thanks to Bill Peebles, owner of the Lumina,
Rialto, Colony and Studio theaters, for the projector
and theater photos and his valuable assistance;
Crawford Harris, owner of Reel
Automation, for his assistance and advice; and the North
Carolina School of Science and Mathematics for
the optical toy photos in the Wileman
Movies are part of every modern culture. And while movies
on VHS and
extremely popular, nothing replaces the larger-than-life
spectacle of a grandiose film, such as "The Patriot,"
filling the big screen. In the United States alone, there are
more than 37,000 movie screens, a clear testament to just how
much we love to go to the movies!
A projector is the key piece of technology in
showing movies in theaters around the
show a modern film-based movie, there are five things you
A way to advance each frame of the film
A way to project the image from the film
A way to read the audio
A surface to project the image on
A system to play the audio
provides the first three items on our list. While movies are
usually projected onto a screen, a large white wall is
all you really need.
What is a Movie Projector? A movie projector
is a device that continuously moves film along a path so that
each frame of the film is stopped for a fraction of a
second in front of a light
source. The light source provides extremely bright
illumination that casts the image on the film through a
lens onto a screen.
There are four major groups of parts that make up a
Audio assembly (optical and digital readers, infrared LED)
In the following sections, we'll look at the first three
assemblies. For information on the audio assembly, check out
Movie Sound Works.
Spooling the Film It takes an amazing amount
of film to make a movie. Most movies are shot on 35mm film stock.
You can get 16 frames (individual pictures) on 1 foot (30.5
cm) of film. Movie projectors move the film at a speed of 24
frames per second, so it takes 1.5 feet (45.7 cm) of film to
create every single second of a movie.
At this rate, you end up needing a lot of film pretty
quickly. Consider these calculations:
One second = 1.5 feet (24 frames per second divided by
16 frames per foot)
One minute = 90 feet (1.5 feet per second multiplied by
One hour = 5,400 feet (90 feet per minute multiplied by
Typical two-hour movie plus five minutes of previews =
2.13 miles (11,250 feet divided by 5,280)
use this formula to figure out just how much film it took to
show the next movie you go see. Just multiply the number of
minutes in the movie by 90 to get the number of feet of film.
The platter sits beside the
a feature length film is so long, distributors divide it into
segments that are rolled onto reels. A typical two-hour
movie will probably be divided into five or six reels. In the
early days, films were shown with two projectors. One
projector was threaded with the first reel and the other
projector with the second reel of the movie. The projectionist
would start the film on the first projector, and when it was
11 seconds from the end of the reel, a small circle flashed
briefly in the corner of the screen. This alerted the
projectionist to get ready to change to the other projector.
Another small circle flashed when one second was left and the
projectionist pressed a changeover pedal to start the
second projector and stop the first one. While the second reel
was rolling, the projectionist removed the first reel on the
other projector and threaded the third reel. This swapping
continued throughout the movie.
In the 1960s, a device called a platter began to
show up in theaters. The platter consists of two to four large
discs, about 4 or 5 feet in diameter, stacked vertically 1 to
2 feet apart. A payout assembly on one side of the
platter feeds film from one disc to the projector and takes
the film back from the projector to spool onto a second disc.
The discs are large enough to hold one large spool of the
entire film, which the projectionist assembles by
splicing together all of the lengths of film from the
different reels. Splicing is the process of cutting the end of
one strip of film so that it carefully matches up to the
beginning of the next strip of film, and then taping the
Once projectionists could put all of the film for a movie
on a single spool, a couple of things happened:
One projector could show the entire film.
One projectionist could easily run movies in several
auditoriums at the same time.
These two factors made
it less expensive to show movies because you needed less
manpower and fewer projectors. This led to the birth of the
multiplex, a group of several auditoriums in one
theater. Since their introduction, multiplexes have grown from
two or four auditoriums to 15 to 20. These super-sized
theaters are often referred to as megaplexes.
Moving the Film Once a projectionist splices
the film and loads it on the feed platter, he
threads the film through the platter's payout assembly
and into the top of the projector. A strip of film has small
square holes along each side called sprocket holes.
These holes fit over the teeth of special gear-like
wheels called sprockets. The sprockets, driven by an electric
motor, pull the film through the projector.
Cambers, small spring-loaded rollers, provide tension
to keep the film from bunching up or slipping off the
from the platter goes to the projector (top). The film
then goes through the projector and back to the platter
The film needs to advance one frame, pause for a fraction
of a second and then advance to the next frame. This is
accomplished using one of two mechanisms. The first one uses a
small lever known as the claw, which is mounted on a
bar next to the film's path. The claw is connected to the
outer edge of a wheel that acts as the crank. The
circular motion of the crank makes the claw lift up and out to
come out of a sprocket hole and then down and in to catch onto
another sprocket hole. This causes the film to advance one
frame. The speed of the sprockets is closely synchronized with
the lever action of the claw to make sure that the claw is
consistently advancing the film at a rate of 24 frames per
The second type uses another sprocket wheel mounted just
below the aperture
gate. This intermittent sprocket rotates just far
enough to pull the film down one frame, pauses and then
rotates again. Intermittent sprockets provide more reliable
performance and do not wear out the sprocket holes as quickly
as the claw.
The film is stretched over a couple of bars as it passes in
front of the lens. The bars serve to keep the film tight and
properly aligned. Depending on the projector's configuration
and the sound format used, the film will pass through an
optical audio decoder mounted before or after the lens
assembly. For digital
sound, the film will travel through a special digital
decoder attached to the top of the projector. As the film
leaves the projector (or the digital-audio decoder), it is
carried on a series of rollers back to the platter's payout
assembly and spooled to a take-up platter.
Projecting the Film The key element in a
projector is the light source. Carbon arc lamps have
been used since the early 1900s but have a very short life.
Xenon bulbs are the most commonly used lamps today.
Xenon is a rare gas with certain properties that make it
especially suited for use in projectors:
In dense enough quantities, it will conduct electricity.
As a conductor, it glows very brightly.
It will continue to provide bright illumination for a
substantial amount of time (2,000 to 6,000 hours).
lamphouse (top) has a xenon bulb and parabolic mirror
mounted inside. A xenon bulb (bottom) is made up of a
quartz shell and has xenon gas, a cathode and an anode
Constructing a xenon bulb is a tricky process. The bulbs
have a quartz envelope instead of a glass one because the
bulbs get very hot. The quartz shell houses a cathode and an
anode. Since the xenon gas itself is conductive, the
bulb doesn't need a filament. Instead, when a current is
applied to the bulb, the charge arcs between the
cathode and anode. For the bulb to shine brightly enough, the
xenon must be pure and the quartz envelope must be vacuum
sealed. Because of the rarity of xenon and the complicated
processes involved in bulb production, xenon bulbs generally
cost $700 or more each.
The xenon bulb is mounted in the center of a parabolic
mirror located in the lamphouse. The mirror
reflects light from the bulb and focuses it on the
condenser. The condenser is a pair of lenses used
together to further intensify the light and focus it on the
main lens assembly. The heat generated by this focused light
is incredible. That's why film melts so quickly when the
projector stops spooling it.
(top) rotates to keep the film from flickering. An
aperture gate (bottom) can come in a variety of sizes
depending on the screen
As the focused light leaves the lamphouse and enters the
projector, it is intercepted by the shutter. The
shutter is a small, propeller-like device that rotates 24
times per second. Each blade of the shutter blocks the path of
the light as it comes to a certain point in its revolution.
This blacking out is synchronized with the advancement of the
film so that the light doesn't project the fraction of a
second when the film is moving from one frame to the next.
Without it, the film would seem to flicker or have faint
impressions of the images out of sync. Many projectors use
double shutters that rotate in opposite directions. This
causes the light to be cut off from both the top and bottom of
each frame, further reducing the possibility of flicker.
Before the light gets to the film, it also passes through
an aperture gate. The aperture gate is a small,
removable metal frame that blocks the light from illuminating
anything but the part of the film that you want to see on the
screen. Two good examples of unwanted images would be the
sprocket holes and audio information along the sides of the
film. Aperture gates come in a variety of sizes that
correspond to the screen format of the movie.
projector lens (top) can be changed to show different
film formats. The viewport (bottom) is at the front
of the projection
From the aperture gate, the light passes through the film
and into the main lens. The lens is removable and can be
changed depending on the format of the film. The two most
common lenses are flat and CinemaScope. Many
projectors have a turret that allows both types of
lenses to be mounted, and the projector will rotate the
required lens into place.
From the projector, the light goes through a
viewport at the front of the projection booth and
travels to the front of the auditorium until it reaches the
screen. Finally, the images from the film appear on the
Automating the Process
Tens of thousands of movie theaters across
the U.S. use projectors like this
have developed many innovative techniques to ensure that the
show proceeds as it should. Cue tape is one of the more
interesting and useful of these. It is a short strip of metal
fastened to the edge of the film at a specific location. At
the appropriate time, the film passes two electrical contacts,
and the cue tape completes a circuit between the contacts.
This circuit acts like a switch, and it can serve a variety of
functions. A cue-tape switch can:
dim the house lights
turn off the house lights
change the lens setting
change the sound format
change the screen masking (masking is the use of
curtains to frame the screen)
The last item on the list is
not very relevant since most theaters now use platters, but
changing projectors is the original reason that cue tape was
invented. With cue-tape switches, manufacturers were able to
automate the process of beginning one reel as the other ended.
Enterprising projectionists soon realized that they could
automate a number of other functions as well by using certain
combinations of cue tape to trigger specific responses.
Cue tape has made it possible to automate many aspects of
movie projection, such as changing sound formats between the
previews and the movie, but new systems like Reel
Automation's Showtimer promise to greatly enhance and
expand automated processes.
History Movies, short for moving
pictures, have been around for more than a century. Movies
work because of persistence of vision, the fact that a
human eye retains an image for about one-twentieth of a second
after seeing it. In the early 19th century, several devices
began to appear that used persistence of vision to create the
illusion of motion from still images. The zoetrope,
invented by William George Horner in 1834, consisted of a
series of pictures on a paper strip arranged on the inside of
a revolving drum. The drum had small slits you could look
through to see the pictures.
In the case of a particular sort of zoetrope called the
praxinoscope, there was a mirrored drum in the middle,
so that you could see the pictures by looking through the top
of the device. The pictures on the drum changed slightly from
one to the next. By spinning the drum, you could make the
pictures move fast enough to fool your eye into thinking it
was looking at one moving picture. These pictures were
typically of some repetitious movement, such as a person
walking or dancing, because this movement could be
looped easily. In a looped strip of images, the last
picture in the series would almost match up to the first one,
so that the images would create a single cycle of the
simulated movement, which could be infinitely repeated to
produce the illusion of continuing movement.
Wileman Collection housed at the North
Carolina School of Science and Mathematics Inventors took advantage
of the eye's persistence of vision to create moving
pictures. Here are some early inventions: (from top) a
zoetrope, picture strips used in a zoetrope, and a
The earliest film projectors, such as the magic
lantern, actually showed up in the late 1600s, but they
presented only still images. Some of the earliest projectors
to show moving images were simply modified zoetropes. Creative
entrepreneurs used translucent strips on the drum and placed a
light source, usually a lantern, in the middle of the box.
They would then project the image through a small hole, or
aperture, onto a blank wall or piece of stretched white
cloth. Obviously, these devices were very limited. They were
operated by hand and were used the same type of looping
animation or photos as the original zoetrope.
Pictures line the inside of the
drum of a
Everything changed with the invention of Thomas Edison's
kinetoscope in 1891. The kinetoscope used a motor to
revolve a strip of film in front of a light source. The light
source projected the image from the film on a screen in a
booth. As it became obvious that people were willing to pay
money for this type of entertainment, many inventors began to
design variations of Edison's original device. One such
variation, the manually operated kinora, was invented
by the Lumiere brothers and enjoyed great success into the
The Lumiere brothers, Louis and Auguste, created the
astounding cinematographe in 1895. This portable device
was a camera, film
processing lab and projector all in one package! The brothers
traveled the French countryside shooting films that lasted a
few minutes at most. They then processed and projected the
film on location! The next year, the vitascope (which
was another variation of the kinetoscope) heralded the dawn of
a new age of entertainment. The vitascope worked like a basic
kinetoscope with one essential difference: The image was
projected onto a large screen in a room instead of a small one
in a booth. Thus began the road to the development of the
first theater, the Nickelodeon in Pittsburgh, PA.
Wileman Collection housed at the North
Carolina School of Science and
Mathematics The magic lantern (top and bottom) was a
forerunner of the projector. The kinora (center) was
invented by the Lumiere brothers and was manually
Throughout the 20th century, movies and projectors grew in
complexity. Engineers outfitted projectors with
sprockets and spools to make it easier to move
the film rapidly in front of the light source. Movies went
from a few minutes in length to an hour or more, and by the
late 1920s, movie-goers were enjoying "talkies," films that
included a soundtrack. The first color movies appeared in the
1930s, and the 1940s and 1950s saw the development of several
new processes and screen
formats. The platter, which revolutionized the
industry, debuted in the 1960s. Automation began to
take hold in the 1970s and 1980s, and the 1990s saw the advent
of digital sound and the growth of LCD
technology. Still, although modern projectors are
brighter, faster and more functional than their predecessors,
and manufacturers have added many bells and whistles over the
years, the essence of the projector has remained the same
since the beginning of the 20th century.
projectors are now springing up in select theaters, the
movie-theater industry is still overwhelmingly analog. This is
simply a matter of practical consideration. Most theaters have
spare parts and local technicians who can service an analog
projector easily. Repairing a digital projector, on the other
hand, often requires flying in a specialized technician in
addition to buying replacement parts. Digital projectors use
LCDs to create the image instead of film. At first, this
sounds great -- no more scratches or spots! But LCD projectors
do have a major drawback: If the LCD has a bad pixel or two
(which happens quite often), that blemish will appear
throughout every movie shown on that projector. With film,
once you replace the scratched film or go to another movie,
you lose all the picture blemishes.
For more information on movie projectors and related
topics, check out the links on the next page!