3-D Films: behind the scenes

Unless you live under a rock or somewhere on Pluto, you have probably seen or at least heard of the film Avatar by now. Perhaps you also watched the Clash of the Titans, or Tim Burton's Alice in Wonderland. Curious about these eye-popping 3-dimensional cinematographic experiences, you may have wondered, how'dey do it?

Films produced for your 3-dimensional watching pleasure depend on special projectors and eye-gear. In everyday life, you are able to perceive the world around you in three dimensions (provided you have fairly good vision in both eyes). This is because your eyes are spaced approximately 6 cm apart, each conveying to your brain a slightly different perspective image from its position on the head. Your brain (nerd) takes these two slightly dissimilar images and fuses them into one three-dimensional picture.

When you watch a conventional film, your brain is unable to appreciate depth of field, because the two eyes send fairly similar information to the brain. However, if the movie projector were to display two slightly dissimilar images to each eye, your brain would be fooled into thinking the film is 3-dimensional. With the RealD technology used in creating Avatar, two slightly dissimilar images are produced as the film's projected light-rays are polarized (oriented) in two opposing directions. Normally light waves bounce randomly in all different ways, but if subjected to a polarizing filter, they will follow in the ordained direction of their filter (clockwise and counterclockwise, in this instance). The opposing polarizing filters on the Avatar projector actually alternate the image light-rays from clockwise to counterclockwise at a rate of 144 times per second! If you were to see both projected images at once, however, the picture would appear fuzzy, not 3-dimensional; so for the low price of $16.50 you are also provided with a pair of 3-D glasses. The right and left lenses on these glasses also contain circular polarizing filters: one oriented clock-wise, the other counter-clockwise. This way, while one eye filters out the clockwise rays the other receives the counterclockwise ones. So each eye sees a slightly different image. These two images are dispatched to your brain, which computes the information into a 3-dimensional picture.  Et, voilá.

Interestingly enough, we use a similar technique in everyday pediatric ophthalmology. With linear polarizing filter glasses (one lens filters only vertical image rays the other only horizontal image rays), children are asked to look at a tablet containing a fly, circles and animals. These images are preprinted on a material that allows light rays to be projected vertically alongside a shadow image that projects horizontally. If the child identifies the correct “3 dimensional” images in the tablet (Titmus test), we know s/he has good binocular vision from both eyes. The test serves as a great screening exam for amblyopia (a condition in which a child does not develop good vision in one or both eyes). It can also be used to assess depth perception in adults.

So, now you know how they work, prepare for an onslaught of 3-D films!