Haidinger's brush
Encyclopedia
Entoptic phenomenon
Entoptic phenomena are visual effects whose source is within the eye itself. In Helmholtz's words:...
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 when looking through polarized sunglasses. It typically occupies roughly 3–5 degrees of vision, about the same size as the tip 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
Liquid crystal display
A liquid crystal display is a flat panel display, electronic visual display, or video display that uses the light modulating properties of liquid crystals . LCs do not emit light directly....
(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 caused by the blue cones, which contain one of the four visual pigments used by human vision xanthophylls (also known as luteinLutein
Lutein is a xanthophyll and one of 600 known naturally occurring carotenoids. Lutein is synthesized only by plants and like other xanthophylls is found in high quantities in green leafy vegetables such as spinach and kale...
). Since xanthophyll is anisotropic (rod shaped), radiation stimulates both electronic and molecular vibrational states, mainly along the molecule direction. That means that xanthophyll is potentially sensitive to the light's polarization. As xanthophyll is responsible for the absorption in the blue range of the light spectrum, the human eye is sensitive to polarization of light in this wavelength.
Haidinger's brush is usually attributed to the dichroism
Dichroism
Dichroism has two related but distinct meanings in optics. A dichroic material is either one which causes visible light to be split up into distinct beams of different wavelengths , or one in which light rays having different polarizations are absorbed by different amounts.The original meaning of...
of the pigment
Pigment
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...
of the macula
Macula
The macula or macula lutea is an oval-shaped highly pigmented yellow spot near the center of the retina of the human eye. It has a diameter of around 5 mm and is often histologically defined as having two or more layers of ganglion cells...
. 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
Fovea
The fovea centralis, also generally known as the fovea , is a part of the eye, located in the center of the macula region of the retina....
is not flat, are almost orthogonal to the fovea in its central part while being nearly parallel in its outer region. As result, two different areas of the fovea can be sensitive to two different degrees of polarization.
Seeing Haidinger's brush
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
Liquid crystal display
A liquid crystal display is a flat panel display, electronic visual display, or video display that uses the light modulating properties of liquid crystals . LCs do not emit light directly....
screen. Look at a white area on the screen, and slowly tilt the head (a CRT
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
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
Marcel Minnaert
Marcel Gilles Jozef Minnaert was a Belgian astronomer.He obtained a PhD in biology at Ghent University in 1914....
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.
Use of Haidinger's brush
The fact that the sensation of Haidinger's brush corresponds with the visual field correlate of the maculaMacula
The macula or macula lutea is an oval-shaped highly pigmented yellow spot near the center of the retina of the human eye. It has a diameter of around 5 mm and is often histologically defined as having two or more layers of ganglion cells...
means that it can be utilised in training people to look at objects with their macula. People with certain types of strabismus
Strabismus
Strabismus is a condition in which the eyes are not properly aligned with each other. It typically involves a lack of coordination between the extraocular muscles, which prevents bringing the gaze of each eye to the same point in space and preventing proper binocular vision, which may adversely...
may undergo an adaptation whereupon they look at the object of attention not with their fovea (at the centre of the macula
Macula
The macula or macula lutea is an oval-shaped highly pigmented yellow spot near the center of the retina of the human eye. It has a diameter of around 5 mm and is often histologically defined as having two or more layers of ganglion cells...
) but with an eccentric region of the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
. 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
Macula
The macula or macula lutea is an oval-shaped highly pigmented yellow spot near the center of the retina of the human eye. It has a diameter of around 5 mm and is often histologically defined as having two or more layers of ganglion cells...
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
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
.
Further reading
- W. Haidinger: Über das directe Erkennen des polarisirten Lichts und der Lage der Polarisationsebene. Poggendorfs Annalen, Bd. 63, 1844, S. 29-39 (Original communication at the Bibliothèque nationale de FranceBibliothèque nationale de FranceThe is the National Library of France, located in Paris. It is intended to be the repository of all that is published in France. The current president of the library is Bruno Racine.-History:...
.) - W. Haidinger: Beobachtung der Lichtpolarisationsbündel im geradlinig polarisirten Lichte. Poggendorfs Annalen, Bd. 68, 1846 S. 73-87 (Original communication at the Bibliothèque nationale de FranceBibliothèque nationale de FranceThe is the National Library of France, located in Paris. It is intended to be the repository of all that is published in France. The current president of the library is Bruno Racine.-History:...
.) - 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.
- Fairbairn, Maxwell B. (2001). Physical Models of Haidingers Brush. Journal of the Royal Astronomical Society of Canada. 95, 248–251.