An Added Dimension in Display Technology

While we see in 3D, most pictures exist only in 2D. Even clever attempts to make convincing three-dimensional representations of objects-Victorian-era stereoscopes, green-and-red-lensed spectacles for 1950s B movies, even sophisticated holographic images-all strain to create the illusion of three dimensions on a two-dimensional surface.

Now Elizabeth Downing, a former Stanford University graduate engineering student turned entrepreneur, has taken a completely different approach by building a true 3D display. Though small and rudimentary, her proof-of-principle invention–a sugar-cube-sized block of special glass-can come alive with dancing colors that exhibit height, width, and, most importantly, depth.

The new technology “doesn’t create an image that appears to be three-dimensional,” Downing says, “it actually produces an image that is drawn in three dimensions.” As a result, it places few restrictions on the viewing angle or the number of people who can observe the images at the same time. Moreover, the images are emissive-they glow rather than reflect-so viewers can easily see them under ordinary room light without special glasses or headgear.

The display’s unique characteristics seem to make it a natural for potential use in, for example, medical diagnostic-imaging systems, arcade games, computer-aided-design tools, and air-traffic-control monitors. The display could also be employed as a scientific-visualization aid for analyzing weather patterns, air flows around an aircraft, and other complex multidimensional sets of data.

The patented device, now being commercialized by Downing’s new company, 3D Technology Laboratories of Mountain View, Calif., uses a pair of infrared lasers to selectively excite fluorescent metallic particles suspended in a clear glass cube, measuring 1.5 centimeters on a side. When these special rare-earth metal additives (also called dopants) are mixed into the molten glass during manufacturing, they “distribute themselves evenly throughout the glass like chocolate chips in a cookie,” Downing says. When a spot inside the solidified glass is illuminated with invisible infrared light, the tiny impurities glow brightly.

The ability to visualize real-time volumetric data in true three-dimensional form has been the Holy Grail of display-development efforts for decades. And while the concept of depicting 3D objects in fluorescent glass dates back at least to the mid-1960s, not until the early 1970s did researchers at Battelle Laboratories in Columbus, Ohio, succeed in generating two faint dots of light inside a crystal of erbium-doped calcium fluoride using high-intensity light from xenon lamps, similar to that generated by halogen sources. But that was as far as they got.

Realizing that inexpensive but powerful lasers and new optical materials had since become available, Downing, working as an engineer on laser-based equipment at FMC Corp.’s Technology Center in Santa Clara, Calif., believed the time to develop the technology was at hand. When she came to Stanford for further graduate studies in 1988, she continued her research on 3D displays with Lambertus Hesselink, a professor of electrical engineering at the university, receiving a $350,000 U.S. Navy grant and additional support from the Defense Advanced Research Projects Agency to pursue the concept.

The prototype display she developed is based on a principle called “upconversion.” Certain rare-earth elements exhibit this phenomenon by emitting visible light when struck in quick succession by two infrared laser beams of given wavelengths. Neither beam has enough energy to cause fluorescence by itself, Downing explains, but the combined energy of the two can cause an ion in the glass to glow.

When the ion, which normally remains at its lowest energy level, absorbs energy from the first laser, it makes a transition to an intermediate excited phase, where it remains for a short time. When an ion in this phase is struck by the second laser beam it absorbs energy at the second wavelength, undergoes a transition to an even more excited state, and re-emits most of its excess energy as a single photon of visible light as it decays back to its ground state.

To enable the prototype display to produce color images, Downing assembled the small glass cube from three layers of fluoride glass developed for commercial fiber-optic lasers and optical amplifiers. Each layer contains ions that emit one of the three additive primary colors-a layer doped with praesodymium glows red, another with erbium glows green, and a third with thullium glows blue.

Downing assigned addresses to precise points on each glass layer. Then by programming a pair of laser scanners she borrowed from optical disk players, she was able to direct the laser beams vertically and horizontally as well as backward and forward through the cube. By controlling exactly where the two invisible laser beams crossed in the transparent glass, she was able to light up a fluorescent additive of a given color-much like an electron beam lights up particular phosphors on a color television screen-to produce the desired image.

Each turned-on point of light-called a volume element, or voxel-is like a World War II bomber caught in the intersection of two searchlight beams. However, voxels are tiny. In fact, beams focused to a diameter of 100 microns, produce roughly 300 voxels around the perimeter of a circle one centimeter in diameter.

The display in Downing’s initial prototype is composed of a stack of only three individual glass layers glued together with an optically compatible adhesive to form a composite structure. However, the inventor intends to build a larger-scale 3D color system by assembling many thin doped layers arranged in a repeated sequence-red, blue, green; red, blue, green; and so on-to enable the creation of high-resolution color images. In fact, Downing has already begun evaluating new display materials and has started work on her next project (for which she says she has received venture-capital funding): building a display using a 6-inch glass cube.