Haidinger's brush

It typically occupies roughly 3–5 degrees of vision, about twice or three times the width of one's thumb held at arm's length.

Haidinger's brush may also be seen by looking at a white area on many LCD flat panel computer screens (due to the polarization effect of the display), in which case it is often diagonal.

Haidinger's brush is usually attributed to the dichroism of the xanthophyll pigment found in the macula lutea.

As described by the Fresnel laws, the behavior and distribution of oblique rays in the cylindrical geometry of the foveal blue cones produce an extrinsic dichroism.

Look at a white area on the screen, and slowly tilt the head (this method generally works only with LCDs, as most other electronic visual display technologies do not emit polarized light).

People with certain types of strabismus may undergo an adaptation whereupon they look at the object of attention not with their fovea (at the centre of the macula) but with an eccentric region of the retina.

One such apparatus utilises a rotating polarised plate backlit with a bright white light.

The reason for such training is that the healthy fovea is far greater in its resolving power than any other part of the retina.