The Pulfrich Effect is an optical phenomenon where objects (or images) moving in a single plane can appear to be in 3D when the light reaching one eye is dimmed, e.g. with a filter. It also has a curious history – Carl Pulfrich (biography – pdf), who discovered the phenomenon, was blind in one eye and never observed it for himself, but nonetheless made many contributions to stereoscopy (the study of 3D vision) in both theory and the construction of apparatus.
Unlike other forms of stereoscopy, this only works with moving objects or animations; it does not work with still images! But what’s really cool is that you don’t need any special equipment to view it, beyond a piece of darkened glass or plastic to act as a filter. Videos exhibiting the Pulfrich effect can be viewed on a normal monitor or TV screen.
- Classic pendulum demonstration
- “Moving scenery” demonstration
- Animated explanation showing lines of sight
The principle is actually quite simple, and the classic demonstration involves a pendulum swinging in one plane, as shown in the videos below:
Here’s my attempt at an explanation: Consider a pendulum moving back and forth in a single plane. Normally our eyes would observe and process their input simultaneously, and our brains subconsciously interpret the information to tell us that the motion is two dimensional. However, what happens if the input for one eye is delayed by a fraction of a second? Recall that in conventional stereoscopy, the 3D effect occurs because the image reaching one eye is laterally displaced with respect to the image reaching the other eye. So if one eye is slower at processing the image than the other eye, for an object moving laterally, this is equivalent to a stereoscopic effect because the combined image that is put together by the brain contains a lateral displacement. This is much easier to grasp as a diagram – have a look at my animation here, which also lets you play around with parameters like speed and time delay.
How do we achieve this delay in one eye? In some people with conditions like multiple sclerosis, this occurs naturally, and so they have problems with stereoscopic vision in their day-to-day life. To simulate this artificially, one can cover one eye with a dark filter. The light is not any slower in reaching the eye (!) but because the overall light intensity is dimmer. The image processing is therefore slower, and this asymmetry between the eyes leads to the appearance of depth.
There is a comprehensive website on the Pulfrich effect at Southern Illinois University Carbondale, which also contains copies of the primary literature. It has an illustrated explanation of the effect, which inspired my own version.
If you search on YouTube for ‘Pulfrich effect’, you’ll find many videos which can illustrate the effect. Most of them appear to be taken from the window on long train or bus rides. Landscape scrolling by is perfect for demonstrating the effect, because there is foreground and background, and constant lateral motion. There’s even an Instructable on how to make such videos.
What if you don’t have a filter handy, you say? Well there’re even more cheapo ways to view the effect! You can either squint with only one eye (a bit tricky to do for some people), or (at the risk of looking completely ridiculous) fan out the fingers of one hand in front of one eye and quickly wiggle it up and down – this “strobe” effect also reduces the overall brightness of the image reaching that eye and trust me, it works!
How did I end up delving into this? I was looking up some old episodes of 3rd Rock from the Sun, a comedy sitcom from the ’90s that I remember watching as a kid. One fun fact that I learned was that the two-part season 2 finale was supposedly shot in 3D. I wondered how that was possible in the days before 3D TVs came on the market, unless they used red/blue anaglyph glasses. This seemed reasonable because I read somewhere that the show spent quite a bit of money on distributing special glasses for viewers to watch that episode.
After a bit of searching online I found videos of these episodes, but it was definitely NOT shot in red/blue anaglyph. I also found out elsewhere that the technique used was the Pulfrich effect. Both episodes contain (spoiler alert) trippy dream sequences. The exaggerated perspective and lateral motion in these sequences are what result in the Pulfrich effect 3D.
This also points to the main limitation of using this effect for stereoscopic entertainment. It requires that the images be in constant lateral motion. This works with scenery scrolling past the window (some arcade-style scrolling video games are said to have exploited this effect), or fantastical dreams with lots of flying and jumping, but in a typical TV show the characters spend most of their time just standing around and talking.
Previous blog posts here about stereoscopy:
- Wiggle-3D – Stereoscopy that works even with one eye
- Auto-autostereograms – Magic-Eye style 3D with additional stereo effect
- 3D-like effect based on chromatic aberration (have since learned that this is a patented product called ChromaDepth)