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Holograms in True Color
7 April 2011 2:01 pm
Researchers have developed a new way to create true-color holograms that can be viewed from any angle using ordinary white light. The advance could lead to a new generation of electronic devices, such as cell phones or miniature televisions that display three-dimensional (3D) images.
True 3D images can be created in several ways. In the 1960s, researchers generated the first holograms by firing a laser at an object and then using a photosensitive material to record the pattern of interference between light waves reflected off the object and those striking the material directly from the laser. This hologram, if later illuminated with the same wavelength of laser light, reproduced a 3D (but monochromatic) image of the object. One well-known hologram of that era captured in red laser light a chessboard on which pieces could be viewed from various angles as a viewer shifted position.
The so-called rainbow holograms now common on credit cards are generated differently, using white light reflected off a silvery backing through a plastic film that contains several images, each stored in a different color in its own layer. As the hologram is viewed from different angles, the shifting view of those colored layers with respect to one another provide a 3D perspective. In many cases, the image produced by these rainbow holograms isn’t a true-color representation of the object depicted.
Now, researchers report today in Science that they can create true-color holograms that can be viewed using only white light. Like the first holograms, the new technique uses lasers to generate an interference pattern, says Satoshi Kawata, a photonics physicist at Osaka University in Japan. To capture colors, Kawata and his colleagues illuminate the original object with three different lasers: red, blue, and green, the three primary colors of projected light. They store the hologram in a light-sensitive material coated with a thin layer of metal such as gold or silver, a veneer that contains free electrons that are easily excited when struck by radiation such as light waves.
To reproduce a 3D image, the researchers bathe the metal-sheathed material in ordinary white light, which contains all wavelengths of visible light (including red, blue, and green). That white light excites the free electrons; their resulting movements and oscillations (so-called surface plasmons) in turn give off light that regenerates the image—an image that combines the red, blue, and green versions of the hologram to generate a true-color representation of the original object. In their lab tests, Kawata and his colleagues created realistically hued holograms of an apple, a flower, a Japanese origami crane, and several other objects. For now, Kawata says, the new technique can produce only static holograms—no pint-sized Princess Leia pleading for help from Obi-Wan Kenobi just yet.
“It’s quite a scientific achievement,” says physicist Pierre-Alexandre Blanche of the University of Arizona in Tucson. The technique may be able to generate brighter images that can be seen through a broader range of viewing angles than holograms produced using other methods, adds media technologist V. Michael Bove Jr. of the Massachusetts Institute of Technology in Cambridge. The problem, he says, may be figuring out how to mass-produce images more cheaply than other techniques can.
Yet another issue may be scaling up the holograms to large size, says photonics physicist Nasser Peyghambarian of the University of Arizona. So far, the researchers apparently have created holograms only about the size of an index card. The prisms used to illuminate the holograms, which in the current scheme are mounted beneath or behind the metal-coated material, could easily become cumbersome in much larger displays, he contends.
Nevertheless, the notion of watching the Super Bowl on a coffee table with an embedded holographic display—complete with little linemen, wee wide receivers, even a tiny blimp floating a few feet above the potato chips and beer coasters—may someday become a reality to sports fans everywhere.