The reason why incomplete cleaning may increase the visual perception of dirt is by increasing the contrast between the patches of thicker grime and the normal colour by removing a uniform covering of thinner dirt. If something is uniformly grimy, then the colour of the covering dirt may be perceived as the thing’s normal hue. Cleaning may remove approximately the same thickness of dirt from all points on the surface. If some patches initially have a thicker layer, then these remain the colour of the dirt after the cleaning, but other areas may be fully cleaned and revert to the original look of the surface. The human visual system mostly perceives contrast, not the absolute wavelength of the reflected light, as various optical illusions demonstrate. Higher contrast between the thicker patches of grime and the rest of the surface then enhances the perception of dirtiness.