Haidinger's brush



Haidinger's brush is an entoptic phenomenon first described by Austrian physicist Wilhelm Karl von Haidinger in 1846.

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. 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 flat panel computer screens, in which case it is often diagonal.

Some arthropods (insects, mantis shrimp), mollusks (cuttlefish, squid, octopuses) and fish are sensitive to polarized light.

Haidinger's brush is usually attributed to the dichroism of the pigment of the macula. 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. As well, the outer layer of the cones is said to be birefringent; this is thought to contribute to the phenomenon. The cornea has also a slight birefringence.

Seeing Haidinger's brush
Many people find it hard 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 your 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.

You can instead use the polarizer that is built into a computer's LCD screen: simply look at a white area on the screen, and slowly tilt your head (a CRT monitor does not involve a polarizer, so it will not work for this purpose unless you look through a separate polarizer).

The accompanying image shows the approximate size of the brush as it appears on a nominal-15" (38 cm) computer monitor located about 18" (46 cm) from the eye. In this example, the polarization is horizontal; the polarizers built into LCD screens are more often diagonally oriented, so the brush would be rotated by 45 degrees with respect to this image. The picture shows the actual intensity of the brush as seen by one observer. Different individuals see it with different degrees of intensity and distinctness, but it is usually very pale.

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. Minneart 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.