Stereoscopy (noun, “Stair-ee-AH-skuh-pee”)
Stereoscopy is a technique to trick the brain into perceiving flat images as three-dimensional scenes. Here’s how it works. Each eye is shown a picture of a flat, or two-dimensional, scene. The two pictures are almost identical but show the scene from slightly different angles. When viewing that pair of 2-D pictures together, the brain interprets them as a 3-D scene.
This mimics what the brain does in the real, 3-D world we live in. In everyday life, each of your eyes can perceive only a flat image of what’s in front of you. And because your eyes are separated by a small distance, they each see the world from slightly different angles. (To see this difference, close one eye and then the other. Watch how the objects in front of you appear to shift a tiny bit.)
The brain combines the visual input from each eye. In the process, the brain uses the slight differences between those two 2-D images to work out the distances to various objects within the field of view. (Say, for instance, you’re looking at a pencil sitting on your desk. Your eyes get two slightly different 2-D views of that pencil. By comparing those two flat images, your brain can estimate how far away the pencil is.) This is called depth perception. And it works whether your eyes are taking in two slightly different views of the real, 3-D world — or two versions of a 2-D scene displayed on a screen.
This effect is behind much of today’s immersive tech. Virtual reality headsets show slightly different images to each eye to make on-screen images feel real. Stereoscopy has also been used to make 3-D movies. In these films, right-eye and left-eye images are both displayed on screen. Special glasses help show the correct image to each eye. Old-school 3-D glasses did this by letting each eye see only specific colors of light. More sophisticated glasses now show each eye only light waves that wiggle at specific angles. Either way, the brain can assemble the 2-D view through each lens into an image that leaps off the screen.
In a sentence
Stereoscopy tricks the brain into thinking the flat images displayed on virtual reality headsets are 3-D scenes.