Mach bands

What can you see? The Mach Bands are named after the physicist Ernst Mach (1830-1916), who was also interested in psychological experiments on the sense of balance and optical illusions.

In the case of adjacent areas with graduated grey values, these bands can be seen at the borders, which increase the contrast between the areas (left picture). The effect disappears as soon as the areas are no longer in direct contact (right picture).

The perception of the actually non-existent bands has long been interpreted as the result of lateral inhibition. Today, however, there are more differentiated interpretations (see Wallis & Georgeson, 2012).

What can you do? With n_bands the number of adjacent areas, with dividing_line the width of the space between them is determined. colour_hue, brightness and saturation can be used to define the colour values. Please note: colour changes are only visible with a saturation > 0! What influence has the number n_bands of adjacent areas? or the spacing between them or their height? What is the smallest distance dividing_line at which the effect disappears? Do colours make a difference?

Mach’s experiment: Mach carried out his experiments with cylinders, which he covered with strips of different lengths. When they rotated quickly, he obtained graduated brightnesses in the form of a ramp between the evenly coloured edge strips. The visual impression, however, is that there is no clear edge at the transitions to the ramp, but a more or less wide band – darker at the left edge and lighter at the right edge of the transition (the transitions are marked here with arrows ).

What can you do? Again with dividing_line the width of the space between the two strips is determined. colour_hue and saturation applies to both evenly coloured edge strips. brightness_left and brightness_right apply differently to these strips and these determine the steepness of the ramp in between. What is the smallest difference in brightness, that makes the effect still visible? Does colour_hue influence its visibility?

Related topics: Hermann grid illusion, Mach discs, flicker colors

Wallis, S.A. & Georgeson, M.A. (2012). Mach bands and multiscale models of spatial vision: The role of first, second, and third derivative operators in encoding bars and edges. Journal of Vision, Vol.12, 18. Available as download.