Mach bands
Encyclopedia
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
gradient
s (even if the lightness itself is the same on both sides of the boundary).
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
. Mach bands may also result in the misdiagnosis of horizontal root fractures because of the differing radiographic intensities of tooth and bone.
Optical illusion
An optical illusion is characterized by visually perceived images that differ from objective reality. The information gathered by the eye is processed in the brain to give a perception that does not tally with a physical measurement of the stimulus source...
named after the physicist Ernst Mach
Ernst Mach
Ernst Mach was an Austrian physicist and philosopher, noted for his contributions to physics such as the Mach number and the study of shock waves...
. The illusion consists of light or dark stripes that are perceived next to the boundary between two regions of an image
Image
An image is an artifact, for example a two-dimensional picture, that has a similar appearance to some subject—usually a physical object or a person.-Characteristics:...
that have different lightness
Lightness
Lightness is a philosophical concept most closely associated with continental philosophy and existentialism, which is used in ontology. The term "lightness" varies in usage but is differentiated from physical weight, such as "the lightness of balsa wood"...
gradient
Gradient
In vector calculus, the gradient of a scalar field is a vector field that points in the direction of the greatest rate of increase of the scalar field, and whose magnitude is the greatest rate of change....
s (even if the lightness itself is the same on both sides of the boundary).
Explanation
The Mach bands effect is due to the spatial high-boost filteringUnsharp masking
Unsharp masking is an image manipulation technique, often available in digital image processing software.The "unsharp" of the name derives from the fact that the technique uses a blurred, or "unsharp," positive to create a "mask" of the original image...
performed by the human visual system on the luminance
Luminance
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square...
channel of the image captured by the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
. This filtering is largely performed in the retina itself, by lateral inhibition
Lateral inhibition
In neurobiology, lateral inhibition is the capacity of an excited neuron to reduce the activity of its neighbors. Lateral inhibition sharpens the spatial profile of excitation in response to a localized stimulus.-Sensory inhibition:...
among its neurons.
The effect is independent of the orientation of the boundary.
Mach bands in radiology
Dental radiography
Dental radiographs, commonly referred to as X-ray films, or informally, X-rays, are pictures of the teeth, bones, and surrounding soft tissues to screen for and help identify problems with the teeth, mouth, and jaw. X-ray pictures can show cavities, cancerous or benign masses, hidden dental...
for evidence of decay, in which grayscale
Grayscale
In photography and computing, a grayscale or greyscale digital image is an image in which the value of each pixel is a single sample, that is, it carries only intensity information...
images of teeth and bone are analyzed for abnormal variances of intensity. A false-positive radiological diagnosis of dental caries
Dental caries
Dental caries, also known as tooth decay or a cavity, is an irreversible infection usually bacterial in origin that causes demineralization of the hard tissues and destruction of the organic matter of the tooth, usually by production of acid by hydrolysis of the food debris accumulated on the...
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
Tooth enamel
Tooth enamel, along with dentin, cementum, and dental pulp is one of the four major tissues that make up the tooth in vertebrates. It is the hardest and most highly mineralized substance in the human body. Tooth enamel is also found in the dermal denticles of sharks...
and dentin
Dentin
Dentine is a calcified tissue of the body, and along with enamel, cementum, and pulp is one of the four major components of teeth. Usually, it is covered by enamel on the crown and cementum on the root and surrounds the entire pulp...
. Mach bands may also result in the misdiagnosis of horizontal root fractures because of the differing radiographic intensities of tooth and bone.
See also
- AcutanceAcutanceIn photography, acutance is the edge contrast of an image. Acutance is related to the amplitude of the derivative of brightness with respect to space...
- Cornsweet illusionCornsweet illusionThe Cornsweet illusion, also known as Craik–O'Brien–Cornsweet illusion and Craik–Cornsweet illusion, is an optical illusion that was described in detail by Tom Cornsweet in the late 1960s...
- Hermann grid illusion
- Lateral inhibitionLateral inhibitionIn neurobiology, lateral inhibition is the capacity of an excited neuron to reduce the activity of its neighbors. Lateral inhibition sharpens the spatial profile of excitation in response to a localized stimulus.-Sensory inhibition:...
Further reading
- Eagleman, DM (2001) "Visual Illusions and Neurobiology." Nature Reviews NeuroscienceNature Reviews NeuroscienceNature Reviews Neuroscience is a review journal covering neuroscience. Its 2008 impact factor is 25.94. The journal covers the following areas:*Biochemical signalling in neurons*Cellular and molecular neuroscience*Neural development...
. 2(12): 920-6.