed and takeup).

The tower system largely resembles the two reel system, except in that the tower itself is generally a separate piece of equipment used with a slightly modified standard projector. The feed and takeup reels are held vertically on the axis, except behind the projector, on oversized spools with 12,000 foot (3,660 m) capacity or about 133 minutes at 24 frame/s. This large capacity alleviates the need for a changeover on an average-length feature; all of the reels are spliced together into one giant one. The tower is designed with four spools, two on each side, each with its own motor. This allows the whole spool to be immediately rewound after a showing; the extra two spools on the other side allow for a film to be shown while another is being rewound or even made up directly onto the tower. Each spool requires its own motor in order to set proper tensioning for the film, since it has to travel (relatively) much further between the projector film transport and the spools. As each spool gains or loses film, the tension must be periodically checked and adjusted so that the film can be transported on and off the spools without either sagging or snapping.

In a platter system the individual 20 minute reels of film are also spliced together as one large reel, but the film is then wound onto a horizontal rotating table called a platter. Three or more platters are stacked together to create a platter system. Most of the platters in a platter system will be occupied by film prints; whichever platter happens to be empty serves as the "take-up reel" to receive the film that is playing from another platter.

The way the film is fed from the platter to the projector is not unlike an eight-track audio cartridge. Film is unwound from the center of the platter through a mechanism called a "brain" which controls the speed of the platter's rotation so that it matches the speed of the film as it is fed to the projector. The film winds through a series of rollers from the platter stack to the projector, through the projector, through another series of rollers back to the platter stack, and then onto the platter serving as the take-up reel.

This system makes it possible to project a film multiple times without needing to rewind it. As the projectionist threads the projector for each showing, he transfers the brain mechanism from the empty platter to the full platter and the film then plays back onto the platter it came from. In the case of a double feature, each film plays from a full platter onto an empty platter, swapping positions on the platter stack throughout the day.

The advantage of a platter is that the film isn't subjected to the stresses of being rewound each show. Rewinding risks rubbing the film against itself, which can cause scratching of the film and smearing of the emulsion which carries the pictures. The disadvantages of the platter system are that the film can acquire diagonal scratches on it if proper care is not taken while threading film from platter to projector, and the film has more opportunity to collect dust and dirt as long lengths of film are exposed to the air. A clean projection booth kept at the proper humidity is of great importance, as are cleaning devices that can remove dirt from the film print as it plays.

=Automation and the rise of the multiplex

= The single reel system can allow for the complete automation of the projection booth operations, given the proper auxiliary equipment. 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. It is the complete automation that has enabled the modern "multiplex" cinema - a single site typically containing from 16 to 24 theaters with only a few projection and sound technicians, rather than a platoon of projectionists. The multiplex also offers a great amount of flexibility to a theater operator, enabling theaters to exhibit the same popular production in more than one auditorium with staggered starting times. It is also possible, with the proper equipment installed, to "interlock", i.e. thread a single reel of film through multiple projectors. This is very useful when dealing with the mass crowds that an extremely popular film may generate in the first few days of showing, as it allows for a single reel to serve more patrons.

Feed and extraction sprockets

Smooth wheels with triangular pins called sprockets engage perforations 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

As with motion picture cameras, the intermittent motion of the gate requires that there be loops above and below the gate in order to serve as a buffer between the constant speed enforced by the sprockets above and below the gate and the intermittent motion enforced at the gate. Some projectors also have a sensitive trip pin above the gate to guard against the upper loop becoming too big. If the loop hits the pin, it will close the dousers and stop the motor to prevent an excessively large loop from jamming the projector.

Film gate pressure plate

A spring loaded pressure plate functions to align the film in a consistent 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 intermittent mechanism. The plate also has spring-loaded runners to help hold film while in place and advance it during motion.

Intermittent mechanism

The intermittent mechanism can be constructed in different ways. For smaller gauge projectors (8 mm and 16 mm), a pawl mechanism engages the film's sprocket hole 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. This is similar to the claw mechanism in a motion picture camera.

In 35 mm and 70 mm projectors, there usually is a special sprocket immediately underneath the pressure plate known as the intermittent sprocket. Unlike the all the other sprockets in the projector, which run continuously, the intermittent sprocket operates in tandem with the shutter, and only moves while the shutter is blocking the lamp, so that the motion of the film cannot be seen. It also moves in a discrete amount at a time, equal to the number of perforations that make up a frame (4 for 35 mm, 5 for 70 mm). The intermittent movement in these projectors is usually provided by a Maltese Cross mechanism.

IMAX projectors use what is known as the rolling loop method, in which each frame is sucked into the gate by a vacuum, and positioned by registration pins in the perforations corresponding to that frame.

Types of projectors

Projectors are classified by the size of the film used, i.e. the film format. Typical film sizes:

8 mm

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

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 8 mm width, not split during processing as is the earlier 8 mm. Magnetic stripes could be added to carry encoded sound to be added after film development.

16 mm

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.

35 mm

The most common film size for theatrical productions during the first half of the 20th century. In fact, the common 35 mm 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.

70 mm

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

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

Sound

Regardless of the sound format, any sound represented on the film image itself will not be the sound for the particular frame it occupies. All optical sound formats must be offset from the image because the image is projected with intermittent motion. If the sound head on the projector was adjacent to the gate, the sound would be a jerky start-stop-start-stop and so on. Therefore the sound head requires continuous motion and will be located a certain number of frames before or after the gate.

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

Optical

With 16 mm and the larger sizes it is practical to add a narrow channel of optically encoded sound track. This is read using an illuminating light or laser and a photocell or photodiode. In 16 mm, this is a single mono track, and the sound head is 26 frames after the gate. In 35 mm, this can be mono or stereo, the latter including several different Dolby sound matrixing systems (including Dolby A and Dolby SR). The sound head is located twenty frames after the gate for 35 mm projectors. Originally optical sound was variable density, where the transparency/opacity level of the sound track was used to represent sound. This had disadvantages because the grain of the film caused a background hiss, and so was replaced with the now-universal standard variable area. In this system, a clear waveform on black background represents the sound, and the width of the waveform is equivalent to the amplitude. Variable area does have slightly less frequency response than variable density, it should be noted. In the 1970s and early 1980s, optical sound Super-8 mm copies were produced mainly for airline in-flight movies. This technology was soon made obsolete by video equipment.

Digital

Modern theatrical systems use optical representations of digitally encoded multi-channel sound. An advantage of digital systems is that the offset between the sound and picture heads can be varied and then set with the digital processors. Digital sound heads are usually above the gate. All digital sound systems currently in use have the ability to instantly and gracefully fall back to the optical sound system should the digital data be corrupt or the whole system fail.

Cinema Digital Sound (CDS)

Created by Kodak and ORC (Optical Radiation Corporation), Cinema Digital Sound was the first attempt to bring multi-channel digital sound to first-run theaters. CDS was available on both 35 mm and 70 mm films. Film prints equipped with CDS did not have the conventional analog optical or magnetic soundtracks to serve as a "back-up" in case the digital sound was unreadable. Another disadvantage of not having an analog back-up track is that CDS required extra film prints be made for the theaters equipped to play CDS. The three formats that followed, Dolby Digital, DTS and SDDS, can co-exist with each other and the analog optical soundtrack on a single version of the film print. This means that a film print carrying all three of these formats (and the analog optical format, usually Dolby SR) can be played in whichever format the theater is equipped to handle. CDS did not achieve wide-spread use and ultimately failed. It premiered with the film Dick Tracy and was used with several other films, such as Days of Thunder and Terminator 2: Judgement Day.

Sony Dynamic Digital Sound (SDDS)

SDDS sound runs on the outside of 35 mm film, between the perforations and the edges, on both edges of the film. SDDS was the first digital system that could handle up to eight tracks of sound. The additional two tracks are for an extra pair of screen channels (Left Center and Right Center) located between the 3 regular screen channels (Left, Center and Right). A pair of CCD's located in a unit above the projector read the two SDDS tracks. The information is decoded and decompressed before being passed along to the cinema sound processor. By default, SDDS units use an onboard Sony Cinema Sound Processor, and when the system is set up in this manner, the theatre's entire sound system can be equalized in the digital domain. In contrast, both DTS and Dolby Digital soundtracks must be passed through to standard analog Dolby cinema sound processors - which are also used for analog optical sound, so equalization of the sound remains in the analog domain. The audio data in a SDDS track is compressed in the 20-bit ATRAC2 compression scheme at a ratio of about 4.5:1. SDDS premiered with the film Last Action Hero.

Dolby Digital

Also known as Spectral Recording Digital or "SR•D." Sound is printed between the perforations and is 26 frames before the picture (the offset can be varied based on processing presets). There are two types of Dolby Digital: SR•D EX, an 8 channel digital system, and SR•D, a 6 channel digital system. The audio data in a Dolby Digital track is compressed in the 16-bit AC-3 compression scheme at a ratio of about 12:1. The images between each perforation are read by a CCD located either above the projector or in the regular analog sound head below the film gate. The information is then decoded, decompressed, and converted to analog by an SR-D processor before going to a standard Dolby analog multi-format cinema sound processor. A consumer version of Dolby Digital is also used on most DVD's, often at higher data rates than the original film. Dolby Digital officially premiered with the film Batman Returns, but it was earlier tested at some screenings of The Undiscovered Country.

Digital Theater Systems (DTS)

DTS actually stores the sound information on separate CD-ROMs supplied with the film. The CDs are fed into a special modified computer (usually a 386 or 486 system) which syncs up with the film through the use of DTS time code, decompresses the sound, and passes it through to a standard analog Dolby processor. The time code is placed between the optical sound tracks and the actual picture, and is read by an optical LED ahead of the gate. The time code is actually the only sound system which is not offset within the film from the picture, but still needs to be physically set offset ahead of the gate in order to maintain continuous motion. Each disc can hold slightly over two hours of sound, so longer films will require a second disc. Three types of DTS sound exist: DTS-ES (Extended Surround), an 8 channel digital system; DTS-6, a 6 track digital system, and a now obselete 4 channel system. The audio data in a DTS track is compressed in the 20-bit APTX-100 compression scheme at a ratio of about 4.5:1. Of the three digital formats currently in use, DTS is the only one that has been used with 70 mm presentations. DTS was premiered on Jurassic Park. A consumer version of DTS is available on some DVDs.

Magnetic sound

70 mm, which had no optical sound, used the 5 millimeters gained between the 65 mm negative and the final release print to place three magnetic tracks on each side of the perforations, for a total of six tracks. Unlike all other non-double head magnetic sound, 70 mm magnetic heads are located before the gate. Until the introduction of digital sound, it was fairly common for 35 mm films to be blown up to 70 mm often just to take advantage of the greater number of sound tracks. 35 mm four-track magnetic sound was used from the 1950s through the mid 1970s for big-budget feature prints. It was of excellent quality, although somewhat prone to damage and erasure over time. As analog optical stereo gained popularity (it was also more durable and far less expensive to include on a film print), 35 mm four-track magnetic sound was increasingly only used for special road show screenings, and the development of digital sound systems made it completely obsolete.

35 mm and 16 mm each are sometimes run in sync with a separate reel of magnetic sound (known as double head projection because two reels are running on one projector in sync); the image goes through a gate while the magnetic reel passes over a sound head. Since the sound is on a separate reel, it does not need to be offset from the image. This system is usually used only for very low-budget or student productions, or for screening rough cuts of films before the creation of a final married print. Sync between the two reels is checked with SMPTE Academy leader, also known as countdown leader. If the two reels are synced, there should be one frame of "beep" sound exactly on the "2" frame of the countdown - 2 seconds or 48 frames before the picture start.

On certain stocks of Super 8 and 16 mm an iron-oxide sound recording strip was added for the direct synchronous recording of sound which could then be played by projectors with a magnetic sound head. It has since been discontinued by Kodak on both gauges.

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.

35 mm VistaVision was a wide screen orthographic system. The wide image was obtained by running the film horizontally 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 70 mm film and a large, curved screen. The 1956 version of Around the World in Eighty Days starring David Niven and Cantinflas was the leading general release production using this process. It could be said that watching the Cinemascope print of this movie instead of the Todd-AO original is like watching a movie on television rather than in a movie theater. Similar 70 mm processes include Super (and Ultra) Panavision and VistaVision.

Fish eye with dome

The IMAX® dome projection method (called "OMNIMAX®") uses 70 mm 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 foreground. Owing to the great area covered by the picture it is not as bright as seen with flat screen projection, but the immersive 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 cities) exhibition of the technology in This is Cinerama, but the only memorable story-telling film of two made for this technology 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 documentary production, limited release locations, and long running 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.

External links

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