Wired Explains: How 3-D Television Works

http://www.wired.com/gadgetlab/2009/10/3d-tv-explainer/

By Priya Ganapati

October 6, 2009  |  6:05 pm  | 

“Larger than life” takes on a whole new meaning when you watch Beowulf swing from a dragon and hack branches that seem to just pop out of the TV.

TV manufacturers want to bring that experience to your living room with 3-D displays that work much like the ones in the theaters. Major consumer-electronics companies, including Panasonic, Mitsubishi and Sony, are betting on 3-D, with compatible TV sets planned for the market in 2010.

To understand why, here’s a short primer on how our vision works.  Our eyes are about 3 inches apart, which means each eye sees a slightly different perspective of the same scene. The brain takes images from both eyes, fuses them together and uses the difference between the images to calculate distance, creating a sense of depth.

Getting the 3-D effect at home involves tricking the brain into doing something similar with the images that it gets from a TV set. But that’s not a trivial problem: TV makers have to figure out a way to precisely show a set of slightly different images to each of your eyes.

So how do they do that? Here are the key technologies that are making their way into 3-D TVs.

Color Filter Glasses

Remember the old red-and-blue glasses at movie theaters that came to define 3-D in the 1950s? The tinting acts as color filters, so the image specific to that particular eye is seen by it. With both eyes seeing slightly different perspectives of the same image, a 3-D effect is created.

But the red and blue glasses can make you sick — literally. And the color filtering limits the colors that can be used to create content, so 3-D content using this technique is not very vivid. That’s why most movie theaters don’t use the tech any more, and neither do TV manufacturers.

Pros: Inexpensive, quick and easy way to watch and create 3-D movies or shows.

Cons: Since the image input to the eye is not controlled, it can cause headaches, nausea and just that icky feeling that kills the thrill of 3-D. It’s not worth the trouble, because the 3-D picture is not much to look at either.

Shutter Glasses

In this method, the left and right images are alternated rapidly on the HDTV. A single sequential imager switches very rapidly between the left and right images when projecting the information on a display.

For the eye to view the right set of images, viewers have to wear a pair of battery-powered glasses with shutters that can open and close rapidly. Each shutter is synchronized to transmit the wanted image and block out the unwanted one.

The two sides open and close in alternation while the screen displays left- and right-eye images in sync with the glasses. The shutter glasses are in sync with the screen’s refresh rate of 120 Hz. The result is that the left eye sees only the intended left view of the image, and the right eye sees the intended right view, and it happens so fast that your brain blends it together into a single, stereoscopic image.

The technique is called active-shutter technology. Active-shutter glasses contain liquid crystal, a receiver-transmitter combination that uses infrared, Bluetooth or radio technology. The HDTV sends a signal to the glasses to synchronize them to the images on the screen. Meanwhile, alternating electrical signals activate the LCD screen in the lens, blocking or transmitting the view.

The active-shutter glass idea has become popular among big TV makers such as Panasonic and Sony. Both  have committed to having at least one model of 3-D ready TVs in stores by the end of 2010.

Pros: Glasses are relatively inexpensive, no ghosting effect or delayed images that results when tinted glasses are used. Reduced viewer fatigue. Most likely to be available next year.

Cons: It’s like watching television with your sunglasses on. There can be up to a 50 percent loss of picture brightness. Add to that the timing lag and 3-D can get a bit rocky. In case of fast-moving sequences like a NASCAR race, the flicker can be noticeable.

Polarized Glasses

An alternative to active shutter glasses are polarized glasses that have lenses similar to those on sunglasses. The lenses have polarization that is adjusted to be orthogonal — set perpendicularly at 90-degree angles — to one another.

The 3-D material is projected by two projectors, which each have polarizing lenses in front of them. The surface on which the images are projected is coated with special chemicals so it does not affect the polarization.

Because each filter passes only the light that is similarly polarized and blocks the orthogonally polarized light, each eye sees only the image intended for it. The brain then puts these images together to create a three-dimensional effect.

Though the big boys of consumer electronics are not yet betting on this, the method offers the best viewing experience we have seen so far. Take the technology from HDI, a startup that can take two separate full-resolution imagers and integrate it as one projector.  For viewers, this means no reduction in image quality, and brightness that is almost as good as what you can get from a traditional LCD TV. Increasingly, movie theaters are offering polarized glasses for 3-D movies.

Meanwhile, LG has said it is working on having a 3-D ready TV next year based with polarized glasses.

Pros: Light weight; pictures with amazing level of detail and color.

Cons: The big TV makers have to still buy into the technology.

No Glasses

If putting on a pair of glasses all day to watch TV sounds annoying, there’s a way to do it without glasses called autostereoscopy. There are two ways to get this right: lenticular lenses or the parallax barrier.

Take LG’s 3D TV intriguingly codenamed M4200D. The idea uses cylindrical plastic lenses known as lenticules. The lenticules are placed on a transparent sheet which is fixed on the LCD screen.

The lenticules must be perfectly aligned with the image underneath. Each lenticule then acts as a magnifying glass to enlarge and display the portion of the image below it.

The viewer’s eye directly perpendicular to the screen sees the portion of the LCD that is directly under each lens. The other eye, observing the screen from a slightly different angle, sees a portion of the LCD that is off-center under each lens. The brain then combines the two views to create the perception of depth.

The idea comes with some real fine print. It requires an optimum viewing distance of 13 feet (or 4 meters), and there’s no messing around with that. Sit outside that zone and you are likely to see a set of muddled images.

The parallax barrier works on a similar principle.  It has a layer of material with some precise slits placed in front of a regular LCD screen. These allow each eye to see a different set of pixels creating the 3-D effect.

For instance, Sharp, which has shown 3-D TVs that don’t require glasses, has developed electrically switchable liquid crystals that are aligned with the columns of pixels in the display. When switched on, the parallax barrier controls the direction at which the light leaves the display and the way it hits your eyes. Even better, the parallax barrier can be switched off for 2-D content.

Both LG and Sharp TVs are still in the prototype stage. Sharp’s 3-D TV technology is unlikely to hit the production line anytime soon. Philips, which has also shown a concept 3-D TV in the past, has said it is not working on bringing the TVs to market.

Pros: No glasses required. It’s like watching TV the old fashioned way.

Cons: You need to sit in one of the “sweet spots” to get the image right. Also sitting in the lotus position may be preferred. Okay, we made the last part up — but this technology definitely requires you to sit in exactly the right spot. No lying down on the floor!