How do 3-D movies work?

Dr. Christopher G. Healey, associate professor of computer science at N.C. State, explains the science behind those 3-D movies showing at a theater near you. Questions and answers have been edited.

Q: How can a pair of glasses make images on a flat cinema screen appear as if three-dimensional?

"Polarization" refers to the orientation of the light we're seeing through our eyes.

It's possible to use filters to only pass through light that's polarized in a specific direction.

3-D movies nowadays are filmed using two cameras that are slightly offset from one another.

Two projectors are used in the movie theater to show the movie. Each projector uses filters to polarize its images to have a unique orientation. The images superimpose on the screen.

Each lens in the eyeglasses lets through only light with a particular polarization.

So, the right lens passes through the light from the right projector but blocks the light from the left projector.

Similarly, the left lens passes through light from the left projector and blocks light from the right projector.

This means you see two slightly offset images in each eye, just as you would if you were looking out into the real world. Our visual system fuses that into the impression of real 3-D.

Q: How does this work for 3-D TV?

The television again shows two offset images, but they're displayed one after another rather than superimposed.

3-D shutter glasses are used. Here, the glasses can darken either lens very quickly. This effectively blocks your ability to see anything through that lens.

When an image offset to the left is shown, the glasses darken the right lens.

When an image offset to the right is shown, the glasses darken the left lens.

The visual system fuses the two offset images into a 3-D image.

For 3-D TV to work, you need whatever you're watching to be properly formatted to have this left-right-left-right image order.

You also need about 120 images per second to be shown in order for the image sequence to fuse and not flicker.

Q: Are 3-D movies going to go away - or will they evolve to stay relevant?

I'm not sure whether 3-D movies are here to stay or whether they'll fade out because people get tired of the effect after they've seen it a few times.

There are certainly some issues with 3-D movies, in terms of not everyone being able to fuse the images properly.

But, as one of my friends pointed out, movie makers may still be learning what they can do with 3-D that they couldn't do with normal 2-D.

So, given time, we may start seeing 3-D movies with cinematic storytelling or effects that are more compelling or just not possible in a 2-D movie.