3D TV not ready for prime time

— -- If you've been shopping for a new TV in the last two years, you've probably heard a lot of hype about 3D. When it first hit the market, manufacturers had dollar signs in their eyes, hoping people would flock to the new technology. But a combination of factors, including a sagging economy, customer confusion, and some 3D theatrical flops (Clash of the Titans, anyone?) has slowed the adoption rate. Nevertheless, 3D has infiltrated the field of TVs, spanning a wide range of prices and screen sizes.

As with most new technologies, customers are awash in baffling terminology and misinformation. Here's a simple guide to walk you through the basics.

What is 3D?

Most people have two functioning eyes in their head. Because they're set slightly apart from one another, each eye receives a different image, known as parallax. Your brain interprets those slight differences to determine how far an object is. Closer objects have a larger parallax, while more distant objects have less parallax. Your eye doctor checks this all the time. Move a finger too close to your eyes and you mistakenly see two fingers (too much parallax!).

In order for these 3D TVs to work, your brain has to be tricked into thinking there's depth in an image that, in reality, is only two-dimensional. It's a simple process, really. The objects on the screen are doubled and set slightly apart from one another. Objects that are meant to appear closer are given a larger parallax (just like that finger in your face), while background objects are set closer together. Without the 3D glasses you would see both sets of images, but the glasses act as a filter, sending one set of images to the left eye and another to the right eye.

Because 3D movies rely on your ability to interpret parallax, if you have any sort of impairment that prevents depth perception—known as "stereo-blindness"—you won't be able to see the 3D effect on your TV either. Experts estimate about 10% of the population falls under this category.

Active versus passive

The 3D TV market is currently split into two basic technologies: active shutter and passive. Our testing has shown that active shutter is far more effective at creating an immersive 3D experience. It comes at a cost, however. Each pair of 3D glasses retails from $50 - $150. That's a major investment for a family of four. Why so expensive? Active shutter glasses are electronically synced with the TV screen, alternating on and off, left and right, 60 times per second. When it works, it's a great trick. When it doesn't, it's a headache-inducing mess.

Passive 3D is closer to what cinemas are currently using. The glasses are so cheap as to be almost disposable and work on a simpler principle. The lenses are polarized so a slightly different image can pass through to each eye. In order to maintain the effect, your head has to be kept perfectly level the whole time. Sorry, but no lying down on the couch for you.

Our testing show has shown passive 3D to create a less immersive experience, though the glasses are certainly more comfortable for a two or three hour stretch.

Why 3D isn't great… yet