Benham Discs

This animation shows an illusion known as Fechner colors. It was first noticed by Charles Benham, a 19th century toy maker, who observed it on spinning tops decorated with certain geometric patterns. Though the patterns were pure black and white, somehow he was seeing colors in the spinning display!

After some experimentation, mostly trial and error, he came up with a design optimized to generate the color illusion. It’s the design you can see in the animation before it starts spinning. This is a 60 fps animation that accelerates from zero to 6000 RPM for the first 10 seconds, then spins at a constant rate of 6000 RPM for the final 20 seconds.

I see pastel red, green, and blue streaks in concentric circles on the spinning disc during the first 10 seconds. Then when the disc reaches full speed for the remaining 20 seconds, I see mostly reds and greens but occasional blues. Your mileage may vary. Here’s why:

The effect was noted by both Gustav Fechner and Hermann von Helmholtz. It’s also known as “pattern induced flicker colors” PIFCs. The exact perceptual mechanism is not fully understood. The most popular theory suggests that color receptors (cones) in the human eye have different decay times for excitations by red, green, and blue photons of light, each of which deliver different amounts of energy. And as I’m sure my readers know, white light contains all those colors.

Natural physiological variations in cone sensitivity means that the illusory colors experienced by individuals will vary. So you may not see the colors I see in the illusion, but I’m sure you’ll see at least some colors. Fechner colors can sometimes appear as the blades of a fan, or the spokes of a wheel, accelerate in RPM. I saw hints of the effect when creating these two rotation animations:

The effect was less pronounced because my geometric patterns were not optimized to produce Fechner colors. But what I saw piqued my curiosity, and my searches ultimately led to Benham discs. Fechner colors may not be totally understood, but they’re a real perceptual phenomenon that provides clues into the workings of the eye-brain system.

Next Week in Sky Lights ⇒ Upgrading the Grid

Q&A: Bicycle Wheel Illusion
Upgrading the Grid