Movie Projector

History

The inspiration for the development of motion pictures and projectors can be traced to a variety of sources including theaters, circuses, and magic shows. Another important factor was the understanding of the phenomenon of persistence of vision. While the process was known for hundreds of years, it was only in the early nineteenth century when Roget introduced the under-lying theory in an article that it developed popular interest. In short, persistence of vision is the phenomena in which the brain retains an image that is observed by the eyes for slightly longer than it is actually seen. Movies take advantage of persistence of vision to create the illusion of motion. When successive still frames are viewed, the brain "connects" the image and they appear to move.

During the early 1800s, hundreds of novelty devices based on this principle were introduced. Some of the most influential include the Thaumatrope and the Phenakistiscope. Dr. John Ayrton Paris is generally regarded as having invented the Thaumatrope in 1825. This device was a toy with a simple design that took advantage of persistence of vision. It consisted of a small round board with a picture on both sides. The original toy had a bird on one side and a cage on the other. The board was held at the side by two strings and when spun it appeared as though the bird was in the cage.

The Phenakistiscope was introduced in 1832 by Joseph Antoine Ferdinand Plateau. This toy was a disc with a fixed center that allowed it to be spun freely. Various images were drawn on the outer edges of the disk depicting sequential movement. The pictures were spaced evenly and slits were cut in conjunction with each. The toy was held between the user and a mirror and images were viewed reflected by the mirror. The persistence of vision created the illusion of movement. Plateau was the first to realize that there had to be a resting period between images for a perfect illusion and determined that 16 images per second was the optimum number. Other inventors introduced similar devices. In 1853, Baron von Uchatius invented a projecting Phenakistiscope by adding a lantern. This was the earliest known moving picture.

One of the most important early moving picture devices was the Zoetrope that was invented by William George Homer in 1834. This device was a rotating drum that had slits cut in its side. A strip of paper, which contained the images, was affixed to the inside and the top of the drum was open. When the drum was spun, the images appeared to move. This was by far the most popular of all these animation toys. It had the added advantage of being able to change the pictures by putting in a different strip of paper with new images. The next device that advanced the technology of animation toys was the kineograph, invented in 1868. This was essentially a flipbook that had drawings or pictures of sequential movement. When the pages were flipped, the illusion of motion was created.

In 1891, Thomas Edison introduced a mechanized version of the Zoetrope he called the Kinetoscope. While similar in principle, it had significant changes. Instead of being moved by hand, the device had a motor attached for automated movement. Also, instead of simple paper images, it utilized a film with that had pictures on it. The film was moved past a fixed light source that projected an image on the wall of a closed booth. When it was found that people would gather to watch these moving pictures, a new industry was born. In 1895, the Lumiére brothers, Auguste and Louis, introduced the Cinematograph. This device was a camera that could take pictures, process it into film and project the image. In 1896, they introduced the Vitascope, which was similar to the Kinetoscope. The primary difference was that the image could be projected onto a much larger screen.

During the course of the twentieth century, movie projector design became more complicated and sophisticated. Spools were added to make it easier for film to move past the light source. The length of movies was significantly increased, and by the 1920s sound was available. In the 1930s, color movies were introduced. The industry was revolutionized in the 1960s by the introduction of the platter that made it possible to show a long movie using a single projector. During the 1970s and 1980s, digital sound was developed. Today, movie projectors are much more impressive and functional than the early counterparts, but the basic principal by which they work remains the same.

Design

Movie projectors consist of four primary sections including the spool assembly, lamp assembly, lens assembly, and audio assembly.

Spool assembly

The primary purpose of the spool assembly is to move the film through the projector. While the motion appears continuous, there is actually a slight pause after each frame. This allows light to be passed through the image and projected on the screen. The spool assembly is made up of all the parts related to storing and moving the film. The platter, which is located on the side of the projector, consists of up to four large discs about 5 ft (152 cm) in diameter vertically stacked between 1-2 ft (30-60.1 cm) apart. Each disc is large enough to hold the length of an entire film. Since every second of film requires 24 frames, a two-hour movie can be as long as 2 mi (3.2 km) when stretched out. Therefore, films are provided to movie theaters on numerous reels that must be spliced together before being loaded on the platter.

A payout assembly on the side of the platter moves the film from the feed disc through the lamp and lens assembly and back to the receiving disc. The film has small holes on its edges that allow it to be held by specialized gears called sprockets. An electric motor turns the sprockets that cause the film to be pulled through the device. Spring-loaded rollers, called cambers, provide tension to keep the film from slipping out of the sprockets. Intermittent sprockets have been developed to pull the film one frame at a time and pause before moving again. They are timed to show 24 frames per second. The film is also stretched between two bars as it passes in front of the lens to keep it tight and aligned. Depending on the projector design, the film is passed through a sound decoding system that is located above or below the lens.

Lamp assembly

The lamp assembly includes all of the parts related to illuminating the image on the film. The key element is the light source. Modern movie projectors use a xenon bulb because they burn brightly for thousands of hours. A xenon bulb is constructed with a quartz outer shell, a cathode, and an anode. When current is applied, the bulb burns bright and hot. The bulb is located in the center of a parabolic mirror which is mounted in the lamphouse. The mirror focuses the light and reflects it onto the condenser. The condenser consists of two lenses that focus the light further and direct it to the main lens assembly. The whole setup not only intensifies the light but also the heat which is why film quickly melts if it is suddenly stopped moving through the projector. Most projectors have a cooling system because of the heat generated by the lamp.

Lens assembly

The light is next passed through the picture head and lens assembly. At the start of this section is the shutter which is a small plate that is rotates 24 times a second. Its movement is synchronized with the advancing film so that dark spaces between the frames are not seen. If the shutter was not in place, the film would appear to flicker. To further reduce flickering, some movie projectors are designed with double shutters. The light is then passed through a small metal frame called the aperture. This ensures that light only shines on the part of the film with the image and not on the sprocket holes.

Light passing through the film causes the image to be projected. The main lens first focuses this image. On most movie projectors, lenses can be removed and changed for different movies. There are primarily two types of lenses available: flat and CinemaScope. A flat lens is more suitable for comedies and dramas while a CinemaScope lens is designed for action movies. Flat lenses typically are between 1.5-1.8 in (37-45 mm) long while CinemaScope lenses are 2.8-3.3 in (70-85 mm). Some movie projectors have a turret system which contains multiple lenses that can be automatically moved into place as needed.

Audio assembly

The audio assembly is the part of the projector that gives the film sound. Two types of technologies can be used: optical or magnetic. Optical systems are the most common. They consist of a light source and a photocell. On one side of the film, a transparent line is recorded. The line varies in width depending on the frequency of sound. As it passes by the light source, varying amounts of light are passed though. A photocell located on the side of the film opposite the light source, picks up the transmitted light. This light is then converted to an audio signal that is then amplified before being sent out to the speakers. Magnetic systems have a recorder head that is in direct contact with the film. The differences in the magnetic field on the film are then converted to the audio signal. Magnetic sound systems are not used as much because they have disadvantages such as being easily damaged, more expensive and a shorter life span.

Raw Materials

Numerous raw materials are used in the manufacture of a movie projector. Aluminum alloys and hard plastics are primarily used to make the housings, sprockets, gears and other structural components. Xenon gas is used for the light bulb. Xenon is a so-called inert gas that creates a tremendous amount of light when it is exposed to an electric current. Quartz is also used to make movie projector light bulbs because it can maintain its structure at high heat better than glass. Other materials used in the construction of a movie projector include rubber, stainless steel, and glass.

The Manufacturing
Process

The major components of a movie projector, including the spooling system, the projector console, the audio reader, and the lenses, are produced by different manufacturers and typically assembled on-site at the movie theaters.

Making the main body

• 1 The main body of the movie projector is basically a rectangular box that houses the lamphouse, the lenses, the picture head, and the audio head. It is made from steel that is loaded onto a conveyor belt. The sheets are then placed in a die with the desired shape of the housing. A hydraulic press is then released. The punch forces the steel sheet to assume the shape of the die. The body is then removed and fitted with an adjustable base that can be changed to modify the angle of viewing.

Making the picture head

These parts are all assembled separately and then put together as a whole.

• 2 The picture head is the area between the lamp and the lens through which the film moves. Its housing is first formed from steel in a punch press process similar to the production of the body.
• 3 Then a series of sprockets and roller pads are manually screwed above and below the framing aperture. An intermittent sprocket is placed below the aperture. This sprocket is then connected to the motor which causes it to start and stop at a frequency of 24 frames per second.
• 4 On the other side of the film, across from the aperture, is the film gate that provides pressure to hold the film in place as the image is projected. The film gate is also formed by the punch press process. Behind the film gate is the shutter blade. This is a small metal device with blades like a fan. It rotates in front of the light condensers at a controlled rate. It is synchronized with the moving film so that dark spaces between frames are not seen.
• 5 The lens turret is placed in front of the aperture. This is a rotating device into which the lenses are placed. It can be moved when a different lens is desired.
• 6 One side of the picture head frame is fitted with a door that can be opened so that film can be loaded. The separate pieces are then assembled and the entire picture head is bolted to the main projector housing.

Making the audio head

• 7 The audio head is constructed in much the same way as the picture head. It is composed of a variety of sprockets and film rollers. On one side of the film path a light source is affixed. This device emits light at a specific wavelength and intensity. On the other side of the film path is a photocell that detects the amount of light that travels through the film. It is connected to a series of amplifiers that are then wired to the theater speakers. The audio head can be located above or below the picture head depending on the design of the movie projector. Like the picture head it is bolted to the main projector body.

Making the lamphouse

• 8 The lamphouse consists of a frame and xenon bulb. Producing a xenon bulb can be a difficult process. Since these bulbs can produce a tremendous amount of heat, their outer housing is made from quartz instead of glass. First a quartz tube is heated and air blown to create the shape required for the bulb. A metal cathode is attached on one end and an anode on the other. Air is replaced in the quartz envelope by xenon gas and the whole unit is vacuum-sealed. The rarity of xenon and the difficulty in construction makes these bulbs expensive, running anywhere from $700 to $2,000 each.
• 9 The bulb is then mounted in the center of an aluminum parabolic shaped mirror. This assembly is then manually attached to a metal frame. The frame has an exhaust pipe and several fans to help remove the large amount of heat generated by the bulb. Wires
  

  A movie projector.
  are hand soldered to the anode and cathode which are then connected to the power supply line. The light assembly is then put in the top of the main projection body. Within the body are the condenser lenses that help focus and intensify the light.

Making the lens

• 10 Lenses are produced from glass. Each movie projection lens is really composed of a number of small glass lenses that have a different magnifying effect. For each component lens, the glass is first cut to the manufacturer's specifications. The glass is then placed on an assembly line and workers polish each piece to the appropriate thickness, then treat it with a special anti-reflection coating. As many as seven component lenses might be used for a single lens system.
• 11 The component lenses are then fitted with metal and then placed into the lens barrel at specific intervals. This is a highly precise process done by specialized workers because the distances between the glass lenses have a profound effect on the image quality. The inside of the barrel is coated with a dark, non-reflective material. The lenses are then attached to the projector body by being screwed into the lens turret.

Making the spool assembly

• 12 The spool assembly begins with the construction of the solid metal frame. The typical frame consists of a tall pillar with two foot bars. Each component is placed on a conveyor belt and passed under a hydraulic punch. This punch is equipped with a sharp metal saw that will cut the proper dimensions from solid steel bars. The foot bars are then manually welded perpendicularly to the bottom of the main pillar. They are positioned such that there is about a 45-degree angle between them. A smaller metal pipe is welded between the foot bars at their midpoints to provide a more stable structure. Finally, metal plates with rubber bottoms are welded to the bottom of the pillar and foot bars to ensure that there is minimal movement during operation.
• 13 Separately, the support arms and accompanying parts are assembled. To one end of the steel support arm a metal bearing is attached. This bearing can spin freely. To the other end of the support arm a hole is drilled through it and an electric gear motor is fitted through. At the end of the motor is a small rubber wheel that spins. It is the motion of this wheel that creates the spinning of the platter that moves the film.
• 14 The support arm assemblies are then attached to the main pillar at set intervals. The arms are welded to a metal plate that is then bolted and secured to the pillar. At specific points on the main pillar, the rollers that hold and guide the film to and from the projector are attached. Sensors that monitor the speed of the platters are bolted to the pillar above each support arm to synchronize the motion of the moving film. The electronic wires are fed into a control box located at the junction between the foot and the main pillar.
• 15 The platters are then placed on the support arms. The platters are made of lightweight aluminum alloy. They can be cut from thick sheets of the metal. A typical dimension is 5 ft (152 cm) in diameter and 0.5 in (1.3 cm) thick. They have a circular cut in the middle that can accommodate the centerpiece. This is a circular device complete with rollers and tension bars that accepts the incoming and outgoing film. A hole is also drilled directly in the center of the platter so it can be held and moved by the bearings on the support arm.

Final assembly

• 16 The main projector console and the film spooling systems are delivered to the movie theatre. They are connected through an electrical cable so that they move the film in a highly controlled motion. They are then ready to be loaded with film and show a movie.

Quality Control

At each step during the production process quality control testing is done to ensure that a working movie projector is produced. Each manufacturer has their own tests specifically related to the part of the projector that they make. These tests include both visual inspection and physical measurements. For example, the lens manufacturer uses computerized laser calipers to measure the thickness of each lens produced. The lamp producers measure various characteristics of the lamps that are produced such as luminosity, heat and power consumption. The components of the main projector are then assembled, and manufacturers run a sample movie through to adjust and pacing of all the moving components and determine whether the device works properly. Even after the movie projector is assembled in the theater, technicians constantly check and adjust parts as necessary.

The Future

The future of movie projectors looks to change dramatically in the coming years. With significant advances in electronic storage mediums, film may not be used to show movies. In fact, some companies are working on a system by which movies are produced on computer hard drives. Stored in this way, movies promise to be much less expensive to distribute and display. Fewer workers could run movie theaters and the movie images will be much clearer and crisper. Currently, theaters are hesitant to adopt the new technology, but it is just a matter of time before computerized digital projectors replace movie film projectors.