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.
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