How 3D films work

I saw Avatar in IMAX 3D the other day. The reviews are correct: the story is somewhat trite; however, watching it in 3D on a big screen is mind-blowing, and seeing it any other way is a waste.

It made me think about the science of 3D movie-watching, though. There have been different technologies to do so for nearly a hundred years. They’ve been rarely employed though, as the gear to produce and project them has always been cumbersome and expensive. And the results – for anyone like me who saw early ’80s Hollywood efforts – were often poor.

But all technology becomes cheaper over time, and new techniques make things easier. So Avatar can blow our minds for the entire feature-film length.

Here’s how it works:

Hold your finger about 15 cm in front of your face. Now close one eye and look at it. Now switch: close the other eye and look at it with the first eye. Switch back and forth, one eye at a time. You can see that the position of your finger seems to move when viewed with each eye. If you open both your eyes and focus on your finger it appears to be in the middle of where it appears when viewed with each eye individually. This is the magic of how your brain has been trained to see: it takes the slightly different collections of light arriving at each eye, figures out that they’re of the same thing, andbuilds a composite picture in your brain. You can see this separate imagery if you leave both eyes open, leave your focus out about 15 cm, but move your finger in toward your nose: you’ll see two fuzzy images of your finger, one from each eye.

Now look at something far away. Close one eye and look at it, like before. Now switch eyes, again like before. The position of the image may change a little, but it’ll change much less than your finger did. If it’s very far away its position may not seem to move at all. This is how your brain tells the difference between something that’s close and something that’s far away: if the distance between the left- and right-eye images is large, your brain assumes that the object is close to you, and if the images from each eye are pretty much the same your brain assumes they’re far away.

Most current 3D movies use this process to trick you into seeing 3D of what is actually a 2D projection on a screen. A film actually projects two sets of images on the screen, either by a single fancy camera or – typically for IMAX – two separate cameras. But the light of the two projections are polarised differently (think of polarisation as the direction that the light waves vibrate in as they move towards your eyes). For the 3D film each projection’s light is polarised differently.

You’re given glasses to watch the 3D film with. These special glasses have different plastic lenses in the left and right frames; they’re filters that have different polarisations that match the polarisations of the projected images. The one on the left only lets through the light from the projection on the left; the one on the right only lets through the light from the projection on the right.

Also, corresponding objects in the two projections are spaced differently: objects that are meant to be close to you are spaced farther apart, and those that are meant to be in the background are closer together.

So the combination of the two projections and the glasses simulates what happens when we view the real world: one set of light arrives at the left eye, one set arrives at the right eye, and different objects have different amounts of separation between them. Your brain interprets these as things that are at different distances from you: voila, 3D.

There’s some other techie details about linearly polarised light (which messed up the 3D effect if you tilted your head) and circularly polarised light (which doesn’t) if you’re interested. We’ve also discovered that shrimp can detect polarised light,which means they might be able to watch 3D films without the glasses (I saw none at my screening of Avatar, though).