3D, or Three Dimensional imagery works on the principle of binocular parralax--the fact that you have two eyes at two slightly different locations. To understand the concept, look at something distant, and hold up your finger at arms length. Keep looking at the distant object while covering, or closing each eye. See how your finger seems to shift from side to side against the background? That's because each eye sees the world from a slightly different angle. Your brain takes the two pictures your eyes create and fuses them into a single mental image. It uses the slight difference in angle between near and far objects to determine how far things are away. The closer something is to you, the more your eyes point towards each other, and the more cross-eyed you become. The further something is away, the more your eyes point parallel to each other, at a certain distance, your eyes stay parallel, no matter how much farther away they are.
Knowing that your brain percieves depth by taking in not one, but two images, one for each eye, we can fool your brain into perceiving a simulated sense of depth. The earliest way this was done was a steriopticon, a device that held two pictures side-by-side, and had magnifiers for each eye. This presented one picture to each eye directly. The pictures were taken side by side, and created an illusion of depth. You likely own or have seen a modern version of the steriopticon, the Viewmaster. This simple toy has a magnifier for each eye. If you look carefully a the disks you put in the toy, you will see that images across from each other are stereo pairs.
After the invention of motion pictures, and the introduction of sound and color, it was inevitable that movies would play with 3D. Stereoscopic filmmakers were faced with a problem. Movies were designed for a single flat image on a screen. Some way of separating each image for each eye was required, and the simplest for projection was color. The technique, called Anaglyph, projected one image through a red filter, and the other image through a blue filter. The audience then wore glasses with a red filter over one eye and a blue filter over the other. The red filter let red light through, but not blue, and the blue filter let blue light through, but not red. Set up properly, with each colored image set to the correct half of a stereo pair, the audience sees a three-dimensional image. The earliest 3D films were released in this Anaglyph format. One of the many drawbacks of this process is that images are black and white (or black and purple really). Different versions were tried, red and green had better separation of the images, and blue and yellow was thought better still. One of the later advantages of color film was that anaglyph can be "encoded" on a single piece of film, instead of using two synchronized projectors. This is also the only reliable method for sending true stereo images over television.
While Anaglyph uses a black and white image for each of the red and blue filters, it has recently been possible to create what is called a "Full Color Anaglyph." It was discovered that the red filter transmits only red light, and filters out blue and green. Blue filters tend to transmit both blue and green, but not red. It is possible to create filters that better transmitt blue and green, while limiting red. By taking two color pictures for the stereo pair, one can separate out the three color channels of each image. The red channel of the left eye image is combined with the blue and green channels from the right eye image. When viewed through anaglyph glasses, it appears 3D, and conveys a good impression of color, but not full-color.
To get full-color 3D movies, a full color image must be sent to each eye, and so a different method of separating images was required. Enter polarizing filters. Polarizing filters work on the principal that light "vibrates" in many planes. Light of only one plane can be filtered out by a polarizing filter. If you have polarized sunglasses, look at the reflection in a window through them. Then rotate the lens, and you will see the intensity of the reflection change, but not the rest of the light. Light reflected off of certain materials becomes polarized in one plane. A filter that allows only one plane of light through can be rotated until the polarized light cannot pass through, or so it can be passed through. Polarized 3D works by projecting each image through oppositely polarized filters. If one filter allows light of a plane from 12 to 6 on a clock dial, the other is at 90 degrees to it, from 3 to 9 on a clock. Similar filters over each eye, each at 90 degrees to the other, allow the appropriate image into each eye. Even with color images. A number of films were shown with this process, and it is still used today, such as "Captain EO" at Walt Disney World (no longer running), and "Terminator2: 3D" at Universal Studios.
Early work on Image Sequential tequniques used syncronized, rotating shutters suspended in bulky machines in front of each spectator's eyes. Syncronized with the projector(s), they opened for one eye while showing it's image, then closed for that eye and opened for the other eye while diplaying it's image. Current Image Sequential techniques use LCD Shutters over each eye. LCDs (Liquid Crystal Displays) are what make the numbers on your pocket calculator. They manipulate the polarization of light to change from transparent to opaque in response to electricity. In the same way, they are transparent for one eye while it's image is shown, and flip to be transparent for the other eye while it's image is displayed. This is the only full-color, truly stereoscopic technique for television. Each frame of television is made up of two fields, filling every other line. Television uses Field Sequential for true color 3D, and the viewer must have a properly-synchronized pair of LCD Shutter glasses. The technique can also be used with computer screens, you can read about my use of this technique on my Virtual Reality page.
A recent invention is the Pulfrich system. It conists of a pair of glasses with one lens clear, and the other slightly darkened. If the camera is moving in the correct direction, left or right, the images give a 3D effect. You can try the Pulfrich system by tying something to a string to make a pendulum. Let it swing left to right. If you cover one eye with half a pair of sunglasses, the pendulum will appear to be coming toward you and away from you. Scientists aren't sure what causes this, but I believe it has to do with our eyes and our brains. My theory is that our brains are hard-wired to react more quickly to things that are brighter. As a result, our brains react to the clear-eye's image, and only later reacts to the darker-eye's image. The effect is that one eye sees the image on the screen, then the other eye perceives the same image a fraction of a second later, possibly even while the clear-eye is now seeing the next image. One eye sees an image, and the other now sees an image from a slightly different location, since the camera is always moving. If the camera is moving to the left, the dark filter should be on the right eye. This technique was used on a SuperBowl half time, a Rolling Stones tv special, and most recently for the Dicoverey Channel's "Sharks 3D."
Perhaps in the future, all televisions will come with glasses-less 3D systems, but I doubt it. It is also possible that Motion Holography could be done in movie theaters, but I doubt the public will pay more for it. The main failing of 3D thus far has not been the process itself, but how people have used it. 3D has become synonomous with what a friend calls "The House of Wax effect" where every few minutes, something is poked at the camera, just so you know it's in 3D. Color films don't go from black and white to intensley vibrant color, nor do sound films go from silence to a roar simply to draw attention to themselves. 3D should, like color and sound, be a device used only to help tell an entertaining story. "Terminator 2: 3D" is an excellent use of 3D to tell a compelling and exciting story. The 3D projection is incorportated with live-action on stage in a seamless illusion.