Weird looking step wedge


I have never given much thought to step wedges used for checking monitor calibration and profiles. Yet now I noticed that the grey-colored steps look themselves like little gradients: always lighter on the left side and darker on the right. For example this.

The far left and right sides are always completely black and white, no gradient effect. Any step block inbetween looks like little gradient with lighter left and darker right side.

What is it? Distortion rooted in the LCD technology or some physiological visual effect?

Could you please link to the actual article? @Elle no longer frequents this forum but feel free to email her.

It’s an optical illusion, a “physiological visual effect”; see Mach bands - Wikipedia. If you download the image and examine the pixel values, you will see they are actually constant within each step. If you mask out adjacent steps so you can see only one step, it doesn’t seem to be a gradient.


Yep, that seems to be it, thanks!
I examined the pixel values of linked step wedge before posting and also tried to search for such effect, but not being native English speaker apparently failed.

I made real setup of 3 grey paper cuts adjacent to each other but to be honest I didn’t such effect. Also I don’t see the effect when only two grey steps are adjacent on LCD monitor. The stacked steps on Wikipedia article seem to have only the slightest amount of effect, but the larger static image of vertical strips looks exactly as “bad” as the linked step wedge. Just speaking out loud what I observe, next person might see differently…