Haidinger's brush is an entoptic phenomenon first described by Austrian physicist Wilhelm Karl von Haidinger in 1844.
Many people are able to perceive polarization of light. It may be seen as a yellowish horizontal bar or bow-tie shape (with "fuzzy" ends, hence the name "brush") visible in the center of the visual field against the blue sky viewed while facing away from the sun, or on any bright background when looking through polarized sunglasses. It typically occupies roughly 3–5 degrees of vision, about the same size as the tip of one's thumb held at arm's length. The direction of light polarization is perpendicular to the yellow bar (i.e. vertical if the bar is horizontal). Fainter bluish or purplish areas may be visible between the yellow brushes (see illustration). Haidinger's brush may also be seen by looking at a white area on many LCD flat panel computer screens (due to the polarization effect of the display), in which case it is often diagonal.
Some arthropods (insects, mantis shrimp), mollusks (cuttlefish, squid, octopi) and fish are sensitive to polarized light.
Physiological causes of Haidinger's brush
Haidinger's brush is due to the blue cones which contain one of the four visual pigments used by human vision xanthophylls (also known as lutein). Xanthophyll is anisotropic (rod shaped) as a result radiation stimulates both electronic and molecular vibrational states, mainly along the molecule direction. That means that xanthophyll is potentially sensitive to the polarization of the light. As xanthophyll is responsible for the absorption in the blue range of the light spectrum, this explains why this is the wavelength for which the human eye is sensitive to polarization of light.
Haidinger's brush is usually attributed to the dichroism of the pigment of the macula. In this Fresnel–Arago laws effect the unguided oblique rays in the cylindrical geometry of the blue cones in the fovea along with their distribution to produce an extrinsic dichroism. The brush's size is consistent with the size of the macula. The macula's dichroism is thought to arise from some of its pigment molecules being arranged circularly. The small proportion of circularly arranged molecules accounts for the faintness of the phenomenon. Xanthophyll pigments tend to be parallel to visive nerves that because the fovea is not flat, are almost orthogonal to the fovea in its central part, while being nearly parallel in its outer region. As result, two different areas of the fovea can be sensitive to two different degrees of polarization.
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