Haidinger's brush

Haidinger's brush, more commonly known as Haidinger's brushes is an image produced by the eye, an entoptic phenomenon, first described by Austrian physicist Wilhelm Karl von Haidinger in 1844. Haidinger saw it when he looked through various minerals that polarized light.^[1][2]

Many people are able to perceive polarization of light.^[3] Haidinger's brushes 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 approximately 90° from the sun,^[4] or on any bright background. Haidinger's yellow bar is observed perpendicular to the direction of light polarization (i.e., if light polarization is vertical, then Haidinger's yellow bar is observed horizontal - like around dawn and dusk, the natural skylight maximum polarization around the north and south horizon is almost vertical as per Rayleigh sky model and so Haidinger's yellow bar is observed horizontal). The perception of this effect vanishes within few seconds due to neural adaptation unless the observer rotates the eyes (like by moving the head sideways).^[3] It typically occupies roughly 3–5 degrees of vision, about twice or three times the width of one's thumb held at arm's length. 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.

Haidinger's brush is usually attributed to the dichroism of the xanthophyllpigment found in the macula lutea. As described by the Fresnel laws, the behavior and distribution of oblique rays in the cylindrical geometry of the foveal blue cones produce an extrinsic dichroism. The size of the brush is consistent with the size of the macula.

It is thought that the macula's dichroism arises 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 the retinal ganglion cell axons that (because the fovea is not flat), are almost orthogonal to the fovea in its central part but nearly parallel in its outer region. As a result, two different areas of the fovea can be sensitive to two different degrees of polarization.^[5]

Many people find it difficult to see Haidinger's brush initially. It is very faint, much more so than generally indicated in illustrations, and, like other stabilized images, tends to appear and disappear.

It is most easily seen when it can be made to move. Because it is always positioned on the macula, there is no way to make it move laterally, but it can be made to rotate, by viewing a white surface through a rotating polarizer, or by slowly tilting one's head to one side.

To see Haidinger's brush, start by using a polarizer, such as a lens from a pair of polarizing sunglasses. Gaze at an evenly lit, textureless surface through the lens and rotate the polarizer.

An option is to use the polarizer built into a computer's LCD screen. Look at a white area on the screen, and slowly tilt the head (this method generally works only with LCDs, as most other electronic visual display technologies do not emit polarized light).

It appears with more distinctness against a blue background. With practice, it is possible to see it in the naturally polarized light of a blue sky. Minnaert recommended practicing first with a polarizer, then trying it without.^[6] The areas of the sky with the strongest polarization are those 90 degrees away from the sun. Minnaert said that after a minute of gazing at the sky, "a kind of marble effect will appear. This is followed shortly by Haidinger's brush." He commented that not all observers see it in the same way. Some see the yellow pattern as solid and the blue pattern as interrupted, as in the illustrations on this page. Some see the blue as solid and the yellow as interrupted, and some see it alternating between the two states.

El hecho de que la sensación del pincel de Haidinger se corresponda con el campo visual de la mácula significa que puede utilizarse para entrenar a las personas a mirar objetos con su mácula. Las personas con ciertos tipos de estrabismo pueden experimentar una adaptación en la que miran el objeto de atención no con su fóvea (en el centro de la mácula) sino con una región excéntrica de la retina . Esta adaptación se conoce como fijación excéntrica . Para ayudar a entrenar a una persona a mirar un objeto con su fóvea en lugar de su zona retiniana excéntrica, se puede utilizar un dispositivo de entrenamiento. Uno de estos aparatos utiliza una placa polarizada giratoria retroiluminada con una luz blanca brillante. Usando gafas azules (para realzar la imagen del pincel de Haidinger) y un oclusor sobre el otro ojo, se espera que el usuario note el pincel de Haidinger donde su mácula se correlaciona con su campo visual. El objetivo del entrenamiento es que el usuario aprenda a observar el objeto de prueba de forma que el pincel de Haidinger se superponga al objeto (y, por lo tanto, el observador lo mire con su fóvea). La razón de este entrenamiento es que la fóvea sana tiene una capacidad de resolución mucho mayor que cualquier otra parte de la retina. Otro método de diagnóstico que utiliza las propiedades birrefringentes del tejido retiniano es el escaneo de birrefringencia retiniana , que puede utilizarse en casos de ambliopía grave o cuando el especialista no cuenta con la colaboración del paciente.

• Flotador
• flequillo de Haidinger
• tanque de aislamiento
• Cine de prisioneros

• W. Haidinger: Beobachtung der Lichtpolarisationsbündel im geradlinig polarisirten Lichte. Poggendorfs Annalen, Bd. 68, 1846 S. 73-87 (Original communication at the Bibliothèque nationale de France.)
• Alcoz, J. "Haidinger's Brush". Polarization.com. Retrieved 2006-03-14.
• Fairbairn, Maxwell B. (2001). "Physical Models of Haidinger's Brush". Journal of the Royal Astronomical Society of Canada. 95: 248. Bibcode:2001JRASC..95..248F.
• Minnaert, M. G. J. (1993) Light and Color in the Outdoors. (translated by Len Seymour from the 1974 Dutch edition). ISBN 0-387-97935-2, Springer-Verlag, New York.
• Misson, G. P. (1993). "Form and behaviour of Haidinger's brushes". Ophthalmic & Physiological Optics. 13 (4): 392–396. doi:10.1111/j.1475-1313.1993.tb00497.x. PMID 8278193. S2CID 23035412.
• Misson, G. P. (2003). "A Mueller matrix model of Haidinger's brushes". Ophthalmic & Physiological Optics. 23 (5): 441–447. doi:10.1046/j.1475-1313.2003.00138.x. PMID 12950890. S2CID 6543779.
• William Shurcliff (1955). "Los pinceles de Haidinger y la luz polarizada circularmente". Journal of the Optical Society of America . 45 (5): 399. Bibcode : 1955JOSA...45..399S . doi : 10.1364/JOSA.45.000399 .
• Beenakker, C. (junio de 2018). "La mariposa de Haidinger: nuestro sexto sentido" . Nederlands Tijdschrift voor Natuurkunde .

• Cómo ver la polarización a simple vista