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 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. Fainter bluish or purplish areas may be visible between the yellow brushes. Haidinger's brush may also be seen by looking at a white area on many LCD flat panel computer screens, in which case it is often diagonal.
Physiological causes
Haidinger's brush is usually attributed to the dichroism of the xanthophyllpigment found in the macula lutea. Pursuant to the Fresnel laws, the behavior and distribution of unguided 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; Xanthophyll pigments tend to be parallel to visive nerves that, 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.
Seeing 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. 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. 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.
Use
The fact that the sensation of Haidinger's brush corresponds with the visual field of the macula means that it can be utilised in training people to look at objects with their macula. People with certain types of strabismus may undergo an adaptation whereupon they look at the object of attention not with their fovea but with an eccentric region of the retina. This adaptation is known as eccentric fixation. To aid in training a person to look at an object with their fovea rather than their eccentric retinal zone, a training device can be used. One such apparatus utilises a rotating polarised plate backlit with a bright white light. Wearing blue spectacles and an over the other eye, the user will hopefully notice the Haidinger's brush where their macula correlates with their visual field. The goal of the training is for the user to learn to look at the test object in such a way that the Haidinger's brush overlaps the test object. The reason for such training is that the healthy fovea is far greater in its resolving power than any other part of the retina. Another diagnostic method that utilises birefringent properties of the retinal tissue is retinal birefringence scanning, that can be used in case of severe amblyopia or when the specialist lacks a cooperation from the patient.
