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
engineerguy.com/preview to sign up. We’d love for you to join us!

100 Replies to “How a Film Projector Works”

  1. 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.

  2. 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!

  3. 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?

  4. 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

  5. ……..it 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?

  6. 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.

  7. 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.

  8. 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?

  9. 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.

  10. 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.

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

  12. 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

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

  14. 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?

  15. 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!

  16. 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)

  17. 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?

  18. 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…

  19. 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!!

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

  21. 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!

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

  23. 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

  24. While I had seen how projectors work before, I knew this would still be worth watching. Amazing technology for people to come up with back then.

  25. Wait how is the sound synchronous with the projection if it’s 26 frames ahead?

    Also if film comes from the supply through the projector then the sound drum, wouldn’t that make the sound *behind not ahead?

  26. If the reels are driven what compensation is there for loss of film on the supply reel (decreasing diameter) as opposed to the take up reel (increasing diameter)? I have often wondered.

  27. The "engineerguy" so clear in his vision, knowledge, and voice that I almost missed my train. Mindblowing. We need people (educator) like you and believe me, you can turn a rock (dull guy) into a diamond. Hats-off.

  28. Very well explained ! First time someone explains me so simply what is the difference betweem an 8 mm and a 35 mm. Other teachers couldn't explain me how it works . I was not dumb . THEY were.

  29. i'm 42 and i had the chance to have an aunt whose uncle had a movie theater in a small city in the countryside here in brazil. i used to go to the projection booth and to the basement under it to fiddle with things when it was closed (i'd always find a way to sneak into the building), and to watch the projectionist do stuff when working. it was still carbon arc, rca projectors. very old ones, but looked good as far as i can remember. the movie theater had a little over 900 seats, two floors, and there was a beautiful and thick red curtain that would open and close. a nice movie theater for a small city in a developing country.

    fascinated by how the sound system worked, i would build a thing just to extract the audio from a 35mm doc i had. i used cardboard paper and a flashlight lamp and a light thingy to convert light to energy and connect it to the mic imput of my mom's stereo. fun stuff. there was a movie theater in another city i lived that had a problem of frame misalignment in one projector that would show part of the optical track, and during a ninja turtles session i still remember how cool it was to see the waves being shown while 'go ninja, go' was playing. the song has basically two frequencies of bass, and one bass would look like waves going up and the other like waves going down, and there was all the crazy mids and treble.

    later, when i was 15, i got a chance to work for three months at a movie theater that was part of a cultural complex run by the state. the complex was built with german technology and they donated a planetarium. the movie theater had unique seats, and the screen could lift up and be hidden for orchestra presentations. the screen was a little smaller than the screen at my uncle's, but still a standard size, and there was no curtains. the building desing was quite different.

    aside from the regular 35mm projectors, they had eiki 16mm and bauer super 8 projectos, and a room filled with a lot of 16mm and super 8 documentaries. there was a week that they had an event showing movies and docs from across brazil and from other countries, little known movies, with most attendees being people from the cinema industry ,and enthuasiasts and stuff.

    the projectionists who took turns let me work alone for three days! they'd be there in the beginning only, and i'd close it. it was all illegal. my age for working there, and they letting me work alone having such a responsibility, but they were fed up of it all, little salary, little attention and care from the administration, etc, and they didn't want to miss the opportunity of take the time off and leave me there. well, i loved it! i was showing movies and docs to artists, directors, and other people of sort. one day there was a problem with the audio and i fixed it! it was awesome for a 15 year old!

    i had two 16mm projectors and would invite a couple friend of mine to come to my home and watch documentaries. i stopped using and owing this kind of stuff in my late 20's.

  30. Wow i remember those projectors. The yr was 1972 i was in 4th grade Mrs Miller class. (she was black) really nice teacher she put me in charge of running the projector in class. Yes bak then i considered myself a technician for projector's. Yes i was. 😀🙂 heh, heh. Thank u for explaining the workings of the long gone projector. I miss em. 😀🙂

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