What is unique about the new generation of volumetric displays is that they work with conventional 3-D graphics programs, so scientists and engineers can easily and intuitively manipulate images of such things as drug molecules, oil fields, and satellite orbits. They also send virtually all of the requisite physical cues to your brain, which is especially important when you try to visualize the much more complex and much less familiar images associated with high-tech applications.

Such a natural 3-D experience has been a long time in the making. Way back in the 1960s, experimenters came up with a display based on a device called a varifocal mirror. This display used a reflective Mylar membrane stretched over a loudspeaker that emitted a low-frequency growl in sync with an image coming from a cathode-ray-tube monitor. As the speaker caused the Mylar to bow out or suck in, the reflected image of the CRT gave the illusion of an image moving toward or away from the viewer.

In the 1990s, researchers made crude 3-D images by exciting atoms inside light-reflecting vapors and solid glass cubes with lasers and using mirrors to create monochromatic, skeletal outlines of objects such as spheres. These laboratory curiosities never made it to market because they could not be manufactured reliably, nor could they produce full-color images or work with existing graphics hardware.

The swept-volume displays developed by Actuality, Felix 3D, and Genex use more commercially viable approaches. Each has two main parts: a projector and a projection surface that is mounted on an axis and driven by a motor to spin it at a high rate. Recent advances in projection technologies, including cheaper, brighter laser diodes and, in particular, Digital Light Processing (DLP) technology from Texas Instruments, in Dallas, have made the swept-volume display an effective piece of machinery for high-end visualization.

The projection surface can be a helix-shaped, white piece of acrylic, as in Felix 3D's and Genex's systems, or a remarkably thin, translucent piece of plastic, as in Actuality's Perspecta display. In Actuality's system, this round piece of plastic bisects a glass dome and is mounted on a platform that is driven by a motor to rotate it at a very high rate. In each case, the projection surface scatters light beams projected from below so that the voxels appear to emanate from particular points inside the dome. The projection screen spins fast enough to render it invisible in a darkened room; all that appears is the projected 3-D image [see diagram, "Swept Away"].

There are two projector technologies currently in use for 3-D displays. The Felix 3D and Genex systems use three lasers: one red, one green, and one blue. The Perspecta uses TI's DLP technology. At the heart of the DLP is a digital micromirror device, a chip hosting an array of a million or so microelectromechanical mirrors. In the three-chip version of the DLP, white light from an arc lamp shines into a four-sided prism, which splits it into red, green, and blue beams and directs each to a dedicated digital micromirror device on one of the prism's faces. The red, green, and blue colors are recombined and sent through the open fourth face of the prism to the projection lens, and then to a mirror, which bounces the pixels up to the projection screen, where the full-color image appears.

A lot of what goes into a good 3-D display is clever use of psychological cues, which essentially trick our minds into seeing flat images as 3-D. For example, imagine that you're gazing into a Perspecta display to view a human brain, which appears to be floating in space. The image is composed of more than 200 different images that are being projected sequentially by the three-chip DLP onto the plastic screen so fast that your eyes literally deceive you through an effect called persistence of vision.

Persistence of vision is another tool in the display engineer's toolbox. The eye holds an image for an instant after the stimulus that produced it disappears. This same phenomenon, which helps us perceive motion in film, as opposed to separate frames flickering by, aids our brains in constructing a single 3-D image from hundreds of discrete images being projected onto a rapidly spinning screen.

These individual images are often likened to slices of an apple arranged around its core. In the Perspecta, the projector acts as a kind of strobe light, illuminating the screen for about 100 microseconds for each image slice, to produce volumetric images measuring up to 25 centimeters in diameter. Because the projection surface is translucent, the image projected onto it is visible on both sides; the full image is produced by a half-revolution of the screen. By spinning the screen at 15 revolutions per second and projecting a different image onto it more than 200 times per revolution, the Perspecta in effect shows 6400 frames per second, more than enough to fool brains and eyes accustomed to watching movies at a mere 24 frames per second.

DLP-based displays have one big advantage over laser-based displays: bandwidth. The three-chip DLP can process about 2.75 gigabytes of image data per second—enough to render complex, full-color, lit, and shaded 3-D images. Laser-based swept-volume displays, which use a single beam scanning the projection surface and process only half a megabyte of image data per second, can at best produce a simple, monochromatic 3-D line drawing. Such an image contains only about 10 000 voxels, compared with the Perspecta's 100 million voxels.

If you've never seen a swept-volume display, you may have a hard time understanding the wonder it can inspire. Suffice it to say, oohs and aahs are pretty commonly heard as first-time viewers stroll around a diaphanous image that just seems to float in space. But there are some significant limitations to the technology. First, any swept-volume machine contains a rapidly spinning component that needs to be carefully balanced to control vibrations. At the very least, vibrations can cause the voxels to smear through the volume, blurring the image. At worst, the entire mechanism could violently pull apart, thanks to what is known as gyroscopic precession, where energy from an applied force, say, a wave hitting a ship, is transmitted at 90 degrees to the direction of rotation.

So using a swept-volume display on a ship, plane, motor vehicle, or intergalactic star cruiser would pose some challenges. In addition, to maintain a smooth 3-D image over the 360-degree field of view, the video projector must produce numerous image slices at a very high frame rate, which is a big problem for laser-based displays and can even challenge the output capacity of the DLP used in the Perspecta. Consequently, swept-volume displays can display at most hundreds of colors and therefore cannot create lit, shaded, and texture-mapped 3-D images.