How LCD Projectors Work


A typical 3LCD projector
A typical 3LCD projector
Image courtesy Amazon.com

Whether suffering through a business pitch or a classroom presentation or enjoying an evening of home theater blockbuster thrills, there's a good chance an LCD (liquid crystal display) projector is responsible for the light show.

The technology behind the LCD projector is nearly three decades old in 2012, but it remains one of the top digital projection technologies, alongside Digital Light Processing (DLP) projectors. Inventor Gene Dolgoff developed the first LCD projector in 1984, and both Epson and Sony continue to employ the technology in 21st century projectors.

The old method of film projection was simple: Each frame of the film was a tiny, translucent photograph. Shine light through the film and then have that light pass through an imaging lens and you'd display a larger version of that tiny image onto a wall or screen.

LCD projectors work a little differently. A beam of high-intensity light travels through thousands of shifting pixels in an LCD display instead of through a frame of translucent film. And these projectors don't just use a single LCD display either -- they use three, which is why they're also called 3LCD projectors. The light splits into three hues, then travels through three LCDs before recombining in a prism to generate the crisp, colorful image projected on the screen.

Still sound like magic? Well, let's walk through the process at a slower pace, beginning with the lamp and ending on the big screen.

So grab your popcorn. Settle back in your seat. Turn off your phone and prepare to break some light.

3LCD: Breaking the Light Fantastic

Follow the light.
Follow the light.
©2012 HowStuffWorks

To understand how an LCD projector works, it's best to start at the beginning -- with a beam of light -- and end on the movie screen itself.

Step one: A powerful light source emits a beam of intense, white light.

Step two: Our beam of white light bounces off a group of mirrors that includes two dichroic mirrors, which are coated in a special film that reflects only a specified wavelength of light. You know how a prism (or a droplet of water) breaks a beam of light into distinct wavelengths (or a rainbow of colors)? The same principle applies here, only each dichroic mirror breaks off a single specified wavelength. So the white light hits the mirrors, and each reflects a beam of colored light on through the projector: one red, one green and one blue.

Step three: The beams of red, green and blue light each pass through a liquid crystal display composed of thousands of tiny pixels. You can read How Liquid Crystal Displays Work for a more detailed explanation of LCD technology, but it comes down to tiny, colorless pixels that either block light or allow it to pass through when triggered by an electric current. All three of the LCD screens in the projector display the same image or moving images, only in gray scale. When the colored light passes through these three screens, they relay three versions of the same scene: one tinted red, one tinted green and one tinted blue.

Step four: But of course the final image we see isn't red, green or blue; it's full color. So inside the LCD projector, the three tinted versions of this scene recombine in a dichroic prism (a finely crafted combination of four triangular prisms) to form a single image composed of not three colors but millions of colors.

Step five: The light of this vibrant, colorful version of the scene then passes through a projector lens and onto the big screen.

That's really all there is to it. But what are the pros and cons of this technology?

The Ups and Downs of LCD Projectors

Watch it on the big screen.
Watch it on the big screen.
Sami Sarkis/Photographer's Choice/Getty Images

So what does an LCD projector do that its other projector peers can't? For starters, you don't have to worry about the film wearing out. LCD projectors depend on either outside data from a computer (connected by USB) or data from an inserted DVD. The digital images sent to the three LCD displays (and then recombined into a single picture) are as clear the first time you show the footage as the hundredth time you show it. The gray scale used in the three LCD displays allows for intense picture detail, and all for 25 percent less electricity than other digital projector technologies [source: 3LCD].

Unlike Digital Light Processing (DLP) projectors -– the other popular digital projection technology we mentioned -- LCD projectors don't experience the "rainbow effect," color flashes caused by the spinning color wheel inside DLP projectors. LCD projectors also benefit from higher contrast, allowing for sharper images.

DLP projectors, however, still dominate the high-end, professional projection scene. Catch a show at a major cinema and you're looking at light from a DLP projector worth more than $35,000. LCD projectors, on the other hand, will cost you anywhere between $200 and $9,000.

Like all gadgets, LCD projectors require a certain degree of care and upkeep. Although you don't have to worry about film quality degrading with use, individual pixels do burn out and reduce image quality. Dust particles can also collect on the LCD screens, smudging the image.

So the next time you take in a projected film at home or work, look and see what kind of projector is in use. If it's an LCD projector, just imagine those three tiny gray scale screens inside there and the recombined beams of light that bring it all to colorful life.

Author's Note

Robert Lamb, Senior Staff Writer
Robert Lamb, Senior Staff Writer
HowStuffWorks 2009

I'm generally more of a fan of the film on the screen than the technological route it takes to get there, but I have to admit that the optical wizardry inside these projectors really impressed me. The idea of dichroically reflected light and recombined triple images feels very magical -- like something the Skeksis might use to drain a Gelfling's mojo.

I've toyed around with one of these to project images during parties and art shows, and the image is indeed quite crisp and vibrant. Plus, as noted in the article, the average price tag is far easier on the wallet than the kind of dough you'd have to drop for a professional-grade DLP projector.

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Sources

  • 3LCD.com. 2012. (May 14, 2012) http://www.3lcd.com/
  • "About us." 3-DVision.com. (May 18, 2012) http://www.3-dvision.com/about-us.htm
  • "How LCD Projectors Work." ProjectorPeople.com. September 2007. (May 14, 2012) http://www.projectorpeople.com/lcd-projectors/
  • LCD Projectors. Bamboo AV. 2011. (May 14, 2012) http://www.bambooav.com/lcd-projectors.html
  • Powell, Evan. "The Technology War: LCD vs. DLP." ProjectorCentral.com. July 28, 2009. (May 18, 2012) http://www.projectorcentral.com/lcd_dlp_comparison.htm?page=LCD-Advantages-and-Limitations