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
Wilhelm Karl Ritter von Haidinger (or Wilhelm von Haidinger, or most often Wilhelm Haidinger) (5 February 179519 March 1871) was an Austrian mineralogist.
Early life
Haidinger's father was the mineralogist Karl Haidinger (1756–1797), who died wh ...
in 1844. Haidinger saw it when he looked through various minerals that
polarized light.
Many people are able to perceive polarization of light.
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 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
Haidinger's brush is usually attributed to the
dichroism of the
xanthophyll
Xanthophylls (originally phylloxanthins) are yellow pigments that occur widely in nature and form one of two major divisions of the carotenoid group; the other division is formed by the carotenes. The name is from Greek (, "yellow") and (, "lea ...
pigment
A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic comp ...
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
visive nerves that (because the
fovea
Fovea () (Latin for "pit"; plural foveae ) is a term in anatomy. It refers to a pit or depression in a structure.
Human anatomy
* Fovea centralis of the retina
* Fovea buccalis or Dimple
* Fovea of the femoral head
*Trochlear fovea of the f ...
is not flat), are almost
orthogonal
In mathematics, orthogonality is the generalization of the geometric notion of '' perpendicularity''.
By extension, orthogonality is also used to refer to the separation of specific features of a system. The term also has specialized meanings in ...
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. 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
A liquid-crystal display (LCD) is a flat-panel display or other electronically modulated optical device that uses the light-modulating properties of liquid crystals combined with polarizers. Liquid crystals do not emit light directly but in ...
screen. Look at a white area on the screen, and slowly tilt the head (a
CRT
CRT or Crt may refer to:
Science, technology, and mathematics Medicine and biology
* Calreticulin, a protein
*Capillary refill time, for blood to refill capillaries
*Cardiac resynchronization therapy and CRT defibrillator (CRT-D)
* Catheter-re ...
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 recommended practicing first with a polarizer, then trying it without.
[Minnaert, M. G. J. (1940). Light and colour in the open air (H. M. Kremer-Priest, Trans.). London: G. Bell and Sons.] 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 (at the centre of the macula) but with an eccentric region of the
retina
The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...
. 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 (to enhance the Haidinger's brush image) and an
occluder 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 (and the viewer is thus now looking at it with their fovea). 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.
See also
*
Floater
*
Haidinger fringe
*
Isolation tank
*
Prisoner's cinema
References
Further reading
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.)
*
*
* Minnaert, M. G. J. (1993) ''Light and Color in the Outdoors.'' (translated by Len Seymour from the 1974 Dutch edition). , Springer-Verlag, New York.
*
*
*
*
External links
How to see polarization with the naked eye
{{DEFAULTSORT:Haidinger's Brush
Polarization (waves)
Vision