Haidinger's brush

Haidinger's brush is an entoptic phenomenon in which polarized light produces a faint yellow bow-tie shape with blue ends in the visual field. It arises from the eye's direction-dependent absorption of polarized light by macular pigments.

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Macular pigment: Yellow carotenoid pigments concentrated in the central retina, especially lutein and zeaxanthin. Their direction-dependent absorption helps create the yellow and blue pattern.
Wilhelm von Haidinger: An Austrian mineralogist who studied optical phenomena and described the visual effect now named after him. His account gave the phenomenon its enduring name.
Polarimeter: An instrument that measures the orientation or rotation of polarized light. It can characterize the polarized stimulus used to elicit the brush.
Blue field entoptic phenomenon: A perception of moving bright dots caused by leukocytes flowing through retinal capillaries. It is another retinally generated visual effect, but it appears as moving dots rather than a polarization-aligned bow tie.
Polarization (waves): The orientation of oscillations in a transverse wave, such as light. Rotating the polarization direction rotates the brush's bow-tie pattern.
Polarization of light: The physical condition in which light's transverse electric field has a preferred orientation. Experiments with polarized light established the stimulus that produces the brush.
Polarized sunglasses: Eyewear containing a filter that selectively blocks light with a particular polarization orientation. Their lenses can alter or reveal the brush when the wearer views a bright, uniform field.
Purkinje tree: An entoptic image of retinal blood vessels produced by shifting illumination across the retina. It reveals retinal anatomy, whereas Haidinger's brush reflects polarization-dependent pigment absorption.
Dichroism: The unequal absorption of light depending on its polarization direction. Macular pigments absorb polarized light differently along different retinal axes.
Brewster's angle: The incidence angle at which reflected light is completely polarized for an ideal dielectric interface. Reflected light near this angle can provide a convenient source of strong linear polarization.