Haidinger's brushes: Perceiving the polarization of light via an entoptic phenomenon.
Ruffato G., Mottes J., Ortolan D.
Entoptic phenomena are visual artifacts arising from the interaction of light with the specific physical structure of the eye. While they are too subtle to actually enable additional visual abilities, their perception can provide indirect information on the physiological conditions of the visual system. Among the most famous ones, Haidinger's brushes consist in the appearance of a yellowish bow tie perceived in the presence of linearly polarized white light and arise from the particular spatial distribution of dichroic carotenoid molecules forming a sort of embedded radial polarizer in the macula. We have developed a compact versatile optical setup for the psychophysical analysis of the perceptual threshold of such entoptic effect. The tests on a group of 113 healthy individuals under conditions of maximum contrast (blue light) reveal the human capability to perceive an average polarization degree around $16%$. The same analyses in white light suggest a polarization sensitivity around $55%$. The developed prototype outlines a new platform to train the user in the perception of polarization and can help infer the condition of macular pigments allowing for early diagnosis of diseases.