How 3-D TV Works

Why can you look at an object in the real world and see it as a three-dimensional object, but if you see that same object on a television screen it looks flat? What's going on, and how does 3-D technology get around the problem?

It all has to do with the way we focus on objects. We see things because our eyes absorb light reflected off of the items. Our brains interpret the light and create a picture in our minds. When an object is far away, the light traveling to one eye is parallel with the light traveling to the other eye. But as an object gets closer, the lines are no longer parallel — they converge and our eyes shift to compensate. You can see this effect in action if you try to look at something right in front of your nose — you'll attain a lovely cross-eyed expression.

When you focus on an object, your brain takes into account the effort it required to adjust your eyes to focus on it as well as how much your eyes had to converge. Together, this information allows you to estimate how far away the object is. If your eyes had to converge quite a bit, then it stands to reason that the object is close to you.

The secret to 3-D television and movies is that by showing each eye the same image in two different locations, you can trick you brain into thinking the flat image you're viewing has depth. But this also means that the convergence and focal points don't match up the way they do for real objects. While your eyes may converge upon two images that seem to be one object right in front of you, they're actually focusing on a screen that's further away. This is why you get eye strain if you try to watch too many 3-D movies in one sitting.

How do you show two different images that appear to only be one? It's all in the lenses.

In the 3-D business, there are two major categories of 3-D glasses: passive and active. Passive lenses rely on simple technology and are probably what you think of when you hear the term 3-D glasses. The classic 3-D glasses have anaglyph lenses.

Anaglyph glasses use two different color lenses to filter the images you look at on the television screen. The two most common colors used are red and blue. If you were to look at the screen without your glasses, you would see that there are two sets of images slightly offset from one another. One will have a blue tint to it and the other will have a reddish hue. If you put on your glasses, you should see a single image that appears to have depth to it.

What's happening here? The red lens absorbs all the red light coming from your television, canceling out the red-hued images. The blue lens does the same for the blue images. The eye behind the red lens will only see the blue images while the eye behind the blue lens sees the red ones. Because each eye can only see one set of images, your brain interprets this to mean that both eyes are looking at the same object. But your eyes are converging on a point that's different from the focal point — the focus will always be your television screen. That's what creates the illusion of depth.

Today, a more popular type of passive lenses in movie theaters can be found in the polarized glasses. Again, if you look at a screen that uses this technology you'll see more than one set of images. The glasses use lenses that filter out light waves projected at certain angles. Each lens only allows light through that is polarized in a compatible way. Because of this, each eye will see only one set of images on the screen. Polarized lenses are becoming more popular than anaglyph glasses because the glasses don't distort the color of the image as much and provide a better audience experience. But it's very difficult to use the polarization technique for home theater systems — most methods would require you to coat your television screen with a special polarizing film first.

Now let's take a look at active glasses.

In the last few years, engineers have come up with a new way to create three-dimensional images in movies and on television sets. You still wear 3-D glasses with this method, but they don't use colored lenses. The method doesn't compromise the color quality of the image as much as anaglyph glasses do. It also doesn't require you to put a polarization film on your television screen. What it does do is control when each of your eyes can view the screen.

The glasses use liquid crystal display (LCD) technology to become an active part of the viewing experience. They have infrared (IR) sensors that allow them to connect wirelessly to your television or display. As the 3-D content appears on the screen, the picture alternates between two sets of the same image. The two sets are offset from one another similar to the way they are in passive glasses systems. But the two sets aren't shown at the same time — they turn on and off at an incredible rate of speed. In fact, if you were to look at the screen without wearing the glasses, it would appear as if there were two sets of images at the same time.

The LCD lenses in the glasses alternate between being transparent and opaque as the images alternate on the screen. The left eye blacks out when the right eye's image appears on television and vice versa. This happens so fast that your mind cannot detect the flickering lenses. But because it's timed exactly with what's on the screen, each eye sees only one set of the dual images you'd see if you weren't wearing the glasses.

For several years, LCD and plasma screens weren't good candidates for this kind of technique. The refresh rates — the speed at which a television replaces the image on the screen — were too low for the technology to work without the viewer detecting a flicker from the glasses. But now you can find plasma and LCD displays with incredibly fast refresh rates.

The refresh rates are just one part of a television qualifying as 3-D ready. Learn more in the next section.

You can't use a standard television and expect active glasses to work. You must have some way to synchronize the alternating images on the screen with the LCD lenses in the glasses. That's where the stereoscopic sync signal connector comes in. It's a standardized connector with three pins that plugs in to a special port on a 3-D-ready television or monitor. The other end of the cable plugs into an IR emitter. The emitter sends signals to your active 3-D glasses. This is what synchronizes the LCD lenses with the action on the screen.

The connector operates using transistor-transistor logic (TTL). One pin on the connector carries low-voltage electricity. A second pin acts as a ground wire. The third pin carries the stereo sync signal.

There are two different types of 3-D active glasses and they aren't compatible with one another. They are the E-D and ELSA style of 3-D glasses. While emitters for both styles work with the stereoscopic sync signal standard, E-D glasses will only work with an E-D emitter. While a pair of ELSA glasses can synchronize with an E-D emitter, the glasses won't perform properly. For example, when the E-D emitter sends a signal for the left lens to be transparent, the ELSA glasses will make the left lens opaque and cause the right lens to be clear.

Even if you have a 3-D-ready television, an emitter and a pair of active glasses, not everything on your television will appear to be three dimensional. Content providers must optimize the signal for 3-D first. While it's possible to modify existing footage into 3-D content, some providers prefer to create video with 3-D in mind beforehand. Currently, the easiest way to view 3-D content is to connect a computer to your 3-D-ready television using an HDMI cable, and then stream the 3-D content from your computer to your television. In the future, we'll probably see more DVD players capable of sending 3-D signals to televisions and perhaps even incorporate 3-D transmissions into cable and satellite services.

Lenticular Displays

While 3-D technology is impressive, some people still want a solution that doesn't require them to wear glasses. There have been several attempts at creating a display capable of projecting images into a three-dimensional space. Some involve lasers, some project images onto a fine mist or onto artificial smoke, but these methods aren't that common or practical.

There's one way to create three-dimensional images that you may see in places like sports arenas or in a hotel during a big conference. This method relies on a display coated with a lenticular film. Lenticules are tiny lenses on the base side of a special film. The screen displays two sets of the same image. The lenses direct the light from the images to your eyes — each eye sees only one image. Your brain puts the images together and you interpret it as a three-dimensional image.

This technology requires content providers to create special images for the effect to work. They must interlace the two sets of images together. If you were to try and view the video feed on a normal screen, you would see a blurry double image.

Another problem with lenticular displays is that it depends upon the audience being in a sweet spot to get the 3-D effect. If you were to move to the left or right from one of these sweet spots, the image on the screen would begin to blur. Once you moved from one sweet spot to another, the image would return to a cohesive picture. Future televisions may include a camera that tracks your position. The television will be able to adjust the image so that you're always in a sweet spot. Whether this will work for multiple viewers of the same screen remains to be seen.

Some people experience a feeling similar to motion sickness after watching a lenticular display for more than a few minutes. That's probably because your eyes have to do extra work as they deal with the discrepancy between focus and convergence. But on the other hand, you don't have to worry about losing an expensive pair of active glasses.

Will 3-D television become the next big trend or is it destined to be a fad that comes back every couple of decades? It's too early to say right now. But the technology continues to improve. It may not be long before you duck out of the way the next time a baseball player hits a line drive toward the camera.