English subtitles for clip: File:How a Film Projector Works.webm

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One of the most impactful pieces of engineering
is the technology of movies.

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They’ve shaped every aspect of our lives.

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Today, of course, they’re created digitally,
but I celebrate here the stunning engineering

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that gave life to movies; the technology that
tricked the mind into seeing a moving image.

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Film came in many sizes from the giant 70mm
— popular in the 1960s for epics like Lawrence

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of Arabia — to 35mm used for most feature
films, to 16mm for schools, and even 8mm used

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by home enthusiasts.

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The larger the film, the greater the resolution,
of course.

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All worked with mechanisms similar to common
16mm projectors.

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I’ll examine this Bell and Howell 1580 16mm
projector — built in 1979.

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We’ll look at the shuttle that starts and
stops the film, the shutter that strategically

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blocks light, and the photo sensor that reads
the sound — all of which operate in harmony.

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To create the illusion of movement, a series
of still images — the film — is pulled

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off the supply reel, threaded in between the
lamp and lens so the image can be projected,

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then run across the sound drum, and finally
coiled onto the takeup reel.

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However, it isn’t as simple as that sounds.

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To see why here’s what happens if you just
move the film continuously past the projector’s

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lamp.

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What you see is a blur — you can just make
out the images.

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Here’s what really happens shown in slow
motion.

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A frame appears on the screen, not moving,
then the screen goes blank, and then the next

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frame is projected on the screen.

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The projector must hold the image on the screen
for a moment and then cover up the image while

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the film moves to the next frame.

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Two mechanisms do this.

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First, the shuttle.

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The shuttle has three teeth which engage the
sprocket holes in the film.

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The shuttle moves back to disengage from the
film, then moves up, then forward to engage

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the film, then moves down pulling the film
with it.

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The film is stationary most of the time and
only moves when the shuttle is moving down.

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This is the intermittent motion of the film
necessary to avoid blurring of the projected

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image.

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Here is slow-motion footage of the shuttle
moving up and down intermittently.

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From this angle, you clearly see the shuttle
move forward and back to engage and disengage

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from the film.

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Two shuttle arms hold the teeth of the shuttle
in place.

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In between the arms is an eccentric cam.

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This cam rotates with an axle and moves the
shuttle arms up and down.

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The outline of the cam has a constant width
so that the distance between the arms doesn’t

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change.

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The cam’s shape holds the shuttle steady
at the top and bottom of its travel.

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To see how the shuttle moves forward and backward,
lets look down from above.

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The shuttle arms act like a third-class lever.

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They pivot on one end, and at the other end
a spring force pushes them forward and an

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effort forces them backwards.

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This backwards effort is created by a disk
tilted a few degrees off of the axle.

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When the axle turns, the disk wobbles.

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A horizontal post connected to the shuttle
arms is pressed into contact with the wobbling

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disk by the spring force.

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As the axle turns and the disk wobbles, the
shuttle arms are rhythmically pressed backwards.

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This movement is synced with the eccentric
cam to create the required motion of the shuttle.

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The shuttle transports the film so that it
stationary most of the time and quickly advances

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to the next frame.

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Though it is rapid, the film movement will
still cause blur in the projected image.

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This blur is eliminated by a shutter.

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The shutter is a disk with a blade that protrudes
from half the circumference.

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The other half is open.

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The shutter rotates once every frame and is
synced so that the shutter blade blocks light

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from the lamp while the shuttle is advancing
the film.

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This prevents the projection of film motion
on the screen.

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The film passes by the lamp at twenty-four
frames per second.

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At that rate the human mind blends the still
frames into fluid motion.

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A projector with a single bladed shutter blocks
light from the lamp once every frame.

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So, half the time, every twenty-fourth of
a second, the screen is dark.

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This switching between a bright projected
image and darkness is called flicker.

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If the flicker occurs at about sixty to seventy
times per second the bright flashes fuse together

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and appear — to the human eye — continuously
bright with no periods of darkness.

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This rate is called the flicker fusion threshold.

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Since twenty-four flickers per second is below
the threshold, the flicker is visible.

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This flicker is the origin of the term “flick”
as slang for movies.

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But modern film projectors don’t have this
problem.

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How did they fix it?

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Originally shutters had a single blade that
covered the advancement of the film with an

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open section that showed the picture.

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Modern shutters have three blades.

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The first blade covers the film motion.

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The second two blades block the light even
when the film is stationary — they only

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serve to increase the flicker rate.

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The three openings allow the image to be projected
half the time.

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Here I’ve labeled the three blades with
one, two and three dots.

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Notice that the shuttle moves downward only
when Blade number one blocks the light.

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The three-bladed shutter is a simple and inexpensive
solution that works well.

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The frame rate stays at twenty-four frames
per second and the flicker rate increases

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to seventy-two flickers per second — above
the flicker fusion threshold — so the movie

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appears to move smoothly and without distracting
flicker.

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This means if you watch a film in slow motion,
you will see that a single frame is flashed

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on the screen three times before the next
frame appears.

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A subtle but important detail of film projectors
is the film loop.

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The loop allows for two kinds of motion of
the film: intermittent and continuous.

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The key is they happen simultaneously.

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The film must pause in front of the lens to
project without blur, but must also move continuously

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for the proper playback of the sound.

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The top sprocket pulls the film from the supply
reel continuously.

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A loop of slack film starts to form.

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This slack allows the shuttle to quickly advance
to the next frame without tearing the film.

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A second loop of slack film at the bottom
also forms.

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The bottom sprocket pulls the film continuously.

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This is important because it allows the sound
to be read correctly.

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Sound in movies is recorded optically on the
edge of the film.

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After the film runs past the lamp, it runs
across the sound drum.

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To read this optical soundtrack, light shines
through a tube with a slit.

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This concentrates the light on a small section
of the film’s soundtrack.

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A photo sensor on the other side of the film
measures the amount of light passing through

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the film at a given time.

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The photo sensor converts the amount of light
transmitted into current and this current

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drives the speakers.

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A soundtrack that oscillates slowly produces
low frequency sounds.

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If it oscillates more rapidly it will produce
higher frequencies.

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The volume is determined by the amplitude
or width of the soundtrack.

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Louder sections are wide and quieter sections
are thinner.

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Because the image is projected here, and the
sound is read down here, the soundtrack is

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offset twenty-six frames ahead of the picture
in 16mm films.

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This offset ensures that the picture and sound
are correctly synced.

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To me the most beautiful aspect of the film
projector is how all the mechanisms are synced.

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The mechanisms are driven by a single rotating
axle.

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The axle rotates the shutter, and simultaneously
turns the cam and advances the film.

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Behind the shuttle is a worm screw that drives
two gears that are coaxial with the top and

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bottom sprockets.

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So this means that with every rotation of
the axle, the shutter blocks and flashes light

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three times, the shuttle pulls down a single
frame, and the worm screw rotates the gears

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and sprockets one-fourteenth of a revolution.

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Since there are fourteen teeth on a sprocket,
the top sprocket pulls one frame’s worth

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of film from the supply reel, and the bottom
sprocket pulls one frame through the projector.

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This setup keeps all the important mechanisms
in sync.

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One thing to keep in mind is that film projectors
were designed and built in parallel with film

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cameras.

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In fact, in many respects the technology in
both cameras and projectors are nearly identical.

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I’m Bill Hammack, the EngineerGuy.

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