Benham's Disk

1. Print out the pattern disk provided here (click to enlarge) onto white card stock.
   
2. Cut out the disk. If your printer does not reproduce good solid blacks, fill in the black areas with a black marking pen. You can reduce or enlarge the pattern disk if you like.
3. Poke a hole in the center of the pattern disk and mount it on the machine screw so that it is between the two washers and held in place by the wingnut. Insert the screw into the drill and tighten the chuck to hold it in place.

Note: For a super-simple alternative, reduce the size of the disk on a copy machine and then mount it on the flat upper surface of a suitable toy top. You could also try spinning the mounted disk on a pencil point or on a pushpin stuck into a pencil eraser. Whatever you can devise to get the disk spinning safely should be fine.

Spin the disk under bright incandescent light or sunlight. Fluorescent and LED lights will work, but there’s a strobe effect that gives the disk a pulsating appearance and makes it harder to look at. The brighter the light, the better the effect.

Notice the colored bands that appear on the disk. Look at the order of the colors. What color do you see at the center? What about the next few bands? Reverse the direction of rotation and compare the order of colors again, from the center of the disk to the outer edge.

Try varying the speed of rotation and the size of the pattern, and compare the results with your initial observations.

Different people see different intensities of colors on this spinning disk. Why people see color here is not fully understood, but the illusion involves color vision cells in your eyes called cones.

There are three types of cones. One is most sensitive to red light, one to green light, and one to blue light. Each type of cone has a different latency time (the time in which it takes to respond to a color), and a different persistence-of-response time (the time it keeps responding after the stimulus has been removed). Blue-sensitive cones, for example, are the slowest to respond (have the longest latency time), but they keep responding the longest (have the longest persistence time).

When you gaze at one place on the spinning disk, you are looking at alternating flashes of black and white. When a white flash goes by, all three types of cones respond. But your eyes and brain see the color white only when all three types of cones are responding equally. The fact that some types of cones respond more quickly than others—and that some types of cones keep responding longer than others—leads to an imbalance that partly explains why you see colors.

The colors vary across the disk because at different radial positions on the disk the black arcs have different lengths. As a result, the duration of the flash on the retina is also different.

A complete explanation of the colors produced by a Benham’s disk is more complicated than the simple one outlined here (for example, the short black arcs on all Benham’s disks must also be thin, or no colors will appear), but this is the basis of much of what you see.

Benham’s disk was invented by Charles Benham, a nineteenth-century toymaker who noticed colors in a black-and-white pattern he had mounted on a top. Even now, tops with Benham’s disks can occasionally be found in toy stores.

Like the cones of the eye, the three different color sensors in some color-television cameras also have different latency and persistence times. When a color-­television camera sweeps across a bright white light in its field of view, it often produces a colored streak across the television screen.