Illusory Mach Bands result when gradients from darker to lighter shades are created. The thin light and dark bands are illusory.
Below the effect is doubled
Mach Bands also occur with a related circular effect.
However, going from darker to brighter in the center gives a different result.
The phenonemon can occur even when gradients are not continuous.
Notice the the slight glow that extends. Addition a second gradient enhances the effect and almost seems to produce an entire glowing area..
Addition of a third gradient produces an even stronger effect.
Now the glow is striking and the center yellow seems much brighter that the yellow in each corner. Also, there is a difuse but strong subjective contour effect.
Move closer to and further from the screen while looking at the center of the figure. There is a change in apparent brightness. This effect is covered in more detail in the section on Dynamic Luminance.