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Stomatopods and DVDs

Odontodactylus scyllarus

Photo: Roy Caldwell

Sometimes, the study of basic biology can lead to technological advances, and a recent discovery about the vision of mantis shrimp is a perfect example, providing insight that could help us improve the technology inside DVD players. What is the connection? Circularly polarized light!

You're probably familiar with linearly polarized light. Fishermen often wear polarized sunglasses to reduce the glare from the water and make it easier to see the fish. Typically a ray of light vibrates randomly in all planes, referred to as e-vectors. When light reflects off water at a certain angle, only waves with certain e-vectors are reflected. A linear polarizing filter can be oriented to block those waves, allowing us to see the rest of the light that has passed through the water and is reflected by the fish below. But light can also be circularly polarized, travelling like a corkscrew, twisting either clockwise or counter-clockwise. We can’t see this property of light, but there is one animal that can!

Odontodactylus scyllarus is a stomatopod, or mantis shrimp, living in the Great Barrier Reef. Stomatopods have the most complex eyes in the animal kingdom. About a year ago, UCMP Director and Faculty Curator Roy Caldwell was part of a team of scientists who discovered that when light bounces off the hard exoskeleton of some stomatopods, that light is circularly polarized. What was particularly surprising was that the stomatopods responded to that light — they were capable of seeing circularly polarized light!  What eluded Roy and others was how.

Now colleagues have discovered that the stomatopods don't see the circularly polarized light directly. Special photoreceptor cells in their eyes, called R8 cells, filter/convert the circularly polarized light into linearly polarized light, which can then be sensed by other photoreceptor cells below it. The R8 cell is quite remarkable and might serve as a model for tiny manmade dual-function microsensors.

Manmade filters that convert polarized light are called quarter-wave retarders and are effective only across a very narrow band of wavelengths. The R8 cell (acting like a quarter-wave retarder) can filter light across a wide band of wavelengths, spanning the entire visual spectrum, into the UV spectrum.

There are lots of applications for a highly effective quarter-wave retarder, including DVD players. As DVD technology advances, people are already using circularly polarized light to create 3D movies — one eye sees the clockwise corkscrews of light, and the other eye sees the counter-clockwise corkscrews (Roy received some prototype 3D glasses using this technology and used them to verify that the stomatopods were producing circularly polarized signals!). Digital cameras along with many other optical devices also include quarter-wave retarders in their sensors.

We can learn a lot about optics from the stomatopod eye, and apply this knowledge to new technologies.

Want to learn more about stomatopods? Watch the UCMP video Field notes: Collecting collecting stomatopods on the Great Barrier Reef. And check out Secrets of the Stomatopod: An Underwater Research Adventure.