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The mirror that we stare into each morning often reveals more than we wish it would. That it absorbs rather than reflects a fair share of the light that strikes it is no consolation. But being seen through a glass darkly isn't good enough for scientists in the field of high-tech mirror applications, where the goal is for more than 99% of the light to be bounced back.
For more demanding reflectors, from computer displays to car headlights, researchers at 3M Corp. have come up with a new type of mirror that outshines the competition. They've found a way to build up layers of inexpensive plastic films, thinner than a sheet of paper, made up of polymers with different optical properties. The development promises to bring important advances in computer and television displays, telecommunications relay, and solar control.
LIGHT FANTASTIC
"Multi-bounce reflections produce surprising and useful optical effects"
CREDIT: 3M
As light passes through the layers of the films, almost all of it is bounced back toward the viewer. Better yet, the new mirrors reflect any light that strikes them equally well, no matter what the angle. "Multi-bounce reflections produce surprising and useful optical effects," says Andrew J. Ouderkirk, who headed 3M's research team.
SKIRTING THE LAW.
Multi-layer mirrors were first developed years ago. But these so-called di-electric mirrors ran afoul of a phenomenon first discovered some 200 years ago, known as Brewster's Law. Mirrors work best when light approaches them straight on, but as the angle increases, they reflect less and less -- and when the path exceeds a certain angle, the mirror stops reflecting altogether. This is what happens when the image disappears from the screen of your laptop as you move your head to the side.
The 3M group found a way around Brewster's Law by creating films made of alternating layers of transparent polymers that split rays of light into two beams that travel at different speeds as they cross from one layer to the next. Depending on the thickness of the layers and their number, the scientists were able to design films that reflect light efficiently no matter what its initial angle of approach.
In the first formal report on their work since it began in 1998, published in a recent issue of the journal Science, Ouderkirk and his colleagues described a "broadband" film consisting of 448 alternating layers of two common plastics: polyester and acrylic. Mirrored tubes lined with the film, known as light pipes, can deliver daylight deep into buildings to reduce lighting and energy costs.
FULL SPECTRUM.
With the new films, light emerges from the end of the pipe at virtually the same intensity and with the full range of the spectrum as when it went in -- unlike the light pipes now in use, which are coated with shiny aluminum.
And if it's color you want, the researchers say they can design films that reflect only certain wavelengths of light. This, says Ouderkirk, could have important benefits in separating signals in optical-communications systems. Or the films may offer security uses, in the same way that holograms are now used on credit cards and currency.
These new films are already in the early stages of commercialization, with formal introduction slated for later this year. The first targets are lightweight mirrors, light pipes, computer displays that are brighter and can be viewed from any angle, and improved mirrors in projectors for high-definition TV. Intense and more focused car headlights are being prototyped -- a promising application because the films can be formed into complex shapes.
HOT TOMATOES.
After that, other applications abound: from solar-control films for car windshields and windows that block out heat to greenhouses that keep tomatoes hot and green beans cool. Other possibilities range from decorative products to scientific optical components. "We've combined the highest reflective performance with the cost and flexibility advantages of polymers," says Ouderkirk.
If 3M's latest innovation reflects its track record, the new films may have a bright future. After all, the Minneapolis company's labs brought the world the adhesive for Post-It Notes, those sticky little yellow messages that are now everywhere. Reflect on this: Mirrors have come a long way since Narcissus was transfixed by his own image in a pool of water.
By Alan Hall in New York EDITED BY BETH BELTON
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