- FAQ

Pulfrich Effect

It sounds a lot fancier than it really is: Get a buddy to drive a car about 10 MPH along a suburban neighborhood where you have things (trees, fences, houses, etc.) both near the road and far away from the road. Your job is to sit in the front passenger seat, hold the recording video camera steady, and just point it out the right window, perpendicular to the direction of travel. Then rush back home and watch the video with only the right eye covered by sunglasses (You can also use polarized flip-down shades, but polarization has nothing to do with it) and you have Pulfrich 3D!

The reason: Things closer to the camera will move faster across the TV screen. The dark lens on the right eye reduces the scene's light intensity to the right eye. The brain takes longer to process dark scenes. So, by the time the brain has processed the fast moving tree in right eye's scene, the left eye's tree has already moved farther to the right. To focus the tree the eyes must cross more, thereby giving the illusion of a closer tree.

So, with the dark lens on the right eye, things that move faster to the right appear closer, and things that move faster to the left appear farther away.

Prove it: Hang a spectacular reflector (like a metal spoon) in the center of a doorway with a string. Swing the spoon side to side like a pendulum (always same distance to you). With the dark lens on your right eye, the spoon will appear to swing in a circle: closer to you when moving to the right, and farther from you when moving to the left (counterclockwise from above). Move the single dark lens to your left eye and the spoon will now appear to swing clockwise from above.

Rotation (i.e. having the CAMERA spin around) doesn't work. If you think about it briefly, you'll see that the camera's orientation never changes in relation to the subject(s), so there isn't any source of stereo information. However having the SUBJECT spin around, like an ice-skater, the bed in the "magic bed" video or in the Goldilocks and the 3(-D) Bears video, works fairly well.

The foreground (moving from left to right) will appear to be in front of the screen and the background (moving right to left) will appear to be behind the screen.

Moving the camera sideways (i.e. stick it out the side window of your car as you drive) also works well. In this case everything is moving left to right (if you stick the camera out of the right-hand window) and will appear to be in front of the screen. Since closer objects will move across the screen faster, they will be more "in front".

For a handy-dandy preview, put on your Pulfrich glasses next time you are in the car (hopefully as a passenger) and look out the window as you move along. The world will look like it's in hyper-stereo.

Since the effect works off motion (due to the dark filter having some sort of effect on persistance of vision, in terms of a mechanism) only objects that are in motion on the screen will show any stereo. Thus; the skater will appear fully 3D when she spins (with a flat background), while as she skates sideways both she and the background will be flat but separated from each other (the infamous cardboard-cutout effect).

One final note: all explanations regarding experiments involving cars are based on right-hand drive cars (steering-wheel on the left). Left-hand drive cars work as well, but you may want to take a back seat behind the driver :-)

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Last modified on May 4, 2005
Copyright © 1999 - by and Alexander Klein. All rights reserved.