Human Polarization Sensitivity

Abstract

Humans can detect the E-vector of incident polarized light using a subtle, transient visual phenomenon known as Haidinger’s brush. The effect is a result of the human macula having the properties of a radial analyser with peak absorption at 460 nm. A number of mechanisms, each capable of generating radial diattenuation, have been proposed: (1) oblique light incident on cone outer segments, (2) form dichroism in the Henle fibre layer (the photoreceptor axons) and (3) a perpendicular arrangement of dichroic carotenoid pigments with respect to the radially oriented Henle fibres. A close correlation between the dichroic ratio of the macula and the optical density spectrum of liposome-bound lutein and zeaxanthin provides strong evidence that macular pigment plays a key role. Corneal birefringence can affect the contrast and perceived angle of the brush, together with the appearance of the phenomenon in circularly polarized light. When the retina is photographed between crossed polarizers, a brush-like pattern is observed; this is a result of the birefringence of the Henle fibre layer and cornea and is distinct from the radial diattenuation that generates Haidinger’s brush. A secondary entoptic phenomenon that allows humans to detect the orientation of polarized light was described by Gundo von Boehm. Boehm’s brush is only visible when a polarized light source rotates in the peripheral visual field against a dark background and results from light scattering off axis into the photoreceptors. Both phenomena allow for the detection of polarized light by the unaided human eye; however, there is no evidence to suggest that such capabilities are adaptive.