Camera Filter Cuts Through Fog, Camouflage
Light has polarizations that the human eye can't see, but a new filter will help visualize these invisible directions of light.
CREDIT: Anette Linnea Rasmussen | Shutterstock.com
Many people know how helpful polarized sunglasses are in cutting the glare coming off water on a sunny day. But a filter that could let people see — or block out — all the polarizations of visible light would do much more than just make a day at the beach more comfortable. Such a filter could also help pilots see through fog, haze or clouds of dust; U.S. military members spot cars and trucks even when they're camouflaged; and doctors see through certain layers of human skin. In a U.S. Air Force-funded project, a group of physicists from the Colorado School of Mines have created a filter that detects circular polarized light, which is more difficult to isolate than the linear polarized light that sunglasses filter. They reported their research Jan. 6 in the journal Optics Express.
The human eye can't detect the different polarizations of light, so it can be difficult to imagine what a circular polarized filter can do. Images from linear polarized light cameras, which scientists have already made, can help. In this brochure by University of Arizona optics researcher Russell Chipman, Figures 8 and 11 show how linear polarized light filters cut through haze and camouflage. "The images that our filter would allow to be made would be similar, but would contain even more information," said David Flammer, one of the physicists who developed the new circular filter.
Flammer and his colleagues haven't yet made a camera that can take a picture using their filter, however, so they aren't sure exactly what might show up with some circularly polarized light filtered. It may be a year or more before they can make a camera, said Russell Hollingsworth, senior scientist at ITN Energy Systems Inc., a company that is working with the School of Mines to commercialize the filter.
The draw is that the information is already there, waiting but undetectable. "Just like there were all sorts of contrasts that came out in Dr. Chipman’s images that we wouldn’t have guessed at before making the linear polarization camera," Flammer told InnovationNewsDaily, "there could be a wealth of information stored in the circular polarization piece of the story."
Light coming from the sun and most light bulbs is unpolarized, which means it vibrates in waves in all directions. When that light hits certain surfaces, however, it becomes polarized: Only certain directions of the light reflect off those surfaces into the eye. Though human eyes can't detect the difference, light reflecting off water, tree leaves, haze, human skin and other surfaces have their own polarization. Linear polarization cameras work by isolating polarized light that moves along a plane, which removes phenomena such as glare. A circular polarization camera would do the same, but for circularly polarized light, which travels in a DNA-shaped helix.
The new filter might also help scientists take pictures of things much smaller than trees or cars. Shining circularly polarized light onto a molecule can tell researchers information about the molecule's chemistry that they can't get any other way. For example, circularly polarized light will show the difference between two forms of the sugar glucose whose chemical building blocks are the same, but are shaped like mirror images of each other. One of the forms, D-glucose, is eaten by every living thing on Earth. The other form, L-glucose, is indigestible.
Such differences can be important in drug-making. Some researchers are looking into whether circularly polarized light can help find extraterrestrial life. Scientists have cameras now that tell them about how molecules interact with circular polarized light, but "they're big bulky pieces of equipment," Flammer said. Flammer's filter would make them smaller and lighter.
A double helix cut out of a very thin sheet of metal, a few hundred nanometers thick, the filter is easy to make in a factory, Hollingsworth told InnovationNewsDaily. ITN Energy Systems Inc. is still in the early stages of the commercialization process, he said, but it plans to make a camera that uses this circular polarization filter, plus three linear polarization filters Flammer has developed, to detect all the polarizations of light in a scene.
Their first goal is to sell the camera to the U.S. Department of Defense. After that, Hollingsworth and Flammer see potential for a polarized light camera in cancer imaging. Polarized light may be able to distinguish cancerous skin cells from healthy ones, or allow doctors to look past the surface of the cervix for cancerous signs.