Tuesday, April 07, 2009

Haidinger's Brush

Wilhelm Von Haidinger discovered a quirky entoptic phenomenon in 1844. Like many other animals (insects, cephalopods) some humans can see the polarization of light. If you stare at the blue sky in the opposite direction from the sun, or at a white space on an LCD and notice some small blue and yellow artifacts, chances are you may have this extra perception. Anyone who's into photography or fishing will know that polarized lenses can help you for example, exclude reflection from the surface of water, allowing you to see what is happening underwater with greater clarity. Those 3D movies that are all the rage these days also use this property by giving you glasses with a different angle of polarization in each eye. It is thought that the underlying reason for this perception may be circular deposition of pigments in the macula. The pigment in question is lutein which is anisotropic and sensitive to the polarization of light. Lutein is also a strong anti-oxidant which gives the yellow colour to egg yolks. It makes we wonder whether this ability is related to a certain gene and if it has been selected for in certain sea-faring populations. Also if anyone has this ability, can you see the movies in 3D without the glasses (I suspect not, unless only one eye is affected)?

Interestingly the visual perception changes in patients afflicted with macular degeneration:
"Haidinger's brushes are an entoptic effect of the human visual system that enables us to detect polarized light. However, individual perceptions of Haidinger's brushes can vary significantly. We find that the birefringence of the cornea influences the rotational motion and the contrast of Haidinger's brushes and may offer an explanation for individual differences. We have devised an experimental setup to simulate various phase shifts of the cornea and found a switching effect in the rotational dynamics of Haidinger's brushes. In addition, age related macular degeneration reduces the polarization effect of the macula and thus also leads to changes in the brush pattern."


Anonymous said...

Are we supposed to see something special in the image?

Anonymous Coward said...

no it's an artistic representation of what it may look like to people who are sensitive to polarized light.

Anonymous said...

It's a decent representation of what I see.

The orientation of the pattern changes based on the polarization. Which is (blue/yellow) vertical, horizontal, or diagonal. Also sometimes the blue parts form the "dumbbell" over the yellow, and sometimes the it appears that the yellow reaches across the blue.

Open an empty browser window, assuming it will be all white and stare at the center for a few moments. If you see whisps of blue or yellow, that's it. Learning to see it is somewhat like learning the night vision practice of averted vision. What I see when I use my LCD this way is about the same size as the image on the web page, perhaps a little smaller.