3-D anaglyph technology was used in the generation of this photo of taken in 1997 by the Mars Pathfinder. Photo courtesy of NASA/JPL-Caltech.

Regardless, the public flocked back to the theaters in droves. A spat of 3-D pictures followed, sucking in everyone from John Wayne to Alfred Hitchcock (despite his own tepid feelings toward 3-D), and J.R. Eyerman immortalized the fad in a series of iconic black-and-white prints. Ultimately, however, the phenomenon was short-lived.

3-D films have come back with a bang, most recently with Harry Potter's domination of airwaves, subway ads, and bus station billboards in anticipation of The Half-Blood Prince. Whether here to stay in film this time or another passing fad, 3-D technology will remain both a fascinating technology and valuable tool in science.

Ideas and techniques for generating a 3-D illusion from 2-D panels date as far back as the mid 1800s, and are grounded theoretically on parallax. Parallax is what you see when you look out sideways from the window of a car or bus, and you see trees and buildings in the foreground whizzing by much faster than the mountains in the background. If you trick your eyes into looking at two different images as opposed to just one, for example by crossing them, then you can exploit parallax and trick yourself into seeing depth in a 2-D picture.

Keeping a person's eyes constantly crossed may be difficult for fixed images. For feature length films, it would be nearly impossible, but this is where those funky glasses come in. Modern films and almost all of the 1950s 3-D films rely on polarized light. Light waves may be visualized for many purposes by shaking the end of a jump rope. Shaking vertically makes vertical polarization, and shaking horizontally makes horizontal polarization. A polarized light projector, coupled with the appropriate glasses, ensures that one eye sees only horizontally polarized light while the other eye sees only vertically polarized light. Modern projectors employ the slight modification of using left-handed and right-handed circularly polarized light, which allows people to maintain a 3-D viewing experience with tilted heads (try this the next time you visit a 3-D film!), but the basic concept is the same.

Although not quite as effective, a 3-D illusion can be obtained with color filters. Red glasses hide red lines and accentuate blue lines, whereas blue glasses tend to do exactly the opposite. With a mixed pair of glasses you can see 3-D aspects of photos like the picture above with minimal eyestrain. This method, known as anaglyph, has gained tremendous popularity because of the relative cheapness of the glasses.

A third method of 3-D viewing, known as lenticular, is also possible. With this method, a grating of narrow lenses is placed in front of a canvass, diverting light to create the 3D illusion. Although not as popular, this does have an advantage because you don't need special glasses.

Regardless of what happens to films, 3-D technology will continue to be a valuable resource in the sciences. 3-D vision has allowed planetary probes such as the Mars Pathfinder to see the surface of Mars almost as a real person would. Parallax is an important concept both for medical imaging and measuring the distances of nearby stars. Circularly and linearly polarized light are crucial elements in the technology that led to scientists being able to generate a Bose-Einstein condensate in 1995, and in the way we characterize electrons in metals and superconductors.

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See if you can tell from the 2 images below:*

A real, spontaneous smile incorporates tiny muscles around the eye that are nearly impossible to contract at will. You can see this for yourself in an exhibit called "Polite Smile, Delight Smile" part of the Exploratorium's new Mind exhibition.

This corners-of-the-eyes giveaway, as well as many other subtle, yet revealing, facial gestures, was discovered by Paul Ekman, now a professor emeritus of psychology from the University of California, San Francisco. Ekman's been studying the universality of facial expressions and the secrets our faces reveal for over four decades. The notion that certain expressions of emotion are programmed into us wasn't so well received when he proposed it in the 1960s. At that time, social scientists believed facial expressions were cultural. Then, in 1967, Ekman embarked on an expedition to Papua New Guinea, where he asked people belonging to an indigenous tribe that had virtually no contact with the developed world to imitate the expressions they would have in certain situations, such as meeting an old friend or discovering a decaying animal. Ekman found that the ways these people's faces expressed sadness, fear, surprise, anger, and disgust involved the same eye and mouth muscle movements that people from Western cultures displayed. The collection of photos he took there will be on display at the Exploratorium from January 22 –April 27, 2008.

Today, Ekman is lauded by psychologists. He's considered the leading expert on detecting deceit, and his ideas are used to train CIA, Homeland Security, and other law enforcement officers to detect when they are being lied to by someone they're questioning and to spot unusual behavior. He devised a tool known as the Facial Action Coding System (FACS), which catalogues the musculature behind thousands of facial expressions. Some of the most subtle of these Ekman calls "microexpressions," fleeting muscle movements that reveal emotions the subject is trying to suppress. With the knowledge that these revealing expressions are universal, FACS allows a trained person to "read" someone's emotions by observing their facial muscles.

When Ekman's book Emotions Revealed came out in 2003, I thought it would be great to master the subject matter. Who wouldn't benefit from learning to understand the fleeting messages people send oh-so-subtly? But the more I thought about it, the more uneasy I began to feel. Something didn't sit right with me about the practice of decoding people without their knowledge. Then again, isn't that what any of us do when we "sense" that someone was nervous or untruthful or secretly overjoyed? It's not like our microexpressions are hidden. We express them in plain sight. They may be the source of an intuitive person's "sixth sense." But to formally study these expressions with the intent of detecting emotions that the subjects themselves are unaware of–is that a violation of privacy? Ekman would say no. He insists that he can't read minds, only emotions, and that leaves out most of the personal details. Still, there's something unsettling about the idea that feelings I've long considered private are written all over my face.

* BTW, the real smile is image 1. Did you guess correctly? Leave a comment to tell us how you knew.

Robin Marks is a journalist and science writer who current serves as a Multimedia Projects Developer for the Exploratorium in San Francisco, CA.

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