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Taking An In-Depth Look at 3-D Glasses

As 3-D content becomes more popular and movie studios push to makemore3-D movies, people are starting to wonder what it will take toupgradetheir home entertainment center to 3-D. One thing people oftendon't consider is the glasses they have to wear. There is actually morethan one kind of 3-D glasses, and the difference in price andperformance isenormous.

First, the basics. All 3-D glasses have the same purpose: to bringdifferent images to each eye. 3-D content has two sets of images, onefor each eye, that are slightly offset. When the separate images hiteach eye, the brain can perceive them together as a three-dimensionalimage, just as the slightly offset views from each eye allows us toperceive our world in 3-D.

Ensuring only one image hits each eye means filtering out the imagemeant for the other eye. There are three main ways of doing this.

[See also "Graphic:How 3-D Glasses Work."]

Anaglyph glasses: Classic red and blue

The most common image of 3-D glasses is the iconic white cardboardframe with one blue lens and one red lens. These so-called anaglyphglasses have been widely used for well over half a century and havebecome the symbol of 3-D.

Anaglyph glasses are a "passive" form of 3-D, meaning they justfilter out certain things naturally. One image is projected on a screen with a blue tint, and the other is projected in red. The blue lens onlylets the red image through, and the red lens only lets the blue image through, so each eye sees a slightly different image. It doesn't have tobe red and blue; there are many color combinations that work, butred/blue and red/green are most common.

There's a problem though.

"Although [anaglyph] can create a good depth sensation, it veryseriously compromises the quality of the perceived color," said Dr. Jim Sheedy of the Pacific University College of Optometry.

Those color problems are one of the main reasons that anaglyph 3-Dcontent has been declining in recent years. The main benefit of anaglyphglasses over other technologies is their price. Anaglyph glasses are socheap that they are usually just given away.

Polarized glasses: Not just a pair of sunglasses

The next type of passive 3-D glasses is polarized, and workssimilarly to anaglyph. Instead of using colored lenses, the lenses arepolarized, meaning they only let certain wavelengths of light through.This gives them a tinted look that can make them easy to mistake forregular sunglasses at first glance.

When light is polarized, the light waves all oscillate in onedirection. By displaying one image that is polarized "up and down,"another image that is polarized "left and right" and having each lensonly let one of the two types of light through, these glasses canachieve a 3-D effect.

"This can be very effective and does not have the color problems associated with anaglyph," Sheedy said.

For this reason, polarized 3-D glasses are the technology of choicefor most 3-D content in movie theaters. In addition, the glassesthemselves are relatively cheap to make, too. While it's common to have simple plastic frames, theaters can even mount the polarized lenses in cardboard frames just like anaglyph glasses. Nicer polarized glasses canrun anywhere from just a few dollars to $20.

Shutter glasses: The next generation

The most recent type of 3-D glasses is also controversial for beingthe most expensive. Really expensive, in fact. Most TV manufacturers areselling shutter glasses for 3-D TVs at over $100 each.

Shutter glasses take a more brute force tactic to create 3-D content.Instead of having two slightly different images displayedsimultaneously and filtering one to each eye, like passive glasses do,shutter glasses make each lens go opaque and then transparent again insync with the images displayed on the screen. When the screen displaysthe image for the left eye, the right lens will go dark so that only theleft eye can see it. Then, when the image for the right eye isdisplayed, the left lens will darken and the right lens will becomeclear again.

These images flip back and forth faster than the human eye canperceive. It just seems as though you are watching a film throughregular glasses, even though in a sense you are watching the movie oneeye at a time. This is why shutter glasses are called an "active" 3-Dtechnology.

There are several ways to make shutter glasses; the most commonformat right now is to use LCD technology that can make each lens godark and clear very quickly. Unfortunately, it takes power to make this transition, which leads to another drawback of shutter glasses: Theyrequire batteries.

In order to sync the shutter for each eye with the proper image onthe screen, shutter glasses usually employ a special infrared emitterplaced on top of the TV and pointed at the viewers. The infrared signal tells the glasses when to darken each lens, keeping the glasses in syncwith the show so that viewers get the proper 3-D effect. If the infraredemitter is obscured, the glasses won't work properly.

So why are there so many different types of 3-D glasses? Why useanaglyph when polarized has less color distortion? And why have shutter glasses when passive glasses are so much cheaper. It turns out that it all depends on what medium the 3-D content is in.

3-D glasses for TVs

The main reason shutter glasses have been developed is for 3-D TVs.The other two main options, polarized and anaglyph, have majordrawbacks.

"With current TV systems, it is not possible to show polarized images- they cannot be projected onto a screen as in movie theaters," Sheedypoints out.

That eliminates the polarized option. And while anaglyph technologyworks and has been used on TV for decades, the color distortion problem makes it a less desirable alternative. But there is a bigger reason to go with shutter glasses.

"[Shutter glasses] allow Sony to provide the very best possible 3-Dpicture quality - full HD 1080p.Passive or polarized glasses onlyallow you to receive 720p or half-definition images," said Greg Belloni,spokesman for Sony Electronics.

With high definition being the standard format for TV these days, shutter glasses were the obvious choice.

The main problem is that shutter glasses from one brand usually don'twork with a different brand of TV, so they can't be used just anywhere.And even with the infrared emitters, the glasses can sometimes fall outof sync by milliseconds, which results in an effect called "flicker."The glasses cause the image to appear to flicker, which ruins theviewing experience and possibly even the 3-D effect.

And then there's no getting around the price. Belloni said that all3-D glasses for Sony's BRAVIA line of 3-D TVs will be priced at $150each. Each TV comes with 2 pair of glasses, but if more than two people want to watch at once, you'll have to crack open the wallet for more.

3-D glasses for movie theaters

All three technologies are usable in movie theaters because the imageis projected onto a large screen instead of displayed on a glowing TV screen. However, it should come as no surprise that most theaters aregoing with polarized glasses. They avoid the color problems of anaglyph glasses and the high price of shutter glasses.

3-D glasses for home theaters

The home theater is still a gray area for 3-D glasses. If you use aTV, then the answer is the same as the one explained above. But someowners use a projector or DLP TV in their home theaters. This makes itpossible to use the cheaper polarized glasses and forgo the shutterglasses.

It's not so simple though. Polarized 3-D content requires twoseparate projectors or a projector that can display two imagessimultaneously. In the end, that might cost more than buying a few extrapairs of shutter glasses.

The other problem is getting content. Since most 3-Dmovies will be encoded for TVs, which use shutter glasses, there may not be asmuch content available for purchase in polarized formats.It remains tobe seen if home projectors will comprise a large enough portion of themarket to make a difference or if TVs will just be the standard displayformat.

3-D glasses for gaming

While most 3-D gaming monitors are quite expensive and requireshutter glasses, some manufacturers such as NVIDIA and iZ3D areincluding an option in their hardware to encode 3-D-capable games inanaglyph. This provides a much cheaper option for gamers, but it onlyapplies to PC gaming. Console gaming - on the Xbox or Playstation 3, forexample - which usually relies on a TV for a display, will likelyalways rely on shutter glasses.

Autostereoscopic: No glasses required

There's one last option, although it's not readily available rightnow. Some manufacturers are looking at autostereoscopic displays, which use a special kind of lens over the screen that allows viewers to see 3-Dimages without glasses.

The first major application of autostereoscopic screens will be hitting the market in the Nintendo3DS portable gaming console later this year.

While it's certainly attractive to think of a 3-D display thatdoesn't require you to rummage through the couch cushions for a pair of shutter glasses, it's not a perfect solution. Autostereoscopic screens are hard to scale to larger sizes without getting flicker and "ghostimage" problems that ruin the image quality.

"They are usually a few times more expensive, they do not offer full resolution in 3-D and are limited to a number of viewing positions,"Anton Belev of the 3D Vision Blog also points out.

These drawbacks mean that autostereoscopic TVs are still at least a few years away.

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