How does a cinema projector work?

For a hundred years a cinema showed films from a strip of 35 mm film. A feature was spliced into one long roll, often 3 km or more, and laid flat on big turntables called platters. The film is pulled from the middle of the feed platter, runs up and over to the projector, and winds back onto the take-up platter.

Inside the projector the film cannot just flow past the lens, or the picture would smear. A Maltese cross (a Geneva drive) does the trick. A wheel spins steadily, and once each turn its pin drops into a slot and flicks the cross a quarter turn. A sprocket on the same shaft pulls the film down one frame. Then the cross is locked, and the frame stands still in the gate for three quarters of the time.

A rotary shutter blocks the light while the film moves. It also has a second blade that cuts the light once more in the middle of each frame. So 24 frames become 48 flashes a second, fast enough that your eye stops seeing flicker (see EyeClear for why). Behind it all glows a xenon lamp, a few kilowatts of arc in a mirror. Below the gate, a light and a sensor read the optical soundtrack printed beside the pictures (FilmSoundClear has the sound story; CineCameraClear shows the claw that filmed each frame).

Since about 2010 almost every cinema has swapped its film projector for a digital one. Most use DLP, a chip invented at Texas Instruments. The film arrives as files (the DCP chapter shows how), and the projector turns each frame into light.

The light starts at a xenon lamp or, in newer projectors, banks of lasers. A glass prism splits it into red, green and blue, and sends each colour to its own chip: a DMD, or digital micromirror device. A 4K chip holds 4096 × 2160 mirrors, almost 9 million, each about 7.5 thousandths of a millimetre across. Each mirror can only tilt about +12°, towards the lens, or −12°, into a black light trap.

So how does a mirror make grey? By timing. In each frame it flips on and off in a pattern, thousands of times a second. On for half the frame looks half as bright, because your eye averages the flashes. This is pulse-width modulation. The prism then adds the three colour pictures back together and the lens throws them onto the screen. TVClear shows how TVs make pixels instead; GradeClear covers the DCI-P3 colours.

The projector sits in a booth at the back, often 20 to 30 metres from the screen. That distance is the throw. The lens spreads the tiny picture from the chip (about 3 cm wide) into a picture metres wide. A short focal length spreads it more. The rule is simple: picture width = throw × chip width ÷ focal length. Lenses and refraction are explained in CameraClear and SnellClear.

The booth is usually higher than the screen. If you just tilt the projector down, light to the bottom of the picture travels further than light to the top, so the bottom comes out wider: a keystone. Cinemas avoid it with lens shift, sliding the lens up or down so the projector can stay level.

Brightness is measured in foot-lamberts (fL). The DCI standard asks for 14 fL (48 candela per square metre) in the middle of the screen. The same light spread over a bigger screen is dimmer, so big screens need more lumens. A silver screen bounces light back in a narrow beam: about 2.4× brighter in the middle, but dim from the side seats. Matte white looks the same from almost everywhere. The tiny holes in the screen for the speakers are in FilmSoundClear.

Your two eyes are about 6.5 cm apart, so each sees the world from a slightly different place. Your brain uses the difference to feel depth. A 3D film is filmed or rendered twice, once for each eye, and the cinema must get the left picture only to your left eye and the right one only to your right eye.

Most cinemas use one projector that alternates: left, right, left, right, three times per frame. That is 144 flashes a second, too fast to notice. The glasses sort them out in one of three ways. Polarised glasses (the idea behind RealD-style systems) have filters that pass only light twisted one way. They need a silver screen, because a matte white screen scrambles the twist. Active shutter glasses blink each eye shut in turn. Colour-filter glasses (Dolby 3D is one) pass slightly different shades of red, green and blue to each eye.

The catch is light. Each eye sees its picture only half the time, and every filter swallows more. Roughly a fifth or less of the 2D light reaches each eye, so many 3D shows looked dim. Newer laser projectors have enough light to fix this. How the eye and brain fuse the two views is in EyeClear.

A digital film reaches the cinema as a DCP, a Digital Cinema Package: a folder of files. Every frame of picture is a separate JPEG 2000 image, squeezed with wavelets that split the picture into a small rough version plus fine details. There are up to 250 megabits for every second of film. The sound travels as uncompressed audio beside it (FilmSoundClear follows it to the speakers). A two-hour feature comes to roughly 150 to 250 GB.

It arrives on a rugged hard drive, over a fast internet line, or by satellite. India was early here: from 2005, UFO Moviez and Qube Cinema began sending films to cinemas, many in small towns, by satellite instead of shipping heavy film cans.

The files are encrypted. To play them, the cinema needs a KDM (Key Delivery Message): a small file holding the keys, made for this one server, and valid only between two dates. Before release day the film sits locked on the server. At showtime the server checks the key, unlocks the pictures, decodes them and sends them to the projector, which adds an invisible watermark to trace any camera piracy.

To pull people away from home screens, cinemas went bigger. IMAX began in 1970 with film 70 mm wide running sideways, each frame 15 perforations long: about ten times the area of a 35 mm frame. A 2.5-hour film on its platter weighs about 250 kg. Since 2014, the biggest IMAX halls use pairs of laser projectors instead. Premium large formats (Dolby Cinema, IMAX and chains’ own brands) add wall-to-wall screens, laser light and stronger sound.

Some directors tried high frame rates. The Hobbit (2012) played at 48 fps; Gemini Man (2019) was shot at 120 fps. More frames mean smoother motion and less blur, but some viewers found it looked like video. Avatar: The Way of Water (2022) switched to 48 fps only in some scenes.

The newest screens need no projector at all. An LED cinema screen, such as Samsung’s Onyx (2017), is a wall of tiny lights like a giant TV (see TVClear), with true black. In India, where roughly 12,000 to 13,000 single-screen halls once stood (a rough 1980s figure), about 10,000 screens were counted in 2025. Many old halls closed as multiplexes grew.