The "3D fad" was a real failure in living rooms, and it has an increasingly smaller presence in cinemas, but that doesn't mean we should stop experimenting. There are different ways to reproduce that effect, and one of them is almost a hundred years old: known as the "Pulfrich effect", its official description talks about a latency problem in the visual system, created by leaving one eye exposed and the other in a dark environment.
In very relaxed terms, standard 3D works by sending one image to each of our eyes simultaneously, performing the corresponding separation with the help of special filters (like the classic glasses, whether passive or active). This stereoscopic pair seeks to optimize (to some extent) the illusion of depth, but the process makes the use of glasses mandatory, otherwise the screen will look horrible. However, there are exceptions to the rule, and today we are going to explore one: the Pulfrich effect. Discovered by the German physicist Carl Pulfrich in 1922, the effect can be reproduced with relative ease, although there are health conditions that generate it spontaneously, such as optic neuritis and cataracts. Do you want to try it? If so, take a pair of sunglasses, cover only your right eye with them, and watch this video that is 2D and 3D at the same time:
How the Pulfrich effect works
The short published by Tom Scott explains it wonderfully. The Pulfrich effect arises from a slight delay that the "eyes-brain" combo suffers when processing dark images. The duration of that delay depends on the relative brightness of the images, but in general it is one video frame. If you place the sunglasses only on your right eye, the received image will arrive with a one-frame delay to the brain. In other words, what we see are two images separated by the time equivalent to a single frame.
Now... that's not enough to create a stereoscopic pair. The last component is the displacement, which in the video appears with the cameraman walking in circles around Tom. Being in constant motion, our right eye covered by the sunglasses maintained the one-frame delay, but once it stops, the effect disappears (the alternative is moving objects within the main frame). Of course, the Pulfrich effect is far from an ideal 3D, and it only serves for small tricks, however, we already know very well that the brain loves filling in gaps.
