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I just learned what 3-D TV is. What’s the deal with passive 3-D TV?

I just learned what 3-D TV is. What’s the deal with passive 3-D TV?


For those who don’t know, the most commonly used technique to create 3-D images is a process called stereoscopy. It involves displaying one image for the left eye and another for the right eye. We see depth when those images merge. In high-definition, TVs must refresh the picture at least 120 times a second with alternating frames for the left and right eye, which tricks your brain into seeing only one image. Most new TVs are fast enough to do this, but to be 3-D-capable, TVs must include a converter chip and software to break down the signal and separate the left and right images.

So how does the correct image reach the eye it is intended for? That depends on what type of 3-D TV you have -- active or passive.

Televisions that use active technologies come with active-shutter glasses. They receive an infrared or radio beam from the set they are assigned to, which tells them when the image is changing (left or right), so that the shutters open and close in sync with the image displayed. Thus, the left eye sees the image for the left eye and the right eye sees the image for the right eye. These goggle-like devices are quite bulky, and some say they ruin the viewing experience.

In a passive setup, glasses are also required, but they are much different, much more like the old red and green glasses handed out in movie theaters for watching 3-D films. And instead of displaying the left and right images alternately, the TV displays the right and left images at the same time. The glasses determine which image the eye sees. Think of passive glasses like a pair of sunglasses that don’t block out light equally. Instead of shutters blocking the eye, the glasses have filtered (polarized) lenses that block different kinds of light from each eye, thus creating the illusion of depth.

As a result, there is no need for any kind of expensive, delicate electronics in the glasses themselves, and you don’t need a proprietary infrared emitter to sync the TV with the glasses. There is a downside, however; since each lens is blocking out light, you’re technically not getting a full 1080p image for each eye, though your brain should perceive a 1080p image when it puts the two together.


About the Expert

Heidi Hoffman is the managing director of 3D@Home Consortium, whose mission it is to speed the commercialization of 3-D into homes worldwide and provide the best possible viewing experience by facilitating industry-wide standards, roadmaps and education for software providers and consumers.

Tags: 3-D , Gadgets , Movies