How a Film Projector Works

How a Film Projector Works

October 15, 2019 100 By Kailee Schamberger


One of the most impactful pieces of engineering
is the technology of movies. They’ve shaped every aspect of our lives. Today, of course,
they’re created digitally, but I celebrate here the stunning engineering that gave life
to movies; the technology that tricked the mind into seeing a moving image. Film came
in many sizes from the giant 70mm — popular in the 1960s for epics like Lawrence of Arabia
— to 35mm used for most feature films, to 16mm for schools, and even 8mm used by home
enthusiasts. The larger the film, the greater the resolution, of course. All worked with
mechanisms similar to common 16mm projectors. I’ll examine this Bell and Howell 1580 16mm
projector — built in 1979. We’ll look at the shuttle that starts and stops the film,
the shutter that strategically blocks light, and the photo sensor that reads the sound
— all of which operate in harmony. To create the illusion of movement, a series of still
images — the film — is pulled off the supply reel, threaded in between the lamp
and lens so the image can be projected, then run across the sound drum, and finally coiled
onto the takeup reel. However, it isn’t as simple as that sounds. To see why here’s
what happens if you just move the film continuously past the projector’s lamp. What you see
is a blur — you can just make out the images. Here’s what really happens shown in slow
motion. A frame appears on the screen, not moving, then the screen goes blank, and then
the next frame is projected on the screen. The projector must hold the image on the screen
for a moment and then cover up the image while the film moves to the next frame. Two mechanisms
do this. First, the shuttle. The shuttle has three teeth which engage the sprocket holes
in the film. The shuttle moves back to disengage from the film, then moves up, then forward
to engage the film, then moves down pulling the film with it. The film is stationary most
of the time and only moves when the shuttle is moving down. This is the intermittent motion
of the film necessary to avoid blurring of the projected image. Here is slow-motion footage
of the shuttle moving up and down intermittently. From this angle, you clearly see the shuttle
move forward and back to engage and disengage from the film. Two shuttle arms hold the teeth
of the shuttle in place. In between the arms is an eccentric cam. This cam rotates with
an axle and moves the shuttle arms up and down. The outline of the cam has a constant
width so that the distance between the arms doesn’t change. The cam’s shape holds
the shuttle steady at the top and bottom of its travel. To see how the shuttle moves forward
and backward, lets look down from above. The shuttle arms act like a third-class lever.
They pivot on one end, and at the other end a spring force pushes them forward and an
effort forces them backwards. This backwards effort is created by a disk tilted a few degrees
off of the axle. When the axle turns, the disk wobbles. A horizontal post connected
to the shuttle arms is pressed into contact with the wobbling disk by the spring force.
As the axle turns and the disk wobbles, the shuttle arms are rhythmically pressed backwards.
This movement is synced with the eccentric cam to create the required motion of the shuttle.
The shuttle transports the film so that it stationary most of the time and quickly advances
to the next frame. Though it is rapid, the film movement will still cause blur in the
projected image. This blur is eliminated by a shutter. The shutter is a disk with a blade
that protrudes from half the circumference. The other half is open. The shutter rotates
once every frame and is synced so that the shutter blade blocks light from the lamp while
the shuttle is advancing the film. This prevents the projection of film motion on the screen.
The film passes by the lamp at twenty-four frames per second. At that rate the human
mind blends the still frames into fluid motion. A projector with a single bladed shutter blocks
light from the lamp once every frame. So, half the time, every twenty-fourth of a second,
the screen is dark. This switching between a bright projected image and darkness is called
flicker. If the flicker occurs at about sixty to seventy times per second the bright flashes
fuse together and appear — to the human eye — continuously bright with no periods
of darkness. This rate is called the flicker fusion threshold. Since twenty-four flickers
per second is below the threshold, the flicker is visible. This flicker is the origin of
the term “flick” as slang for movies. But modern film projectors don’t have this
problem. How did they fix it? Originally shutters had a single blade that covered the advancement
of the film with an open section that showed the picture. Modern shutters have three blades.
The first blade covers the film motion. The second two blades block the light even when
the film is stationary — they only serve to increase the flicker rate. The three openings
allow the image to be projected half the time. Here I’ve labeled the three blades with
one, two and three dots. Notice that the shuttle moves downward only when Blade number one
blocks the light. The three-bladed shutter is a simple and inexpensive solution that
works well. The frame rate stays at twenty-four frames per second and the flicker rate increases
to seventy-two flickers per second — above the flicker fusion threshold — so the movie
appears to move smoothly and without distracting flicker. This means if you watch a film in
slow motion, you will see that a single frame is flashed on the screen three times before
the next frame appears. A subtle but important detail of film projectors is the film loop.
The loop allows for two kinds of motion of the film: intermittent and continuous. The
key is they happen simultaneously. The film must pause in front of the lens to project
without blur, but must also move continuously for the proper playback of the sound. The
top sprocket pulls the film from the supply reel continuously. A loop of slack film starts
to form. This slack allows the shuttle to quickly advance to the next frame without
tearing the film. A second loop of slack film at the bottom also forms. The bottom sprocket
pulls the film continuously. This is important because it allows the sound to be read correctly.
Sound in movies is recorded optically on the edge of the film. After the film runs past
the lamp, it runs across the sound drum. To read this optical soundtrack, light shines
through a tube with a slit. This concentrates the light on a small section of the film’s
soundtrack. A photo sensor on the other side of the film measures the amount of light passing
through the film at a given time. The photo sensor converts the amount of light transmitted
into current and this current drives the speakers. A soundtrack that oscillates slowly produces
low frequency sounds. If it oscillates more rapidly it will produce higher frequencies.
The volume is determined by the amplitude or width of the soundtrack. Louder sections
are wide and quieter sections are thinner. Because the image is projected here, and the
sound is read down here, the soundtrack is offset twenty-six frames ahead of the picture
in 16mm films. This offset ensures that the picture and sound are correctly synced. To
me the most beautiful aspect of the film projector is how all the mechanisms are synced. The
mechanisms are driven by a single rotating axle. The axle rotates the shutter, and simultaneously
turns the cam and advances the film. Behind the shuttle is a worm screw that drives two
gears that are coaxial with the top and bottom sprockets. So this means that with every rotation
of the axle, the shutter blocks and flashes light three times, the shuttle pulls down
a single frame, and the worm screw rotates the gears and sprockets one-fourteenth of
a revolution. Since there are fourteen teeth on a sprocket, the top sprocket pulls one
frame’s worth of film from the supply reel, and the bottom sprocket pulls one frame through
the projector. This setup keeps all the important mechanisms in sync. One thing to keep in mind
is that film projectors were designed and built in parallel with film cameras. In fact,
in many respects the technology in both cameras and projectors are nearly identical. I’m
Bill Hammack, the EngineerGuy. Thank you to our advanced viewers who helped shape this
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