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A movie projector is an opto-mechanical device for displaying motion picture film by projecting it onto a screen. Most of the optical and mechanical elements, except for the illumination and sound devices, are present in movie cameras. Modern movie projectors are specially built video projectors. (see also digital cinema)

History

Simulation of a spinning zoopraxiscope
An early projector and seats from a movie theater

The main precursor to the movie projector was the magic lantern. In its most common setup it had a concave mirror behind a light source to help direct as much light as possible through a painted glass picture slide and a lens, out of the lantern onto a screen. Simple mechanics to have the painted images moving were probably implemented since Christiaan Huygens introduced the apparatus around 1659. Initially candles and oil lamps were used, but other light sources, such as the argand lamp and limelight were usually adopted soon after their introduction. Magic lantern presentations may often have had relatively small audiences, but the very popular phantasmagoria and dissolving views shows were usually performed in proper theatres, large tents or especially converted spaces with plenty seats.

Both Joseph Plateau and Simon Stampfer thought of lantern projection when they independently introduced stroboscopic animation in 1833 with a stroboscopic disc (which became known as the phenakistiscope), but neither of them intended to work on projection themselves.

The oldest known successful screenings of stroboscopic animation were performed by Ludwig Döbler in 1847 in Vienna and taken on a tour to several large European cities for over a year. His Phantaskop had a front with separate lenses for each of the 12 pictures on a disc and two separate lenses were cranked around to direct light through the pictures.[1][citation needed]

Wordsworth Donisthorpe patented ideas for a cinematographic film camera and a film presentation system in 1876. In reply to the introduction of the phonograph and a magazine's suggestion that it could be combined with projection of stereoscopic photography, Donisthorpe stated that he could do even better and announce that he would present such images in motion. His original Kinesigraph camera gave unsatisfactory results. He had better results with a new camera in 1889 but never seems to have been successful in projecting his movies.

Eadweard Muybridge developed his Zoopraxiscope in 1879 and gave many lectures with the machine from 1880 to 1894. It projected images from rotating glass disks. The images were initially painted onto the glass, as silhouettes. A second series of discs, made in 1892–94, used outline drawings printed onto the discs photographically, then colored by hand.[2]

Ottomar Anschütz developed his first Electrotachyscope in 1886. For each scene, 24 glass plates with chronophotographic images were attached to the edge of a large rotating wheel and thrown on a small opal-glass screen by very short synchronized flashes from a Geissler tube. He demonstrated his photographic motion from March 1887 until at least January 1890 to circa 4 or 5 people at a time, in Berlin, other large German cities, Brussels (at the 1888 Exposition Universelle), Florence, Saint Petersburg, New York, Boston and Philadelphia. Between 1890 and 1894 he concentrated on the exploitation of an automatic coin-operated version that was an inspiration for Edison Company's Kinetoscope. From 28 November 1894 to at least May 1895 he projected his recordings from two intermittently rotating discs, mostly in 300-seat halls, in several German cities. During circa 5 weeks of screenings at the old Berlin Reichstag in February and March 1895, circa 7.000 paying visitors came to see the show.[1]

In 1886 Louis Le Prince applied for a US patent for a 16-lens device that combined a motion picture camera with a projector. In 1888, he used an updated version of his camera to film the motion picture Roundhay Garden Scene and other scenes. The pictures were privately exhibited in Hunslet.[citation needed] After investing much time, effort and means in a slow and troublesome development of a definitive system, Le Prince eventually seemed satisfied with the result and had a demonstration screening scheduled in New York in 1890. However, he went missing after boarding a train in France and was declared dead in 1897. His widow and son managed to draw attention to Le Prince's work and eventually he came to be regarded as the true inventor of film (a claim also made for many others).

After years of development, Edison eventually introduced the coin-operated peep-box Kinetoscope movie viewer in 1893, mostly in dedicated parlours. He believed this was a commercially much more viable system than projection in theatres. Many other film pioneers found chances to study the technology of the kinetoscope and further developed it for their own movie projection systems.

The Eidoloscope, devised by magic lantern. In its most common setup it had a concave mirror behind a light source to help direct as much light as possible through a painted glass picture slide and a lens, out of the lantern onto a screen. Simple mechanics to have the painted images moving were probably implemented since Christiaan Huygens introduced the apparatus around 1659. Initially candles and oil lamps were used, but other light sources, such as the argand lamp and limelight were usually adopted soon after their introduction. Magic lantern presentations may often have had relatively small audiences, but the very popular phantasmagoria and dissolving views shows were usually performed in proper theatres, large tents or especially converted spaces with plenty seats.

Both Joseph Plateau and Simon Stampfer thought of lantern projection when they independently introduced stroboscopic animation in 1833 with a stroboscopic disc (which became known as the phenakistiscope), but neither of them intended to work on projection themselves.

The oldest known successful screenings of stroboscopic animation were performed by Ludwig Döbler in 1847 in Vienna and taken on a tour to several large European cities for over a year. His Phantaskop had a front with separate lenses for each of the 12 pictures on a disc and two separate lenses were cranked around to direct light through the pictures.[1][citation needed]

Wordsworth Donisthorpe patented ideas for a cinematographic film camera and a film presentation system in 1876. In reply to the introduction of the phonograph and a magazine's suggestion that it could be combined with projection of stereoscopic photography, Donisthorpe stated that he could do even better and announce that he would present such images in motion. His original Kinesigraph camera gave unsatisfactory results. He had better results with a new camera in 1889 but never seems to have been successful in projecting his movies.

Eadweard Muybridge developed his Zoopraxiscope in 1879 and gave many lectures with the machine from 1880 to 1894. It projected images from rotating glass disks. The images were initially painted onto the glass, as silhouettes. A second series of discs, made in 1892–94, used outline drawings printed onto the discs photographically, then colored by hand.[2]

Ottomar Anschütz developed his first Electrotachyscope in 1886. For each scene, 24 glass plates with chronophotographic images were attached to the edge of a large rotating wheel and thrown on a small opal-glass screen by very short synchronized flashes from a Geissler tube. He demonstrated his photographic motion from March 1887 until at least January 1890 to circa 4 or 5 people at a time, in Berlin, other large German cities, Brussels (at the 1888 Exposition Universelle), Florence, Saint Petersburg, New York, Boston and Philadelphia. Between 1890 and 1894 he concentrated on the exploitation of an automatic coin-operated version that was an inspiration for Edison Company's Kinetoscope. From 28 November 1894 to at least May 1895 he projected his recordings from two intermittently rotating discs, mostly in 300-seat halls, in several German cities. During circa 5 weeks of screenings at the old Berlin Reichstag in February and March 1895, circa 7.000 paying visitors came to see the show.[1]

In 1886 Louis Le Prince applied for a US patent for a 16-lens device that combined a motion picture camera with a projector. In 1888, he used an updated version of his camera to film the motion picture Roundhay Garden Scene and other scenes. The pictures were privately exhibited in Hunslet.[citation needed] After investing much time, effort and means in a slow and troublesome development of a definitive system, Le Prince eventually seemed satisfied with the result and had a demonstration screening scheduled in New York in 1890. However, he went missing after boarding a train in France and was declared dead in 1897. His widow and son managed to draw attention to Le Prince's work and eventually he came to be regarded as the true inventor of film (a claim also made for many others).

After years of development, Edison eventually introduced the coin-operated peep-box Kinetoscope movie viewer in 1893, mostly in dedicated parlours. He believed this was a commercially much more viable system than projection in theatres. Many other film pioneers found chances to study the technology of the kinetoscope and further developed it for their own movie projection systems.

The Eidoloscope, devised by Eugene Augustin Lauste for the Latham family, was demonstrated for members of the press on 21 April 1895 and opened to the paying public on May 20, in a lower Broadway store with films of the Griffo-Barnett prize boxing fight, taken from Madison Square Garden's roof on 4 May.[3] It was the first commercial projection.

Max and Emil Skladanowsky projected motion pictures with their Bioscop, a flickerfree duplex construction, from 1 to 31 November 1895. They started to tour with their motion pictures, but after catching the second presentation of the Cinématographe Lumière in Paris on 28 December 1895, they seemed to choose not to compete. They still presented their motion pictures in several European cities until March 1897, but eventually the Bioscop had to be retired as a commercial failure.

In Lyon, Louis and Auguste Lumière perfected the Cinématographe, a system that took, printed, and projected film. In late 1895 in Paris, father Antoine Lumière began exhibitions of projected films before the paying public, beginning the general conversion of the medium to projection. They quickly became Europe's main producers with their actualités like Workers Leaving the Lumière Factory and comic vignettes like The Sprinkler Sprinkled (both 1895). Even Edison, joined the trend with the Vitascope, a modified Jenkins' Phantoscope, within less than six months. [4]

Decline of film projectors

In 1999,[5] digital cinema projectors were being tried out in some movie theatres. These early projectors played the movie stored on a computer, and sent to the projector electronically. Due to their relatively low resolution (usually only 2K) compared to later digital cinema systems, the images at the time had visible pixels. By 2006, the advent of much higher 4K resolution digital projection reduced pixel visibility. The systems became more compact over time. By 2009, movie theatres started replacing film projectors with digital projectors. In 2013, it was estimated that 92% of movie theatres in the United States had converted to digital, with 8% still playing film. In 2014, numerous popular filmmakers—including Quentin Tarantino and Christopher Nolan—lobbied large studios to commit to purchase a minimum amount of 35 mm film from Kodak. The decision ensured that Kodak's 35 mm film production

Both Joseph Plateau and Simon Stampfer thought of lantern projection when they independently introduced stroboscopic animation in 1833 with a stroboscopic disc (which became known as the phenakistiscope), but neither of them intended to work on projection themselves.

The oldest known successful screenings of stroboscopic animation were performed by Ludwig Döbler in 1847 in Vienna and taken on a tour to several large European cities for over a year. His Phantaskop had a front with separate lenses for each of the 12 pictures on a disc and two separate lenses were cranked around to direct light through the pictures.[1][citation needed]

Wordsworth Donisthorpe patented ideas for a cinematographic film camera and a film presentation system in 1876. In reply to the introduction of the phonograph and a magazine's suggestion that it could be combined with projection of stereoscopic photography, Donisthorpe stated that he could do even better and announce that he would present such images in motion. His original Kinesigraph camera gave unsatisfactory results. He had better results with a new camera in 1889 but never seems to have been successful in projecting his movies.

Eadweard Muybridge developed his Zoopraxiscope in 1879 and gave many lectures with the machine from 1880 to 1894. It projected images from rotating glass disks. The images were initially painted onto the glass, as silhouettes. A second series of discs, made in 1892–94, used outline drawings printed onto the discs photographically, then colored by hand.[2]

Ottomar Anschütz developed his first Electrotachyscope in 1886. For each scene, 24 glass plates with chronophotographic images were attached to the edge of a large rotating wheel and thrown on a small opal-glass screen by very short synchronized flashes from a Geissler tube. He demonstrated his photographic motion from March 1887 until at least January 1890 to circa 4 or 5 people at a time, in Berlin, other large German cities, Brussels (at the 1888 Exposition Universelle), Florence, Saint Petersburg, New York, Boston and Philadelphia. Between 1890 and 1894 he concentrated on the exploitation of an automatic coin-operated version that was an inspiration for Edison Company's Kinetoscope. From 28 November 1894 to at least May 1895 he projected his recordings from two intermittently rotating discs, mostly in 300-seat halls, in several German cities. During circa 5 weeks of screenings at the old Berlin Reichstag in February and March 1895, circa 7.000 paying visitors came to see the show.[1]

In 1886 Louis Le Prince applied for a US patent for a 16-lens device that combined a motion picture camera with a projector. In 1888, he used an updated version of his camera to film the motion picture Roundhay Garden Scene and other scenes. The pictures were privately exhibited in Hunslet.[citation needed] After investing much time, effort and means in a slow and troublesome development of a definitive system, Le Prince eventually seemed satisfied with the result and had a demonstration screening scheduled in New York in 1890. However, he went missing after boarding a train in France and was declared dead in 1897. His widow and son managed to draw attention to Le Prince's work and eventually he came to be regarded as the true inventor of film (a claim also made for many others).

After years of development, Edison eventually introduced the coin-operated peep-box Kinetoscope movie viewer in 1893, mostly in dedicated parlours. He believed this was a commercially much more viable system than projection in theatres. Many other film pioneers found chances to study the technology of the kinetoscope and further developed it for their own movie projection systems.

The Eidoloscope, devised by Eugene Augustin Lauste for the Latham family, was demonstrated for members of the press on 21 April 1895 and opened to the paying public on May 20, in a lower Broadway store with films of the Griffo-Barnett prize boxing fight, taken from Madison Square Garden's roof on 4 May.[3] It was the first commercial projection.

Max and Emil Skladanowsky projected motion pictures with their Bioscop, a flickerfree duplex construction, from 1 to 31 November 1895. They started to tour with their motion pictures, but after catching the second presentation of the Cinématographe Lumière in Paris on 28 December 1895, they seemed to choose not to compete. They still presented their motion pictures in several European cities until March 1897, but eventually the Bioscop had to be retired as a commercial failure.

In Lyon, Louis and Auguste Lumière perfected the Cinématographe, a system that took, printed, and projected film. In late 1895 in Paris, father Antoine Lumière began exhibitions of projected films before the paying public, beginning the general conversion of the medium to projection. They quickly became Europe's main producers with their actualités like Workers Leaving the Lumière Factory and comic vignettes like The Sprinkler Sprinkled (both 1895). Even Edison, joined the trend with the Vitascope, a modified Jenkins' Phantoscope, within less than six months. [4]

In 1999,[5] digital cinema projectors were being tried out in some movie theatres. These early projectors played the movie stored on a computer, and sent to the projector electronically. Due to their relatively low resolution (usually only 2K) compared to later digital cinema systems, the images at the time had visible pixels. By 2006, the advent of much higher 4K resolution digital projection reduced pixel visibility. The systems became more compact over time. By 2009, movie theatres started replacing film projectors with digital projectors. In 2013, it was estimated that 92% of movie theatres in the United States had converted to digital, with 8% still playing film. In 2014, numerous popular filmmakers—including Quentin Tarantino and Christopher Nolan—lobbied large studios to commit to purchase a minimum amount of 35 mm film from Kodak. The decision ensured that Kodak's 35 mm film production would continue for several years.[6]

Although usually more expensive than film projectors, high-resolution digital projectors offer many advantages over traditional film units. For example, digital projectors contain no moving parts except fans, can be operated remotely, are relatively compact and have no film to break, scratch or change reels of. They also allow for much easier, less expens

Although usually more expensive than film projectors, high-resolution digital projectors offer many advantages over traditional film units. For example, digital projectors contain no moving parts except fans, can be operated remotely, are relatively compact and have no film to break, scratch or change reels of. They also allow for much easier, less expensive, and more reliable storage and distribution of content. All-electronic distribution eliminates all physical media shipments. There is also the ability to display live broadcasts in theaters equipped to do so.

In 1912 Max Wertheimer discovered beta movement and the phi phenomenon. In each the brain constitutes an experience of apparent movement when presented with a sequence of near-identical still images. This theory is said to account for the illusion of motion which results when a series of film images is 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 beta movement and phi movement. A critical part of understanding these visual perception phenomena is that the eye is not a camera, i.e.: there is no frame rate for the human eye or brain. 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 (frame/s) is regarded as the lowest frequency at which continuous motion is perceived by humans. This threshold varies across different species; a higher proportion of rod cells in the retina will create a higher threshold level. Because the eye and brain have no fixed capture rate, this is an elastic limit, so different viewers can be more or less sensitive in perceiving frame rates.

It is possible to view the black space between frames and the passing of the shutter by rapidly blinking ones eyes at a certain rate. If done fast enough, the viewer will be able to randomly "trap" the image between frames, or during shutter motion. This will not work with (now obsolete) cathode ray tube displays due to the persistence of the phosphors nor with LCD or DLP light projectors because they refresh the image instantly with no blackout intervals as with film projectors.

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. This was more a function of hand-cranked projectors than the silence. When the electric motor supplanted hand cranking in both movie cameras and projectors, a more uniform frame rate became possible. Speeds ranged from about 18 frame/s on up – sometimes even faster than modern sound film speed (24 frame/s).

16 frame/s – though sometimes used as a camera shooting speed – was inadvisable for projection, due to the risk of the nitrate-base prints catching fire in the projector. Nitrate film stock began to be replaced by cellulose triacetate in 1948. A nitrate film fire and its devastating effect is featured in Cinema Paradiso (1988), a fictional film which partly revolves around a projectionist and his apprentice.

The birth of sound film created a need for a steady playback rate to prevent dialog and music from changing pitch and distracting the audience. Virtually all film projectors in commercial movie theaters project at a constant speed of 24 frame/s. This speed was chosen for both financial and technical reasons. A higher frame rate produces a better looking picture, but costs more as film stock is consumed faster. When Warner Bros. and Western Electric were trying to find the ideal compromise projection speed for the new sound pictures, Western Electric went to the Warner Theater in Los Angeles, and noted the average speed at which films were projected there. They set that as the sound speed at which a satisfactory reproduction and amplification of sound could be conducted.

There are some specialist formats (e.g. Showscan and Maxivision) which project at higher rates—60 frames/sec for Showscan and 48 for Maxivision. The Hobbit was shot at 48 frames/sec and projected at the higher frame rate at specially equipped theaters. Each frame of regular 24 fps movies are shown twice or more in a process called "double-shuttering" to reduce flicker.[7]

As in a slide projector there are essential optical elements:

Light source

Incandescent lighting and even limelight were the first light sources used in film projection. In the early 1900s up until the late 1960s, carbon arc lamps were the source of light in almost all theaters in the world.

The Xenon arc lamp was introduced in Germany in 1957 and in the US in 1963. After film platters became commonplace in the 1970s, Xenon lamps became the most common light source, as they could stay lit for extended periods of time, whereas a carbon rod used for a carbon arc could last for an hour at the most.

Most lamp houses in a professional theatrical setting produce sufficient heat to burn the film should the film remain stationary for more than a fraction of a second. Because of this, absolute care must be taken in inspecting a film so that it should not break in the gate and be damaged, particularly necessary in the era when flammable cellulose nitrate film stock was in use.

Reflector and condenser lensIncandescent lighting and even limelight were the first light sources used in film projection. In the early 1900s up until the late 1960s, carbon arc lamps were the source of light in almost all theaters in the world.

The Xenon arc lamp was introduced in Germany in 1957 and in the US in 1963. After film platters became commonplace in the 1970s, Xenon lamps became the most common light source, as they could stay lit for extended periods of time, whereas a carbon rod used

The Xenon arc lamp was introduced in Germany in 1957 and in the US in 1963. After film platters became commonplace in the 1970s, Xenon lamps became the most common light source, as they could stay lit for extended periods of time, whereas a carbon rod used for a carbon arc could last for an hour at the most.

Most lamp houses in a professional theatrical setting produce sufficient heat to burn the film should the film remain stationary for more than a fraction of a second. Because of this, absolute care must be taken in inspecting a film so that it should not break in the gate and be damaged, particularly necessary in the era when flammable cellulose nitrate film stock was in use.

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

It is the gate and shutter that gives the illusion of on

It is the gate and shutter that gives the illusion of one full frame being replaced exactly on top of another full frame. The gate holds the film still while the shutter is open. A rotating petal or gated cylindrical shutter interrupts the emitted light during the time the film is advanced to the next frame. The viewer does not see the transition, thus tricking the brain into believing a moving image is on screen. Modern shutters are designed with a flicker-rate of two times (48 Hz) or even sometimes three times (72 Hz) the frame rate of the film, so as to reduce the perception of screen flickering. (See Frame rate and Flicker fusion threshold.) Higher rate shutters are less light efficient, requiring more powerful light sources for the same light on screen.

objective with multiple optical elements directs the image of the film to a viewing screen. Projector lenses differ in aperture and focal length to suit different needs. Different lenses are used for different aspect ratios.

One way that aspect ratios are set is with the appropriate 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. All films, even those in the standard Academy ratio, have extra image on the frame that is meant to be masked off in the projection.

Using an aperture plate to accomplish a wider aspect ratio is inherently wasteful of film, as a portion of the standard frame is unused. One solution that presents itself at certain aspect ratios is the "2-perf" pulldown, where the film is advanced less than one full frame in order to reduce the unexposed area between frames. This method requires a special intermittent mechanism in all film handling equipment throughout the production process, from the camera to the projector. This is costly, and prohibitively so for some theaters. The anamorphic format uses special optics to squeeze a high aspect ratio image onto a standard Academy frame thus eliminating the need to change the costly precision moving parts of the intermittent mechanisms. A special anamorphic lens is used on the camera to compress the image, and a corresponding lens on the projector to expand the image back to the intended aspect ratio.

Viewing screen

In most cases this is a reflective surface which may be either aluminized (for high contrast in moderate ambient

One way that aspect ratios are set is with the appropriate 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. All films, even those in the standard Academy ratio, have extra image on the frame that is meant to be masked off in the projection.

Using an aperture plate to accomplish a wider aspect ratio is inherently wasteful of film, as a portion of the standard frame is unused. One solution that presents itself at certain aspect ratios is the "2-perf" pulldown, where the film is advanced less than one full frame in order to reduce the unexposed area between frames. This method requires a special intermittent mechanism in all film handling equipment throughout the production process, from the camera to the projector. This is costly, and prohibitively so for some theaters. The anamorphic format uses special optics to squeeze a high aspect ratio image onto a standard Academy frame thus eliminating the need to change the costly precision moving parts of the intermittent mechanisms. A special anamorphic lens is used on the camera to compress the image, and a corresponding lens on the projector to expand the image back to the intended aspect ratio.

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). A switchable projection screen can be switched between opaque and clear by a safe voltage under 36V AC and is viewable from both sides. In a commercial theater, the screen also has millions of very 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

The size of the reels can vary based on the projectors, but generally films are divided and distributed in reels of up to 2,000 feet (610 metres), about 22 minutes at 24 frames/sec). Some projectors can even accommodate up to 6,000 feet (1,800 metres), which minimizes the number of changeovers (see below) in a showing. Certain countries also divide their film reels up differently; Russian films, for example, often come on 1,000-foot (300 m) reels, although it's likely that most projectionists working with changeovers would combine them

The film being wound on the takeup reel is being wound "head in, tails out." This means that the beginning (or "head") of the reel is in the center, where it is inaccessible. As each reel is taken off of the projector, it must be re-wound onto another empty reel. In a theater setting there is often a separate machine for rewinding reels. For the 16 mm projectors that were often used in schools and churches, the projector could be re-configured to rewind films.

The size of the reels can vary based on the projectors, but generally films are divided and distributed in reels of up to 2,000 feet (610 metres), about 22 minutes at 24 frames/sec). Some projectors can even accommodate up to 6,000 feet (1,800 metres), which minimizes the number of changeovers (see below) in a showing. Certain countries also divide their film reels up differently; Russian films, for example, often come on 1,000-foot (300 m) reels, although it's likely that most projectionists working with changeovers would combine them into longer reels of at least 2,000 feet (610 metres), to minimize changeovers and also give sufficient time for threading and any possibly needed troubleshooting time.

Films are identified as "short subjects," taking one reel or less of film, "two-reelers," requiring two reels of film (such as some of the early Laurel & Hardy, 3 Stooges, and other comedies), and "features," which can take any number of reels (although most are limited to 1½ to 2 hours in length, enabling the theater to have multiple showings throughout the day and evening, each showing with a feature, commercials, and intermission to allow the audiences to change). In the "old days" (i.e., ca. 1930–1960), "going to the movies" meant seeing a short subject (a newsreel, short documentary, a "2-reeler," etc.), a cartoon, and the feature. Some theaters would have movie-based commercials for local businesses, and the state of New Jersey required showing a diagram of the theater showing all of the exits.

Because a single film reel does not contain enough film to show an entire feature, the film is distributed on multiple reels. To prevent having to interrupt the show when one reel ends and the next is mounted, two projectors are used in what is known as a "changeover system," after the switching mechanism that operates between the end of one reel on the first projector and the beginning of the next reel on the second projector. The two-reel system was used almost universally for movie theaters before the advent of the single-reel system 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, starting the first reel of the show on projector "A." While this reel is being shown, the projectionist threads the second reel on projector "B."

As the reel being shown approaches its end, the projectionist looks for

As the reel being shown approaches its end, the projectionist looks for cue marks at the upper-right corner of the picture. Usually these are dots or circles, although they can also be slashes. Some older films occasionally used squares or triangles, and sometimes positioned the cues in the middle of the right edge of the picture.

The first cue appears twelve feet (3.7 metres) before the end of the program on the reel, equivalent to eight seconds at 24 frames/sec. This cue signals the projectionist to start the motor of the projector containing the next reel. After another ten and a half feet (3.2 m) of film is shown (seven seconds at 24 frames/sec), 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 (460 mm), or one second at 24 frame/s, to make the changeover. If it doesn't occur within one second, the tail leader of the reel coming to an end will be projected on the screen.

Twelve feet before the "first frame of action," countdown leaders have a "START" frame. The projectionist positions the "START" in the gate of the projector. When the first cue is seen, the motor of the starting projector is started. Seven seconds later the end of the leader and start of program material on the new reel should just reach the gate of the projector when the changeover cue is seen.

On some projectors, the operator would be alerted to the time for a change by a bell that operated when the feed reel rotation exceeded a certain speed (the feed reel rotates faster as the film is exhausted), or based on the diameter of the remaining film (Premier Changeover Indicator Pat. No. 411992), although many projectors do not have such an auditory system.

During the initial 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 changeover douser on the outgoing projector is closed in sync with the changeover douser on the incoming projector opening. 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.

Once the changeover has been made, the projectionist unloads the full takeup reel from projector "A," moves the now-empty reel (that used to hold the film just unloaded) from the feed spindle to the takeup spindle, and loads reel #3 of the presentation on projector "A." When reel 2 on projector "B" is finished, the changeover switches the live show from projector "B" back to projector "A," and so on for the rest of the show.

When the projectionist removes a finished reel from the projector it is "tails out," and needs to be rewound before the next show. The projectionist usually uses a separate rewind machine and a spare empty reel, and rewinds the film so it is "head out," ready to project again for the next show.

One advantage of this system (at least for the theatre management) was that if a programme was running a few minutes late for any reason, the projectionist would simply omit one (or more) reels of film to recover the time.

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 (non-rewinding; 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 (3,700 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.

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 (3,700 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 payout unit 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, the payout unit is transferred 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 need not be rewound after each show, which can save labor. 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 ta

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 of projection that has enabled the modern "multiplex" cinema – a single site typically containing from 8 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 length 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 print to serve more patrons.

Feed and extraction sprockets

Smooth wheels with triangular

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 o

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

The

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 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,

In 35 mm and 70 mm projectors, there usually is a special sprocket immediately underneath the pressure plate, known as the intermittent sprocket. Unlike 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 Geneva drive, also known as the 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.

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, exposing one side, then removed from the camera, the takeup and feed reels are switched, and the film run through a second time, exposing the other side. The 16 mm film is then split lengthwise into two 8 mm pieces that are spliced to make a single projectable film with sprockets holes on one side.

Super 8