Haidinger’s Brush is a subtle visual phenomenon that allows certain individuals to perceive polarized light. It appears as a small, yellowish “brush” or hourglass shape, often with fainter bluish or purplish areas, in the center of one’s vision. This optical effect originates within the observer’s own eye. Many people can learn to observe this phenomenon with practice.
How to See Haidinger’s Brush
Observing Haidinger’s Brush requires specific viewing conditions. One effective method involves looking at a uniform blue sky, particularly when facing away from the sun. The sky’s naturally polarized light helps highlight the brush. A white area on an LCD computer screen also works, as these displays inherently emit polarized light.
To make the brush more noticeable, try rotating a linear polarizing filter, such as a lens from polarizing sunglasses, while gazing at the light source. As the filter rotates, the yellowish brush-like shape will appear to rotate in your central vision. Slowly tilting your head from side to side can also produce a similar rotating effect, helping to bring the brush into focus. The brush occupies about 3 to 5 degrees of your visual field, roughly two to three times the width of your thumb held at arm’s length.
The Science of Polarized Light
The perception of Haidinger’s Brush is rooted in the interaction of polarized light with specific structures within the human eye. Light waves, unlike sound waves, vibrate in various planes. When light is polarized, its vibrations are restricted to a single plane. The human eye, particularly the macula, can detect this orientation.
This ability is attributed to the macular pigment, a dense layer of xanthophyll carotenoids like lutein and zeaxanthin, located in the inner retina. These pigment molecules are not randomly arranged; a portion of them are oriented in a circular or radial pattern within the Henle fiber layer, which surrounds the foveola. This specific alignment allows the macular pigment to preferentially absorb blue light (wavelengths around 400-500 nm) that is polarized parallel to the orientation of the pigment molecules. The differential absorption of polarized light by these oriented pigments creates a faint pattern of light and shadow on the retina, perceived as the yellowish-blue Haidinger’s Brush.
Why This Phenomenon Matters
Haidinger’s Brush, while seemingly a mere curiosity, holds practical significance in vision science. Its visibility is directly linked to the density and distribution of macular pigment. Researchers utilize this phenomenon to assess macular pigment optical density (MPOD), a measurement that can indicate macular health.
Changes in the perception of Haidinger’s Brush can provide insights into certain eye conditions. For instance, individuals with macular diseases like age-related macular degeneration (AMD) or macular telangiectasia type 2 may have reduced or absent perception of the brush due to lower macular pigment levels or impaired foveal function. This makes Haidinger’s Brush a simple, non-invasive tool for demonstrating light polarization and potentially assisting in the early detection or monitoring of macular health.