When we say that stereoscopy techniques and effects have been with us for a long time, in some cases it's literal. The human eye is wonderful, but a slight change in its normal “working conditions” can make us see something completely different. A good example is the so-called Pulfrich illusion, first documented more than ninety years ago. What do you need to appreciate this illusion? Only a pair of sunglasses with one lens removed from the frame, and play the videos that accompany this text.
How Does the Pulfrich Illusion Work?
The original demonstration — at least the most common one — of the Pulfrich illusion involves a pendulum and a darkened lens. When the pendulum begins to swing, the eye covered by the lens interprets the motion in such a way that it “seems” like an orbit, generating a sense of depth. By reducing the light reaching the retina of the covered eye, a delay in signal processing is caused.
The name “Pulfrich illusion” or “Pulfrich effect” comes from Carl Pulfrich, a German physicist who first recorded the effect in 1922. The effect also works with circular motion, so if you don't have something nearby to act as a pendulum, try this video.
The effect is well documented, and there are examples of its application in commercials (Coca-Cola) and TV series (Doctor Who, 3rd Rock From The Sun), but the effect also depends on the type of motion, so its large-scale use is not feasible. Another example of the Pulfrich illusion?
The same effect can occur naturally, so to speak, in people with cataracts, optic neuritis, and multiple sclerosis. The effect happens because of the apparent reduction in retinal illumination (relative to the eye that isn't covered with the dark lens), which causes a delay in signal transmission and a spatial disparity in moving objects. It's like having different latencies in each eye, so to speak. That's why it only works with things that move.
Remember: to appreciate the effect, you must have one eye behind a dark lens and the other with nothing covering it. The illusion is more impressive if you try the pendulum demonstration mentioned above live and in person.
