View source for Haidinger's brush

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. 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. 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.

==Physiological causes of Haidinger's brush==
Haidinger's brush is usually attributed to the dichroism of the xanthophyll pigment of the macula. In this Fresnel–Arago laws effect, the unguided oblique rays in the cylindrical geometry of the foveal blue cones, along with their distribution, 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 a result, two different areas of the fovea can be sensitive to two different degrees of polarization.

==Seeing Haidinger's brush==

Simulated appearance of a computer screen viewed through a polarizer, showing typical size and intensity of Haidinger's brush
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. Since 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 (a CRT monitor has no polarizer, and will not work for this purpose unless a separate polarizer is used).

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 recommends practicing first with a polarizer, then trying it without. The areas of the sky with the strongest polarization are those 90 degrees away from the sun. Minnaert says 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 comments 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.



==References==


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==See Also==

[[CCW Female]]

[[CW Male]]



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