Holographic projector works at video rates

At the heart of many projectors used to display computer presentations on boardroom screens is the digital micromirror chip developed by Texas Instruments (TI; Dallas, TX). In the chip, an array of tilting micromirrors modulates a beam of incoherent light to form a two-dimensional (2-D) image complete with grayscale information. Now, researchers at the University of Texas (UT) Southwestern Medical Center at Dallas (Dallas, TX) are using the chip in a very different way to project three-dimensional (3-D) dynamic images at video rates.¹

In the conventional 2-D projector, the TI chip forms an image on its surface, which is then directly imaged onto a screen. In the newly developed 3-D projector, the chip serves instead as a hologram, either creating a virtual image (one that can be seen only when looking into the chip itself, and that appears to be behind the chip) or projecting a real image into space.

The UT researchers use a chip with an array of 1024 × 768 mirrors driven by a standard computer video driver card. The mirrors are each 16 × 16 µm in size and have a 1-µm gap between them.

A holographic projection is actually a diffraction order produced by a phase- and/or amplitude-modulated element (in this case, the chip); unavoidably, some light is lost into other diffraction orders. But up to 88% of the incoming light can be coupled into the brightest diffraction order, which then becomes the projected image. The hologram on the chip is highly detailed (see Fig. 1).

FIGURE 1. Normally used to create two-dimensional images directly on its surface, a Texas Instruments digital micromirror chip can instead be used to form dynamic holograms that project true three-dimensional (3-D) images. Here, the complexity of a bit-mapped hologram on the chip is evident.

Click here to enlarge image

The projector system also includes a 15-mW HeNe laser, a spatial filter, and a beam expander to illuminate the whole chip. Viewing a virtual image requires only looking into the chip. For viewing a real image, a convergent lens focuses the proper diffraction order onto an imaging medium, which can be either a screen or a camera to capture a 2-D slice of the image, or a translucent block of a material such as agarose gel (used in biochemistry for electrophoresis) to reveal the image in true 3-D.

2-D slices

The computer-generated hologram can be modeled either as a single hologram of the entire object or objects to be reconstructed, or as a superposition of many holograms of 2-D slices of the objects. The researchers chose the latter approach for its reduced computation, as well as the ease with which they can subtract and add objects within a projected 3-D scene.

FIGURE 2. A 3-D video-rate image of a helicopter and two jets was created by a hologram on a micromirror chip and captured by a charge-coupled-device (CCD) camera. In this case, the projected image was a virtual image (viewed by looking into the micromirror chip). The image seen here was degraded somewhat from the actual (visual) image because the CCD camera's depth of focus was less than the depth of the 3-D scene.Click here to enlarge image

In one example, a holographic movie of a helicopter and jets was created (see Fig. 2). The pixels in the 2-D slices of projected images were calculated as having a 51 × 51-µm size. The helicopter was 80 pixels wide by 65 pixels high and the object was 120 pixels wide by 20 pixels high.

"We envision the first applications of this device for pilot head-up displays without goggles, airborne-warning-and-control systems, air-traffic-control systems, and orbital monitoring systems," says Michael Huebschman, one of the UT researchers. "These applications do not need high-resolution images, but do need quick updates of simulated targets. We envision these applications as virtual image viewers. The next applications need higher-resolution simulated images in real time, including 3-D multi-user computer games, battlefield/naval/air-battle interactive displays, 3-D flight simulators, 3-D scientific work stations, 3-D airport x-ray machines, seismology 3-D exploration, 3-D medical (x-ray or sonic) imaging, and holographic movies. Finally, the application that requires both real-time recording of digital high-resolution holograms and high-resolution replay is 3-D television."

Huebschman notes that the UT group is working to improve resolution by optimizing their computer-generated holograms. "However, we have a physical limitation due to the mirror size on our digital micromirror device and the number of shades of gray each mirror can present (256)," he says. "Just as film holograms have improved with smaller grain size, digital-micromirror-device holograms will improve with smaller mirror size." He adds that the group has used red and green lasers simultaneously to project red, green, and yellow images.