How do TV projectors work? (2024)

by Chris Woodford. Last updated: May 19, 2022.

Everyone knows real life is nothing like television—possiblybecause TV screens are so much smaller than the things we see around us. Youcouldn't show life-sized people, cars, sharks, trees, and skyscraperson a glass-fronted box 30cm (12 inches) higheven if you wanted to. But if you'd like your entertainment to feel more realistic, one option isto swap your TV set for a projector that throws giant images of TVpictures onto the wall. Watching TV then becomes more likewatching a movie—in the comfort and privacy of your own home.Projection TV is also very useful in business meetings and collegelectures where a whole room full of people need to watch a picture atthe same time. You can use it to show live TV pictures, video and DVDrecordings, or even the output from a computerscreen. Let's take a closer look at the different kinds of TV projector and how they work.

Photo: A typical LCD TV projector with its cover removed. Photo by NASA Langley Research Center (NASA-LaRC) courtesy of Internet Archive.

What is projection?

There's nothing new about projecting images onto a screen. Back inancient times, Greek philosopher Plato (429–347 BCE) described afamous idea called the "allegory of the cave" in which he likenedour everyday experiences to those of a group of cave-bound prisonerswatching distorted shadows of puppets flickering on a wall. Thanks to Plato, we cansay fairly confidently that people have understood the basic idea ofprojecting simple images onto screens for thousands of years.

Shadow play like Plato described is something all children enjoyand, simple though it is, it's the basis for all forms of projectiontechnology, no matter how sophisticated. Think for a moment how itworks. You have a light source, you put animage in front of it, and a shadowy image of the object is thrown onto the wall in front ofyou. If you move the object around, you create an animated image.

Front projection and back projection

There are two basic kinds of projection. Ifthe light is behind you and the screen is in front of you, you makean image through front projection. You can also make projected imagesa different way. You might have walked down the street at night andseen shadows of people dancing around on their blinds as they walkaround inside brightly hit homes. In this case, the light source andthe object being projected are behind the screen (the blinds) andyou're looking from the opposite direction in what's known as backprojection.

Artwork: In front projection (left), the image is projected in front of you.You see light reflected off the screen into your eyes. In back projection (right), the image isprojected through the screen from behind. The light you see is traveling directly through the screen.

"Cine" movie projectors, which were developed in 1895 by two Frenchbrothers named Auguste and Louis Lumière (1862–1952 and1864–1948), work by front projection. The projector is positionedbehind the audience and throws an image over their heads onto ascreen in front of them. Televisions, which became popular a fewdecades later, work by back projection. You sit in front of the boxand watch a pattern of light that's being created by a verysophisticated electronic mechanismbehind the screen.

Photo: The Lumière brothers pioneered the movie projector andopened the world's first cinema in the 1890s.

What is projection TV?

Projection TVs are a cross between the two technologies:they use television technology to build up a picture and projector technology to throw that picture onto the screen. You've probably noticed howtelevisions have evolved and developed in recent years: huge,old-style cathode-ray tube (CRT) TV setshave gradually given way toflatter, squarer LCD (liquid-crystal display)and plasma TVs thatwork an entirely different way. Projection TVs have evolved in muchthe same way.

Artwork: Projection meets TV—a TV projector throws a large TV picture onto your wall instead of squeezing a small one into a screen.

CRT projectors

The first TV projectors were a bit like super-powerful CRTtelevisions. Although basic CRT TV projectors were available in the1950s, they became really popular in the 1980s thanks to manufacturers such as Barco. Instead of shining three colored electron guns onto a phosphor screen from behind (thatis, by using back projection), they use three hugely powerful lightguns to shine separate red, blue, and green images onto a screen(through front projection). The images fuse together into a single,large colored image. The trouble with projectors like this is thatthey are huge and heavy (so they're not easily portable), they canuse lots of electricity (to power the three light guns), and the CRTtubes inside them get very hot. But although they can be fiddly toset up initially and adjust, they're neither unreliable nor obsolete, as many people suppose: they give excellent picture quality (as good as or better than newertechnologies) and they're still compatible with new developmentslike HDTV and Blu-ray DVD players.

Photo: An old-fashioned Barco 801 CRT projector from the early 1990s, with its distinctive blue, green, and red lenses shining out from the front.

LCD projectors

Just as CRT televisions have now largely being replaced by LCD sets, so CRTprojectors have gradually gone the same way—and for exactly the samereason: LCD screens are smaller, lighter, cheaper, more reliable, anduse much less power than CRTs. In an LCD TV projector, a very bright lightshines through a small LCD screen into a lens, which throws ahugely magnified image of the screen onto the wall.

Photo: An ASK Impression 960 LCD TV projector weighingin at about 12.5kg. This one uses a powerful 575-watt metal halide lamp to throw the image of an internal, 25cm (10-inch) LCD screen onto a screen up to 4 meters (13ft) away. Photo by NASA Langley Research Center (NASA-LaRC) courtesy of Internet Archive.

The technology is sometimes called LCLV (liquid crystal light valve).While CRT projectors were popular with businesses and colleges, lower-cost LCDprojectors are small, cheap, and portable enough for home use.That doesn't necessarily mean they're superior, however. The imagequality is often poorer than that produced by CRT projectors and thebright lamps used inside LCD projectors to throw the image still havea limited life.

Photo: Inside an ASK Impression 960 LCD TV projector, modified by NASA. Photo by courtesy of NASA Langley Research Center (NASA-LaRC) with annotations by Explainthatstuffcourtesy of Internet Archive.

DLP® projectors

Even LCD projectors are looking old-hat now. The latest TV projection technology, DLP® (digital light processing), uses anentirely different method of making images using microscopic mirrors.

Photo: A Christie Mirage 5000: a typical modern DLP TV projector. Photo by courtesy of Dave Pape, published on Flickrunder a Creative Commons Licence.

Have you ever used a mirror to send a light signal to a friendsome distance away? The basic idea is simple: you angle the mirror soit catches light, then tilt it slightly so the light travels whereyou want it to go. By tilting the mirror back and forth, you can sendprecise light pulses of either long or short duration—and transmitcomplex messages using something like Morse code. The latest projection TV system, called DLP® (digital lightprocessing) technology, works in almost exactly the same way.

What is DLP® technology?

Developed in the mid-1980s by Texas Instruments scientist Dr Larry J. Hornbeck, DLP technology is based on an amazingly clever microchip called adigital micromirror device (DMD). A DMD chip contains about two million tinymirrorsarranged in a square grid. Each mirror is less than one fifth thediameter of a human hair, and it's mounted on a microscopic hinge soit can tilt either one way or another. A bright lamp shines onto theDMD mirror chip and an electronic circuit makes the mirrors tilt backand forth. If a mirror tilts toward the lamp, it catches the lightand reflects it off toward the screen, creating a single bright dotof light (equivalent to a pixel of light made by a normal TV); if amirror tilts away from the light source, it can't catch any light, so itmakes a dark pixel on the screen instead. Each mirror is separatelycontrolled by an electronic switch so, working together, the twomillion mirrors can build up a high-resolution image from twomillion light or dark dots.

Artwork: DLP® chips make pixels with tiny tilting mirrors.In the original design by Larry Hornbeck, shown here, each pixel is a tiny cloverleaf-shaped plate of aluminum copper alloy (red) that can be electrically attracted by a second plate directly underneath (blue), so it tilts one way or the other on a central hinge (green).Artwork from US Patent 4,710,732: Spatial light modulator and method by Larry Hornbeck, Texas Instruments, 1987, courtesy of US Patent and Trademark Office.

To make color images, DLP projectors needan extra bit of technology: they have a spinning colored wheelinserted into the light path, which can color the pixels red, blue,or green. Combined with the tilting mirrors, the color wheel makes afront-projected TV picture from millions of pixels of every possiblecolor. This is explained more fully in the box below.

Laser projectors

The latest TV projectors use lasers to produce bright, high-definition (4K), full color pictures at short range but gigantic size—2.5–5m (8–16ft) from a distance of about 1m (~3ft) or so is achievable. Some use a single laser to reproduce all colors; others are more like a modern-spin on CRT (Barco-style) projectors, with three tuned color lasers producing the red, blue, and green parts of each image instead of three powerful light guns. Different types of projectors use either diode lasers (similar to the ones in laser printers and DVD players)or large gas lasers (like the ones used in industrial machines). DMD mirrors are typically used to "scan" the laser light around to make a picture.

Find out more

On this website

• History of communication
• Light
• Liquid-crystal displays (LCD televisions)
• Television (general principles and cathode-ray tube TVs)
• 3D television

On other websites

• Texas Instruments: DLP: The official DLP website has lots more details.

Articles

• How a Projector Can Substitute for a Television Set: by Damon Darlin, The New York Times, 17 May 2017. A useful set of FAQs about projection TV.
• Stuff a Big Screen in Your Pocket With This Tiny Projector by Tim Moynihan. Wired, July 24, 2015. Sony announces a tiny, laser-based pico projector.
• Moving beyond the whiteboard: BBC News, 21 January 2010. How DLP projectors are helping school teachers to present their lessons in imaginative new ways.
• Rear-Projection TV? Who, Us? by Eric A. Taub. The New York Times, April 9, 2009. Why are some manufacturers playing down their projection TVs?
• Video projectors embrace HD by Marc Cieslak, BBC News, 13 April 2007. Compares home DLP and LCD projectors.
• Films without film arrive in Europe: by Marc Smith, BBC News, 3 February 2000. Explores the arrival of DLP digital projection at movie theaters (cinemas) in Europe.
• Wall-size TV from Tiny LCDs by Mark Fleischmann, Popular Science, June 1991, page 94. A simple introduction to how liquid-crystal light valve (LCLV) projectors work.

Books

• Polarization Engineering for LCD Projection by Michael D. Robinson, Gary Sharp, Jianmin Chen. Wiley, 2005. A detailed introduction to LCD projection, with some coverage of DLP.

Videos

• Barco Graphics 801 CRT projector preview: A quick video tour of an old-style Barco CRT projector.
• LCD projectors: YouTube's Georgef551 walks us through the optical system of an LCD projector.

Patents

Try here for greater technical detail:

LCD

• US Patent 4,906,071: Liquid crystal display device and video projector incorporating same with particular driving circuit connection scheme by Hiroshi Takahara et al, Matsush*ta Electric Industrial Co., March 6, 1990.
• US Patent 5,092,671: Device for projection television pictures by Petrus J. M. Van Os, Philips Corporation, March 3, 1992.
• EP0880287 A1: LCD-projector by Bart Van Den Bossche et al, Barco NV, November 25, 1998.
• US Patent 7,354,161,B2/: Projector by Akitoshi Kuroda, Seiko Epson, April 8, 2008. Another general description of a modern LCD projector

DLP

• US Patent 4,710,732: Spatial light modulator and method by Larry J. Hornbeck, Texas Instruments, December 1, 1987. One of TI's original micro mirror patents, later used in DLP technology.
• US Patent 5,706,061: Spatial light image display system with synchronized and modulated light source by Stephen W. Marshall and Austin L. Huang.Texas Instruments Incorporated, January 6, 1998.
• US Patent 6,856,446: Digital micromirror device having mirror-attached spring tips by Anthony DiCarlo, Texas Instruments, February 15, 2005. More details of the Digital Micromirror Device™ used in DLP projectors.

Other

• US Patent 7,357,518,B2: Projector by Kazuyuki Iinuma, Seiko Epson, April 15, 2008. A more general description of a modern projector that doesn't specify the type of light source. There's quite a lot of detail about the optical system and user controls.