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Foliage-Penetration LADAR Systems
Lincoln Laboratory has developed and demonstrated a number of flash laser detection and ranging (LADAR) systems using single-electron-sensitive Geiger-mode avalanche photodiode (GMAPD) focal planes. In flash LADAR, the system flood-illuminates the scene with a single (or multiple) laser flash(es), rather than relying on mechanical scanning as is done with a number of commercial systems. The flash LADAR system has the advantage of providing very short exposure (typically subnanosecond laser pulses are used) three-dimensional (3D) pictures of a target that may be changing its position or configuration rapidly.
A challenge of a flash LADAR system is that it typically has very weak return signals at each pixel since the illumination is spread out over an extended area covering many pixels. This weak signal drives the need for single-photon-sensitive focal planes, and so the GMAPD technology actually enables practical flash LADAR systems. The GMAPD is sensitive to single-photon LADAR returns and the timing information is digitized in the pixel, eliminating readout noise. These attributes enable compact and high-performance systems.
Figure 1 shows a 32 × 32 GMAPD detector array, mounted on a thermoelectric cooler. This unit was hermetically sealed into a detector package to stabilize the detector’s operating temperature near 20°C. A photomicrograph of the light-sensitive portion of the APD array is also shown.
A rugged and compact LADAR system, called the Gen III system (Figure 2), was developed using the 32 × 32 focal plane. The transmitter in the Gen III system operates at 532 nm and uses a doubled Nd:YAG passively Q-switched microchip laser. In the Gen III system, a diffractive optical element was incorporated into the transmitter optics, which projects a 32 × 32 array of spots onto the scene. These spots are imaged onto the APDs. While this scheme requires tight alignment tolerances in the optics, it has the advantage of good background-light rejection because of the low fill factor of the APDs (about 5 percent). The transmitted light is concentrated on the detectors, whereas most of the background light is not. No microlens array is needed to improve the APD fill factor.
One of the uses of a high-sensitivity LADAR system is "seeing" through foliage from an airborne surveillance platform. The LADAR acquires several 3D images of an area of interest on the ground as the platform flies over a partially obscuring canopy. At any given point in the flight, only a small percentage of the ground area is visible through the openings in the foliage. By merging images taken from different angles, however, it is possible to see a large percentage of the ground area.
To test this type of capability, the Gen-III system was placed on an elevator of a 300-ft tower and used to acquire 3D images of a nearby ground scene under the trees. Figure 3 shows the view from the tower, and Figure 4 shows the ground scene below the obscuring canopy of trees.
Figure 3. View of tree canopy with obscured objects on ground.
Figure 4. Objects placed on ground below tree canopy.
The LADAR system was mounted at four different heights on a tower overlooking the canopy, and 3D imagery was collected at each height. The imagery was then combined into a single composite image in a common coordinate system. The result showed very effective foliage penetration; even when the foliage obscured most of the ground, 3D imagery obtained from multiple look angles filled in much of the scene.
Figure 5 is a picture of the resulting composite 3D image. The original image is dominated by laser returns from the tops of the foliage, obscuring the few returns from the forest floor. Clicking on this image plays a movie that shows the result of processing the image to discard all returns greater than a certain height above the forest floor. In the movie, the cutoff height is continuously decreased to strip away the obscuring returns of the high foliage canopy to reveal the gazebo, vehicles, and picnic tables originally obscured by the trees.
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