How a Film Projector Works

How a Film Projector Works

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
video. Will you help us make our next video?
Will you become an advanced viewer? Go to to sign up. We’d
love for you to join us!

100 thoughts on “How a Film Projector Works”

  1. Hats off, well done! Now you should take the next step (if you haven't already) and explain film to video transfers. With a " film chain" or Telecine technology. Converting 24 frames per second (fps) film to NTSC 30 frame fps video (NDF – too technical to explain NDF and DF in a comment, just ignore this for now) is also impressive. Another interesting fact is that the flicker technique is also incorporated.

  2. I watched another video about the Geneva mechanism (, and in it was said that the shutter sound from old film projectors come from this mechanism. Not only was it not mentioned here, but I also couldn't see it anywhere in the diagrams or internal viewings of the machine, why is that?

  3. Great video Bill, with the added bonus of clear animation, and proper and exact explanation of how projectors work. Not too tech, and a joy to watch right to the last frame! -in super quality…

  4. Worked in a cinema once, up in the projector room.
    One day we were supposed to show Schindler's List.
    The film literally desintegrated between my fingers so I sent somebody from the cash staff to hurry, buy the bluray and hooked up our digital projector.

  5. That was cool.  Man!  The guys that came up with these machines were brilliant. And expertly demonstrated and narrated.  Thank you.

  6. Clear and interesting explanation. I was an "AV Boy" in grammar school for three years in the 1960s, and threaded 16mm projectors hundreds of times, but never knew the details of the underlying technology until now.

  7. I can't tell you how many of your videos I have watched. I've learned so many little things that I've thought of but have never researched and because of your easy to understand videos you have made family time less awkward. I can now keep the conversation. Thank you!

  8. Great video. I'm guessing the reels themselves aren't driven directly, but rather use some sort of spring tension. The tension would have to be enough to keep the film reeled neatly, but not so tight that it causes unnecessary wear on the film or skipping from the sprockets. Is that in any way correct?

  9. I remember these projectors from school in the sixties and early seventies. We used to watch alot of films and it was always great to see one set up with a big reel on it when you came to class. That meant an easy period and some times fun things happened like a reel fell off in the dark or the film skipped or bad like the projector bulb burnt out and no spare. Some times a teacher with a sense of humor would run it backwards and we would all laugh at everybody walking backwards. These is a very good explanation of how these projectors work thanks for making. Cheers

  10. …… had never occurred to me that it wasn't a steady continuous movement of the film reel across the lamp, there's a shuttle and there's a shutter….. Interesting that if the film reel projector would have continued till say 15 years ago it's likely the led light would have replaced the intensely hot incandescent light and if flashed at appropriate rate would have displaced the shutter also……would some re-engineering be happening of these projectors to help with longevity of old film reels? I heard once that in cinemas the film reels only were good for around 80 showings before they had to be replaced cos of wear and tear , is that right?………..I recall the rattling sound of these projectors and that would be the shuttle mechanism. It all is starting to fall into place…
    ….a thing called the digital projector is now in use in cinemas ( movie theatres for Americans )..I think since their introduction that movies appear sorta darkish lacking an atmospheric brightness. I'm thinking around the year 2000 when Lord of rings and the Bond movie die another Day was on the big screen cos they were sort of darkish lighting… Would there be anyone here with experience who might confirm that the dynamic range of luminance is limited for digital projectors?

  11. The end of recess always sucked but the grief was profoundly ameliorated when you saw the teacher setting up the movie projector upon returning to the classroom.

  12. Four thousand years of civilization, and not a single one of us thought of this, or anything else for that matter. Yes, the bleeding wheel.

  13. so the reason movie theatre films seem to be often out of sync with sound is because someone has made too little or too much slack film after the projector, but before the sound drum?

  14. How do people come up with technology like this? I can’t imagine the feelings I would have after seeing a motion picture for the first time in the 19th century.

  15. Man, Engineers and Physicists are the unsung heroes of our world, from the soles of your shoes, to the satellites in orbit, everything is Engineered to make our daily lives better, and more comfortable. The amazing thing is how affordable technology gets as time progresses. Truly, our owes much to these brilliant men.

  16. Wow, fantastic presentation, wish you were teaching when I was a kid, riveting instructional program, well done!

  17. A fabulous video.!
    But how do they sync the sound.?

    If the camera starts to record an image, and the actors start to speak, don't the actors voices get printed right next to their image.?
    But if the sound is actually read 26 frames later, how do they get the voice and image to Match/Sync.?
    Thank You

  18. Very well presented and you kept it interesting right to the end. I really enjoyed it very much Bill.

  19. So if the "editor" cut out a section of film, he would cut between two frames of video, cut along the length of the whole 26-frame offset between video and audio and then cut the audio?

  20. This is not a typical intermittent movement. Most real projectors (i.e. 35mm) use a Geneva type movement. And frames are only projected twice–48cps. The top loop is call the Latham Loop, and the bottom is called the sound loop. There are many interesting little "in the weeds" things, such as, the aperture plate (which framed each frame, if you will) was beveled at the edges by the projectionist using a file, so that the projecting edge was razor thin. This way you didn't project both the front and back edges , which would make the edges on the screen blurry, the back edge of the aperture plate being a few thousandths of an inch back from the focal plane.
    Most people don't realize that a projector is a powerful microscope. In movie theaters a picture .745 inches wide could be 40 or more feet across on the screen.
    Needless to say, I was a projectionist for about 25 years!

  21. Amazing explanation, as usual! Really makes me rethink the capability of tech in the mid-late 1900's (and god does that statement itself make me feel old af)

  22. Fascinating. The only thing you did not address is how as the film moves from one reel to the other, the speed of the reels must vary, because of the circumference of each is constantly changing. Very much like the gears on a 10-speed bike. Is there a clutch system?

  23. The modern, digital era makes ordinary problem solving so easy, that this level of creativity is rarely seen any longer, except in the most demanding of circumstances. And now, with the advancement of machine learning and AI, even these instances are becoming rarer…

  24. I think my projector’s shuttle isn’t syncing up correctly because even though you can see that there is a flicker, all the movies I put through it just appear like a blur. Anyone know how to fix this? Much thanks in advance for an amazingly informative video!!

  25. Superb video. There I was suddenly wondering how old film projectors worked and here I find a perfectly explained answer. Thank you!

  26. This is fantastic! I worked in a theatre with 35 mm and 70 mm projectors, and you still showed me stuff I didn’t know. Your videos are great!

  27. Learnt something new today. I had always assumed that the soundtrack was stored on magnetic tape somehow. Never knew it was all optically-encoded.

  28. ฉันชอบวิดีโอ​นี้มาก​ เสียดายที่ฉันไม่เก่งภาษา​ต่างประเทศ​มากนัก​ ฉันกำลังศึกษา​เกี่ยวกับระบบของเครื่องฉาย​ภาพยนตร์​ ที่ประเทศไทย​มีการทำเครื่องฉาย​ภาพยนตร์​เป็นโมเดล​ขนาดย่อส่วน​ ไว้โชว์​ในงานต่างๆ
    English​ :
    I like this video a lot. Unfortunately, I am not very good at foreign languages. I am studying about the systems of film projectors. In which Thailand has made a movie projector as a mini model To show in various events
    Form​ Thailand

Leave a Reply

Your email address will not be published. Required fields are marked *