Vision-Demo: Purkinjes Blue Arc Phenomenon
Purkinjes Blue Arc is a cool and not well known visual effect. It consists of illusionary blue arcs, emanating from a (typically) red stimulus. It has been rediscovered at least half a dozen times in the last 200 years and goes back to Purkinje. The exact physiological reason for the Blue Arcs is still not now. A detailed write-up of the demonstration with more tipps can be found in Moreland 1967.
Modern screen technology make it much easier for you, to experience this effect. Just follow these simple instructions!
Purkinjes Blue Arc Recipe
- You need an OLED screen – ~50% of modern mobile phone screens are OLED. To check, go to a dark room and open a complete black image. Is the display pitch black (OLED), or is there backlight coming through (LCD)?
- Display this gif in fullscreen – you probably need to download it. No other UI-elements should be visible
- Go to a reasonable dark room
- Close the left eye and stare at the red dot
- Purkinjes Blue Arcs should appear
It is hard to see the blue arcs if you do not know what to look for, therefore I added a small visualization
I can’t see it?!
- Maybe you flipped your phone (or closed the wrong eye ;)) – be sure that the straight line points to the right
- Maybe the room is still too bright. Also be sure that the red-color of the stimulus doesnt brighten up the background of the room
- I don’t have an OLED screen: The demonstration should also work with just a red dot – maybe you have one at your stereo? Any bright red LED should work, the effect is smaller but still there. Your mileage on an LCD screen will vary…
What’s going on?
The nerve fibers from the photoreceptors are bundled and leave the retina through the optic discs. These nervebundles are called the Raphe. As visible in the next screen, Purkinjes Blue Arc follow the raphe perfect.y
Why exactly such a red bright stimulus activates the blue ganglio cells is currently not know.
I experienced the Purkinje blue/purple arcs as a child. I saw that the size of the arcs depended on where I looked at the light source; the arcs originated at the source and converged at my blind spots. At first I thought they were just eyeball reflections, but experiments with colored lights yielding the same color arcs suggested it was nerve crosstalk and not optical reflections. Each stimulus lasts under two seconds, but slight shifting of my direction or blinking my eyes regenerates them. At 78 years old I still see them, though they are slightly weaker.
For decades I though maybe I had discovered the phenomenon, but an article in Scientific American around 1980 ended that notion. Purkinje had studied it in the early 19th century!