Mach bands is an optical illusion named after the physicist Ernst Mach. The illusion consists of light or dark stripes that are perceived next to the boundary between two regions of an image that have different lightness gradients (even if the lightness itself is the same on both sides of the boundary).
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The Mach bands effect is due to the spatial high-boost filtering performed by the human visual system on the luminance channel of the image captured by the retina. This filtering is largely performed in the retina itself, by lateral inhibition among its neurons.
The effect is independent of the orientation of the boundary.
This visual phenomenon is important to keep in mind when evaluating dental radiographs for evidence of decay, in which grayscale images of teeth and bone are analyzed for abnormal variances of intensity. A false-positive radiological diagnosis of dental caries can easily arise if the practitioner doesn't take into account the likelihood of this illusion. Not only do Mach bands manifest adjacent to metal restorations or appliances, as in the radiograph at right, but they can also present at the boundary between enamel and dentin.[1] Mach bands may also result in the misdiagnosis of horizontal root fractures because of the differing radiographic intensities of tooth and bone.[2]