As 3-D content becomes more popular and movie studios push to make more 3-D movies, people are starting to wonder what it will take to upgrade their home entertainment center to 3-D. One thing people often don't consider is the glasses they have to wear. There is actually more than one kind of 3-D glasses, and the difference in price and performance is enormous.
First, the basics. All 3-D glasses have the same purpose: to bring different images to each eye. 3-D content has two sets of images, one for each eye, that are slightly offset. When the separate images hit each eye, the brain can perceive them together as a three-dimensional image, just as the slightly offset views from each eye allows us to perceive our world in 3-D.
Ensuring only one image hits each eye means filtering out the image meant 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 cardboard frame with one blue lens and one red lens. These so-called anaglyph glasses have been widely used for well over half a century and have become the symbol of 3-D.
Anaglyph glasses are a "passive" form of 3-D, meaning they just filter 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 only lets 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 to be red and blue; there are many color combinations that work, but red/blue and red/green are most common.
There's a problem though.
"Although [anaglyph] can create a good depth sensation, it very seriously 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-D content has been declining in recent years. The main benefit of anaglyph glasses over other technologies is their price. Anaglyph glasses are so cheap 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 works similarly to anaglyph. Instead of using colored lenses, the lenses are polarized, meaning they only let certain wavelengths of light through. This gives them a tinted look that can make them easy to mistake for regular sunglasses at first glance.
When light is polarized, the light waves all oscillate in one direction. By displaying one image that is polarized "up and down," another image that is polarized "left and right" and having each lens only let one of the two types of light through, these glasses can achieve 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 choice for most 3-D content in movie theaters. In addition, the glasses themselves 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 can run anywhere from just a few dollars to $20.
Shutter glasses: The next generation
The most recent type of 3-D glasses, called shutter glasses, is also controversial for being the most expensive. Really expensive, in fact. Most TV manufacturers are selling shutter glasses for 3-D TVs at over $100 each.
Shutter glasses take more of a brute force-approach to create 3-D content. Instead of having two slightly different images displayed simultaneously and filtering one to each eye, like passive glasses do, shutter glasses make each lens go opaque and then transparent again in sync with the images displayed on the screen. When the screen displays the image for the left eye, the right lens will go dark so that only the left eye can see it. Then, when the image for the right eye is displayed, the left lens will darken and the right lens will become clear again.
These images flip back and forth faster than the human eye can perceive. It just seems as though you are watching a film through regular glasses, even though in a sense you are watching the movie one eye at a time. This is why shutter glasses are called an "active" 3-D technology.
There are several ways to make shutter glasses; the most common format right now is to use LCD technology that can make each lens go dark and clear very quickly. Unfortunately, it takes power to make this transition, which leads to another drawback of shutter glasses: They require batteries.
In order to sync the shutter for each eye with the proper image on the screen, shutter glasses usually employ a special infrared emitter placed 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 sync with the show so that viewers get the proper 3-D effect. If the infrared emitter is obscured, the glasses won't work properly.
So why are there so many different types of 3-D glasses? Why use anaglyph 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 major drawbacks.
"With current TV systems, it is not possible to show polarized images – they cannot be projected onto a screen as in movie theaters," Sheedy pointed out.
That eliminates the polarized option. And while anaglyph technology works 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-D picture quality – full HD 1080p. Passive or polarized glasses only allow 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't work 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 out of sync by milliseconds, which results in an effect called "flicker." The glasses cause the image to appear to flicker, which ruins the viewing experience and possibly even the 3-D effect.
And then there's no getting around the price. Belloni said that all 3-D glasses for Sony's BRAVIA line of 3-D TVs will be priced at $150 each. Each TV comes with two 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 image is projected onto a large screen instead of displayed on a glowing TV screen. However, it should come as no surprise that most theaters are going 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 a TV, then the answer is the same as the one explained above. But some owners use a projector or DLP TV in their home theaters. This makes it possible to use the cheaper polarized glasses and forgo the shutter glasses.
It's not so simple though. Polarized 3-D content requires two separate projectors or a projector that can display two images simultaneously. In the end, that might cost more than buying a few extra pairs of shutter glasses.
The other problem is getting content. Since most 3-D movies will be encoded for TVs, which use shutter glasses, there may not be as much content available for purchase in polarized formats. It remains to be seen if home projectors will comprise a large enough portion of the market to make a difference or if TVs will just be the standard display format.
3-D glasses for gaming
While most 3-D gaming monitors are quite expensive and require shutter glasses, some manufacturers such as NVIDIA and iZ3D are including an option in their hardware to encode 3-D-capable games in anaglyph. This provides a much cheaper option for gamers, but it only applies to PC gaming. Console gaming – on the Xbox or Playstation 3, for example – which usually relies on a TV for a display, will likely always rely on shutter glasses.
Autostereoscopic: No glasses required
There's one last option, although it's not readily available right now. Some manufacturers are looking at autostereoscopic displays, which use a special kind of lens over the screen that allows viewers to see 3-D images without glasses.
The first major application of autostereoscopic screens will be hitting the market in the Nintendo 3DS portable gaming console later this year.
While it's certainly attractive to think of a 3-D display that doesn'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 "ghost image" 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 pointed out.
These drawbacks mean that autostereoscopic TVs are still at least a few years away.
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