Haidinger´s Brush: A Little Known Polarization Sense in Human Vision

Jorge Alcoz, Ph..D., www.polarization.com/

Polarization is a fundamental property of light with as much significance as color and intensity. However, we are generally completely oblivious to it in spite of being surrounded by partially polarized light. But, thanks to a small aberration or "defect" of the eye, we are not completely polarization-blind.  

In fact, without the help of any instrument many people can learn to see a pattern known as Haidinger's brush that reveals not only if light is strongly polarized or not but also if it is linearly polarized or circularly polarized and in which direction it vibrates or rotates. This ability is apparently common, for some researchers have reported that all of their healthy subjects have been able to see the Haidinger pattern in laboratory conditions. But more than a century and a half after its discovery, this phenomenon remains little known even by scientists working in optics!

Experiments and Studies for the Amateur Scientist

The exact physiological basis of this entoptic phenomenon is not yet settled, but investigators have discovered some good clues, as described later. Although this capability should be widespread, I am not aware of any population studies on its subjective manifestation. Some open questions that could be easy studied by amateur scientists include the following.

Does this capacity change in strength and frequency between people of different ethnicity, age, etc.?

How easy is it to train a person to see Haidinger's brush? (The late optical scientist M. G. J. Minnaert seemed to had acquired an exceptionally vivid sense of it.)

What are the ranges in the appearance of the pattern's color and shape?

There is also the possibility of doing experiments under different light stimuli that would help weed out the diverse theories that have been proposed. Moreover, almost all studies have been on linear polarization, with scarcely any on the circular polarization phenomenon. Besides the research potential of the Haidinger´s Brush, I think that one may obtain some degree of gratification in developing this “polarization sense.”

Discovery of the Phenomenon

Wilhelm K. von Haidinger (1795-1871), an Austrian mineralogist and geologist, made several important contributions to the science of optics, including the first report of dichroism for circular polarization (in amethysts). In 1846 Haidinger was studying minerals under polarized light, carefully trying to discern any special pattern in the refracted light. He perceived a faint yellowish stain or brush that remained when he looked directly at the light without interposing the crystal. The brush rotated together with the polarizer, thus, proving that he was "seeing" the polarization state. That stain is now known as the Haidinger's Brush.

In 1954, William Shurcliff, who was then employed by Polaroid, pointed out that the Haidinger's brush can also detect circular polarized light and distinguish the sense of rotation.

The Brush

Observers generally describe the Haidinger's brush as a diffuse, elongated, yellowish pattern that is pinched at the center. Bluish leaves, generally shorter, cross it at 90 degrees. Some people see the yellow continuously until color fatigue makes the blue continuous. Others claim that the continuous color is whichever is perpendicular to the line joining the eyes. Some only see the yellow part (but I generally notice the blue first). The pattern is considerably more diffuse and fainter than shown in the drawings (Figs. 1-3) and requires some practice to recognize.

The yellow branches point in a direction perpendicular to the vibration plane for linearly polarized light. Thus, horizontal polarization (Fig. 1) causes a vertical (with respect to the ground) yellow brush irrespective of the inclination of the head. On the other hand, circularly polarized light generates a yellow brush slanted with respect to the line bisecting the face, even if the head leans sideways. For right-handed circularly polarized light (Fig. 2) it will go up to the right and down to the left, while the contrary will happen for left-handed light (Fig. 3). This is true for both the right and left eye: no bilateral symmetry here (although there is some change between the eyes in the exact angle the brushes make, which depends on the individual). The way of distinguishing between circular polarized light and slanted linearly polarized light is, of course, to lean the head sidewise and note if the brush is fixed with respect to the source of light or to the eyes.

The Haidinger's brush is relatively small, occupying 3 to 5 degrees (about the size of the figures at arms length on a standard monitor).

A Short Explanation

In 1866 H. von Helmoltz related the effect to dichroism of a pigment of the retina. This early explanation has proven well founded, although the exact mechanism is complicated and a full explanation is still lacking.

Although the brush is centered on the center of the visual field, its size is much larger than the fovea, the region of the retina where we see images with the highest resolution. Thus, the phenomenon must originate in the surrounding macular area of the retina. The fact that it has color means that the cones (and not the rods) are involved. The brush is invisible in the red, a region where the macula pigment is transparent.

The main suspect is the pigment lutein, which is a long chain molecule that absorbs light polarized with the electric vector parallel to the molecular axis more than the perpendicular polarization. That lutein is dichroic is not enough; the molecules should be aligned for the effect not to average out. A partial alignment of the molecules as concentric circles around the fovea will do, as shown in Fig. 4.

If, for example, light is polarized vertically, sector A will have the pigment molecules on average perpendicular to the light vibration, absorbing little light. The reverse is true for sector B. Thus, a four-leaf pattern similar to the Haidinger brush would be produced. In the example, sector A would be blue and sector B yellow.

The macular pigment is located between the outer and inner limiting membranes of the retina, a region containing radially arranged nerve fibers. This would explain the partial alignment of the molecules. The percentage of molecules aligned is small, explaining the weakness of the effect.

This explanation is not complete, as a detailed simulation does not exactly reproduce Haidenger's brush. The outer layer of the cones is birefringent, which undoubtedly contributes to the effect, and more studies are needed. Interestingly, the cornea also has a slight birefringence, with the slow axes generally slanted 20 to 30 degrees down towards the nose. This would create two preferential directions for detection of linear polarization.

Photographs of the retina with crossed polarizers in the stimulating and recording light paths reveal a brush pattern over the macula. This pattern occupies the same retinal area where Haidinger brushes are seen.

Some hints on how to observe the Haidinger brush can be found in my web article http://www.polarization.com/haidinger/haidinger.html

References

Minnaert, M. G. J, “Light and Color in the Outdoors” (translated by Len Seymour from the 1974 Dutch edition), Springer-Verlag, New York, 276-278, 1993.

Hoechlheimer B. F. and Kues H A, “Retinal polarization effects,” Applied Optics 21, 3811-3818, 1982.

Dodt E. and Kuba M., “Visually evoked potentials in response to rotating plane-polarized blue light,” Ophthal. Res. 22, 1319-1322, 1990.

Horváth G. and Varjú D., “Polarized Light in Animal Vision ,” Springer-Verlag, Berlin., 355-361, 2004.