STANFORD -- Within the heart of a piece of clear glass the size of a sugar cube, a three-dimensional ring of neon-pink light twists and dances.
The cube is surrounded by lasers, scanners and the other components of a prototype video display that produces 3-D images in a whole new way -- by creating actual three-dimensional color images inside a solid cube of fluorescent glass.
The technology, although still rudimentary, has a number of potential applications:
- Physicians could use it to gain a fuller view of images of the inside of the body that are produced by medical devices such as MRI, CAT scanners and ultrasound.
- Air traffic controllers might use it to track aircraft positions in three dimensions.
- Engineers could find the technology useful for viewing new products that they create with computer-aided design tools.
The concept of displaying three-dimensional objects in fluorescent glass dates back at least to the mid-1960s. But the materials problems involved have only now been solved, reports Elizabeth Downing, a graduate student working with Stanford electrical engineering Professor Lambertus Hesselink, in the Aug. 30 issue of the journal Science. Robert Macfarlane of IBM Almaden Research Center and John Ralston of SDL Corp. in San Jose also made important contributions to the development.
Over the years, researchers have come up with a number of different ways to produce three-dimensional images. Most of these rely on various tricks to fool the eye into converting two-dimensional images into three-dimensional scenes. They range from the paper glasses with red and blue lenses used to view 3-D movies to the virtual reality display systems that create the illusion of depth by employing two small televisions to deliver slightly different perspectives of the same view.
Another basic approach has been to use holography, which stores three-dimensional information in invisible patterns on a special film. When this film is illuminated by laser light, three-dimensional images appear to a viewer looking through the film.
"There are a number of different 3-D display technologies, but this technology has some unique features," Downing said. "For one thing, it doesn't create an image that appears to be three dimensional, it actually produces an image that is drawn in three dimensions. As a result, there are few restrictions on the viewing angle and a number of people can view the images at the same time. Also, the images are emissive they glow rather than reflective, so they can be seen easily in ordinary room light."
But the technology also has some important limitations. The objects that it forms are transparent, not opaque like most common objects. As a result, additional processing would be required before it would be suitable for entertainment purposes like television or video games. It also takes 500 times as much data to construct a three-dimensional object as it does to draw the same object in only two dimensions.
Hesselink's group has been a leader in the study of three-dimensional video systems. In the 1980s, working with Stanford electrical engineering Professors Joseph Goodman and Al Macovski, Hesselink developed "volume representation" software that allowed ordinary computer monitors to display three-dimensional images. The researchers also created 3-D displays of medical data by stacking plates containing two dimensional images. In 1994, Hesselink's group demonstrated the first fully automated, digital holographic storage system for video, data and sound, a technology that has the high-speed data transfer capability needed to drive the new 3-D display.
Graduate student Downing got the basic idea for the novel display while working at FMC Corp.'s Technology Center in Santa Clara. When she came to Stanford in 1988 and began researching the concept, however, she discovered that she wasn't the first to think of it. "There was a group at Battelle Memorial Institute in Columbus that worked on this in the early 1970s without success. The right components weren't available then. They didn't have semiconductor lasers and they didn't have the variety of fluorescent glasses that are available today," she said.
The fluorescent glass display is based on a scientific principle called "upconversion." Certain atoms in the rare earth family emit visible light when struck in rapid succession by two infrared laser beams of slightly different wavelengths. Different kinds of atoms emit different colors of light when stimulated in this way. To make a display, small amounts of these atoms are mixed (doped) into the glass as impurities. When the two infrared laser beams, which are invisible to the naked eye, are directed through the glass, a point of visible light is created where the two beams intersect.
For the prototype, Downing used surplus scanners from optical disk players to scan the two laser beams vertically, horizontally, and backward and forward through the volume of the cube. Visible images are produced by controlling the beams so that they intersect at points that lie on the images' surface. In this fashion she has successfully created three-dimensional wire figures, surfaces and simple solid shapes.
The technology can generate color images by employing what Hesselink refers to as "the Trinitron approach." Rare earth impurities that create red, green and blue colors are mixed into the glass in separate layers that are very close together. When the laser beams stimulate adjacent layers at nearly the same time, the different colors fuse into a single colored dot.
The current testbed device consists of three relatively thick layers, one for each color. So a pink object appears in front of a green object that appears in front of a blue object. An actual display would consist of thousands of groupings of red, green and blue layers so that 3-D objects of any color could be created. IBM's Macfarlane provided considerable assistance in identifying the right rare-earth impurities to add to create the different colors.
Downing began by making a cube of clear plastic mixed with small rare earth crystals. This was sufficient to demonstrate the upconversion effect, but did not generate crisp images. It was enough, however, to help Hesselink and Downing obtain a $350,000 grant from the U.S. Navy in 1993. They subsequently received additional support from the Advanced Research Projects Agency at the Department of Defense through Stanford's Center for Nonlinear Optical Materials. When Downing was ready to build a prototype using much smaller semiconductor lasers, SDL Corp. in San Jose furnished without charge the diode lasers and some of the other electronics that were required.
Downing considers medical imaging to be the most natural application for the new display technology. Currently, MRI, CAT scans and ultrasound produce complex 3-D images that are studied by stacking a series of flat pictures. The practitioners prefer transparent images because they do not hide any of the features. Moreover, the medical benefits of this improved display technology are likely to be great enough to pay for its expense: Downing calculates that it would cost about $80,000 to make a prototype 10-inch display of this type.