Ladar Images in Three Dimensions
Technology is increasingly important on the battlefield. Now scientists at Massachusetts Institute of Technology's Lincoln Laboratory in Lexington have developed a three-dimensional imaging ladar system for tactical applications. The system offers single-photon sensitivity, depth precision in the 3-cm range, adjustable angular resolution and 128 x 128 pixels.
Three-dimensional imaging ladar enables users to peer through and around obstacles. A nighttime scene imaged with traditional 2-D intensity ladar reveals one camouflage-painted utility vehicle (left). The 3-D ladar images, however, incorporate range information (right, top), enabling the user to rotate the scene and reveal the presence of a second vehicle behind a net blind (right, bottom). Reprinted with permission of MIT Lincoln Laboratory.
The idea behind the system is relatively straightforward. "I like to compare this technology with a digital flash camera," said researcher Richard M. Heinrichs. "But rather than a flashbulb, we have a short-pulse laser, which illuminates the area."
The light that reflects from an area of interest is imaged onto an array of detectors. However, in contrast with CCD and conventional two-dimensional cameras, which measure intensity, the detectors in the ladar system measure the time of flight of the photons and thereby the range to the target. Because this information is encoded in each pixel, the ladar can produce three-dimensional, or "angle-angle-range," images.
The laboratory initially developed the technology for the US Department of Defense for use in ballistic missile seekers. "Shape was identified as a way to tell the target from something that the seeker would not want to hit," Heinrichs explained. "The goal was to develop a highly sensitive array of detectors that would give you a 3-D image" with which to determine shape.
But as the technology progressed, other tactical applications suggested themselves, notably using the ladar to look through obscurants such as trees, towers and camouflage. With conventional, two-dimensional imaging, it is possible to see only through the holes in a tree covering, for example. But because it produces images from multiple angles, the ladar essentially sees around the trees. Other potential applications include mapping and autonomous vehicle navigation.
The ladar system employs a diode-pumped, passively Q-switched Nd:YAG microchip laser operating at the 532-nm second harmonic as a light source. To detect the returning signal, the researchers built a passively quenched, 4 x 4 array of avalanche photodiodes that operate in Geiger mode to enable single-photon detection. The array is scanned to produce images of up to 128 x 128 pixels.
Single-photon sensitivity brings significant benefits, Heinrichs said. "Because it's just looking for a single event, its circuitry can be very simple; all it has to do is measure a single time of flight."
Development of the technology has reached the prototype stage, and Heinrichs noted that its ability to see through trees has been tested on a helicopter platform. In addition, the researchers have discussed the possibilities of vehicle navigation with various robotics companies.
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