Take a look at the following two circles. At the center, they’re both the identical bright white. But which one seems brighter?
Let’s make this a poll:
I’m not sure if this illusion will work when respondents know the objects are the same brightness, but naive viewers will reliably rate the circle on the left as brighter — this is called the “glare effect,” and it occurs whenever there’s a gradual gradient around a circle or other shapes (the gradient must approach the color of the shape as it gets closer to the shape itself). I found the illusion so powerful that I had to close the picture window I was using to create the image while I wrote this post.
To me, the glare effect is so strong that the image feels like it is burning into my retina — even though I know the white background of my word processor is just as bright as the white circle. In fact, when Daniele Zavagno and Giovanni Caputo were first researching the phenomenon, subjects complained that it was very uncomfortable to look at images with the glare effect — they were seeing afterimages, as if a bright light had been flashed in their eyes. But are viewers really seeing stronger afterimages when they experience the glare effect?
Hongjing Lu, Daniele Zavagno, and Zili Liu developed two experiments to test this question. In the first experiment, they showed viewers plain circles and squares, without the glare-effect-causing gradient, for varying periods of time, at two different levels of brightness. When the objects disappeared, viewers were instructed to press a button when they could no longer see an afterimage. While results varied for each individual, they followed the same pattern. Here’s a typical result:
As you can see, the longer the image was presented, the longer the afterimage persisted. And brighter images led to longer afterimages. These results — both statistically significant — were as expected; the experimenters just wanted to confirm that the relationship between afterimages and image viewing time was measurable at the brightness levels their equipment could display.
The procedure for the second experiment was the same, but a different set of images was used:
The image on the left is a control while the image on the right is the glare effect image. The reverse gradient was used for the control to ensure that the overall luminance for control images was similar to the glare effect images. Here’s the surprising result:
The afterimage actually persisted significantly longer when there was no glare effect! Even though the glare effect makes images seem brighter, the apparent increase in brightness had no impact on the afterimage, and in fact actually seemed to diminish the afterimage’s strength.
The authors suggest that the glare effect isn’t actually occurring in the retina or some other low-level visual processing area. Instead, it’s probably a reflex to protect us from looking at truly bright light sources like the sun. The discomfort I feel when looking at the pattern that causes the glare effect is simply the result of an over-ambitious protection mechanism.
Lu, H., Zavagno, D., & Liu, Z. (2006). The glare effect does not give rise to a longer-lasting afterimage. Perception, 35, 701-707.