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Mach Band Illusion 

The eye accentuates abrupt changes in brightness, so that regions of constant brightness near the edges appear to have varying brightness.

Note that the gray level bands, both in the LUT window and in the image window, appear to be shaded or have a "Venetian Blind" appearance.

Despite their appearance, the bands each consist of only one gray level.

Note at this point, that the change is in the LUT window, and not in the image itself. The image has been changed in ImageJ

(Alternatively, use the line selction tool , then the Analyze -> Plot Profile menu.

Note - the line selection tool  and Analyze / Plot Profile are available in ImageJ.  The line profile has steps, as shown in the last plot, below.

The resulting profile (in the Plot window) shows that the image really has continuous gray levels:

 

 

Now the image is unchanged in appearance, but the LUT is a gray level ramp:

and the profile shows steps, as expected:

The profile shows that the intensity across any band is constant, yet each band appear darker on the left side than on the right. This is the Mach Band illusion.