How Do 3-D Glasses Work?

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With 3-D glasses, explosions, gore, or magical creatures jump off the screen. But these spectacles aren’t magical. Most of the technology making 3-D movies work exists inside our skulls. Jenny Read, a vision scientist at Newcastle University explains how filmmakers use the brain’s natural functioning to create the 3-D experience.

How do the “classic” 3-D glasses with the red-blue lenses work?

We see the world from two, shifted views, one provided by each eye. Hold a finger in front of your face while covering one eye at a time — the position of your finger jumps. Your brain uses the offset in those views to determine an object’s distance, triangulating between both eyes. Scientists think that computation occurs in the visual cortex, where individual brain cells seem sensitive to specific distances between the eyes and use those distances to compute depth.

The glasses recreate that triangulation by feeding distinct images to the eyes. They approximate the offsets, depending on how far things are, that your eyes expect in life. Look at a movie character without the glasses — two outlines extend from the character, identical except one’s blue, the other red, and they’re slightly offset. With the glasses back on, your brain merges those images to create the perception of depth. The lenses control what each eye sees by filtering the light going to each eye, only letting certain wavelengths pass.

This is how a scene might look if you took off your glasses during a 3D movie. Two outlines extend from the cacti, identical except one’s blue, the other red, and they’re slightly offset.

U.S. Department of the Interior - U.S. Geological Survey - Geology Discipline / Public domain.

Filmmakers consider how the degree of offset between these images translates to depth inside our brains. By drawing the images on top of each other, viewers will see a flat image on screen (the offset between the eyes is zero). Increasing the offset a little, so the left eye’s image goes to the right and the right eye’s image goes to the left, pulls the image out in front of the screen. Shifting in the opposite direction pushes the image back. However, this system depends on color filtering that distorts the movie’s color quality. Nowadays, we avoid this problem by using glasses that work with polarization.

How do the newer 3-D glasses work?

Light is an electromagnetic wave traveling along a particular plane. That plane — the wave’s orientation — is what we refer to as polarization. Humans aren’t sensitive to light polarization, so image quality isn’t disrupted. Theaters use two forms of polarization for 3-D movies — linear and circular. Digital IMAX theaters use linear polarization. They align two projectors so images line up on the screen. One projector displays images intended for the left eye, and the other for the right, with a polarizing filter in front of each projector. Light from one projector is polarized in one direction and light from the other is polarized along the perpendicular direction. The 3-D glasses have polarizing filters matching to the projectors’ filters. Your brain merges the images to see depth.

But tilting your head puts the filter at the wrong angle — each eye may start seeing a weak version of the other’s image. Circular polarization avoids this problem. A device in front of one project flips rapidly between the two forms of circular polarization. This, combined with the glasses, sends images in rapid alternation to the eyes.

Can everyone use 3-D glasses?

No. Some people are stereoblind — they can’t triangulate between their eyes. It’s not a daily issue because there are many depth cues. For example, a close-up object occupies more of your field of view and gets smaller moving away. Because my brain assumes its size hasn’t changed and I know roughly how big it is, that’s one cue about its distance. Other cues include occlusion (if object A blocks object B, then A is in front), shading, depth cues from shadows, and converging lines — in the distance, rooftops and sidewalks converge at a point on the horizon.

How does the brain deal with 3-D movies?

These movies aren’t geometrically correct. Everyone has a different distance between their eyes. The calculation of how much offset between images would correspond to real life changes depending on where you’re sitting in the theater. You can’t reconstruct the 3-D world in a geometrically perfect way — you’d need perfect measurements for everyone’s eyes. Since we approximate an average, it winds up being rough and ready. Still, the brain figures it out.

What rules do filmmakers follow when creating 3-D films?

Avoid situations that force viewers’ eyes to diverge. When looking at something very far away, the lines of sight from our eyes are parallel. To simulate looking out to infinity, you need to recreate that on screen. But, say you portray a distant object by setting the lines of sight seven centimeters apart on screen for someone whose eyes are seven centimeters apart. For another viewer whose eyes are six centimeters apart, her lines of sight will point outward to see this object—they’ll diverge. That’s uncomfortable. The separation between the left and right images shouldn’t exceed the smallest distance between anyone’s eyes.

Since everyone has a different distance between their eyes and sees the screen from a different place, the image is somewhat wrong for everybody. The brain forgives these issues of geometry though we don’t know exactly how.

Are you investigating how the brain triangulates between the eyes?

As scientists, we trick your vision. Convincing you that you’re seeing something you’re not can be diagnostic. We give participants artificial, messed up images and they guess what they’re seeing. That tells us about the algorithms the brain uses to process visual input.

My lab studies praying mantises. We give them 3-D glasses to fool their vision. Mantises strike at things they think are in range. If they strike while wearing the glasses, then we’ve fooled them into thinking something was floating in front of the screen.

Mantises may compute this vision differently than humans. Human eyes match the scene’s details — edges, colors, and lighting — to align the left and right views and create depth. Mantises’ eyes look for what’s changing in the scene. If we show them two different images with changing aspects in the same place, they’ll match those elements and interpret the result like it’s 3-D. This might help mantises find camouflaged prey. It may also mean they can’t see 3-D unless something’s moving.

About the Author

Lindzi Wessel

Lindzi Wessel is a freelance science journalist who graduated from the UC Santa Cruz Science Communication Program. Before turning her sights on journalism, she studied the mind, obtaining a bachelor’s degree in psychology and a master’s in neuroscience from UC Davis. For the moment, she resides in Santiago, Chile, where she’s learning about science coming out of South America.