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The phenomenon owes its name to Wilhelm Karl Ritter von Haidinger, who first described it between 1844 and 1854. Haidinger's brush is a color pattern in the center of some people's field of vision, indicating the direction of polarization.
The shape is usually a yellow dumbbell with fuzzy ends, or a blue or purple circle in the center, also fuzzy. It appears against a bright background, occupying about 3-5 degrees of the field of view when the light is polarized, such as when looking at a screen facing away from the sun or an LCD screen. Its formation is attributed to the dichroic character of a certain xanthophyll pigment of the yellow spot. The brush is to scale of the yellow spot.
Their sensitivity is explained by the fact that some of the visual pigments are circular. Also, it is pale because there are relatively few visual pigments. Many people have a hard time recognizing her at first, as she is much paler than the pictures show. It appears and disappears compared to other similar forms. Therefore, it is easier to see when moving. You can't move it horizontally, but you can rotate it.
To do this, you need to slowly turn the head to the side or rotate a polarizer in front of a white surface. Next, hold the polarizer against an unpatterned surface and look through the swarm evenly, without straining your eyes. In front of an LCD screen, the head slowly tilts to the side, but old-style screens are not suitable. It is more observable on a blue background, so looking at the sky can produce the phenomenon. Places at right angles to the Sun are the most polarized areas of the sky. Minnaert says that if you look at the sky for a minute, you will see a marble effect and then the phenomenon itself. It is described as non-uniform in appearance, with some saying that the blue is continuous and the yellow is interrupted. Others say that the phenomenon is reversed and some have two states.
Since the brush belongs to the yellow spot, it allows you to see where the yellow spot is. Eccentric fixation is the phenomenon by which the eye adapts to look not with the yellow dot but with the outer part of the retina, another type of strabismus. Based on this, the Haidinger brush can be used to learn to look with the yellow dot.
Another use is when the machine emits white light, before turning the polarizer. The subject looks through blue glasses with one eye covered and the other fully covered. The purpose of this is to make the patient recognize Haidinger's brush, that is, the yellow spot. Subsequently, the test subjects are looked at for the brush to appear on them. This exercise improves vision resolution because the pins are more densely located in the yellow spot. Another method takes advantage of the birefringence of the retina, which can be used in cases of severe strabismus or in uncooperative patients.
Written by Dezső Sándor.