Sensors Magazine - January 1998 - Charge-Coupled Device Image Sensors

SIDEBAR:

The Vision of the Sojourner

Keith E. Wetzel, Eastman Kodak Co.

The stunning images from Mars have rekindled our interest in space exploration. But then we've come to expect great images from the efforts of NASA and the Jet Propulsion Laboratory (JPL). Those images, though, have usually come from specially developed instruments consisting of custom fabricated and tested components.

In mid-1992, JPL sought off-the-shelf image-sensing hardware for the Sojourner rover to more cost-effectively meet the mission's requirements. The sensors had to be suitable for machine vision: they had to use progressive scan architecture and interline format with low dark current and be enclosed in a rugged, reliable package. They also had to be available in both color and monochrome to accomplish the mission.

The mission required a machine vision system that could detect objects for collision avoidance, capture high-quality color images of rocks and soil samples, and transmit images of the Martian landscape to provide a view of the terrain from Sojourner's perspective. Meeting these objectives required electronic shuttering, full-frame capture on each shot, and high spatial frequency response. NASA found that the Kodak Digital Science KAI-0371M and KAI-0371CM image sensors met these requirements.

The Eyes of Sojourner
NASA placed two KAI-0371M monochrome image sensors in the front of the rover to serve as the eyes of the vehicle. Using "seek" coordinates from JPL mission control, the twin KAI-0371M sensors measured the topography and recorded visual maps of the surface, enabling the rover to traverse the Martian terrain and reach its destination while navigating around rocks and other geographic elements on the planet's surface.

This was accomplished using a progressive scan, interline transfer image process that allows the sensors to deliver still/video imaging of fast-moving objects without breakup. Image sensors based on the progressive-scan architecture capture the entire frame in each shot, as opposed to sensors based on the interlaced architecture, which capture only alternating lines with each shot and are susceptible to motion artifacts as images are captured one after the other. The sensors on the Sojourner were designed to continuously capture images while simultaneously transferring the previous image out of the sensor memory.

Kodak KAI-Series image sensors separate the photodetecting and readout functions by forming isolated photosensitive regions between lines of nonphotosensitive or light-shielded parallel readout registers. After integrating a scene, the signal collected in every pixel is transferred—all at once—to the light-shielded parallel register for transfer to the processor. During readout, the next frame is being integrated, achieving a continuous operation and a higher frame rate.

The progressive-scan architecture therefore significantly reduces the image smear associated with the interlaced architecture. This architecture is more complex and lower in sensitivity than interlaced devices, but for applications that require a high level of quality and certainty of scene content, as was the case with the Sojourner, the tradeoff is worthwhile.

While KAI-0371M image sensors provided Sojourner with its sight, a KAI-0371CM image sensor located in the rear of the vehicle provided full-color images of the ground and soil samples inspected by the rover. Using a progressive-scan color CCD with square pixels, the KAI-0371CM provided the uncompressed 768 by 512 pixel still images sent back to the Pathfinder lander and transmitted to scientists at JPL for examination.

The KAI-0371CM mounted on the rear of the Sojourner was filter-coated with the same three-green pattern used in the original Kodak professional digital camera system. The pattern involves coating vertical rows of pixels for spectral sensitivity in red (r), green (g), and blue (b). The pattern, r,ggg,b,ggg, repeats across the sensor. Kodak scientists developed the pattern and coating techniques to facilitate single-frame capture of color images that would approximate the spatial frequency response, luminance characteristics, and performance of transitional photographic materials.

Conclusion
Although this marks the first time Kodak's interline sensors have been used by NASA, the company has a long history of involvement in the U.S. Space Program. From the very beginning of U.S. space exploration, Kodak products, engineering, and services have played an important part in NASA programs. While recording each launch from the ground, Kodak films have been aboard every U.S. manned space flight, beginning with John Glenn's first successful orbit in 1962.

Recently, Malin Space Science Systems, Inc., of San Diego, CA, announced that it would provide cameras for the 1998 Mars Orbitor and Lander missions and that Kodak Digital Science KAI-1001M image sensors would be used in the cameras. The sensors have 1024 by 1024 pixel resolution in the same progressive-scan interline architecture as the sensors used on the Sojourner rover.

Keith E. Wetzel is Marketing & Sales Manager, Microelectronics Technology Div., Eastman Kodak Co., 1669 Lake Ave., Bldg. 81 MC 02010, Rochester, NY 14650-2010; 716-477-4047, fax 716-477-4947, kwetzelmtds00@kodak.com

Return to Main Article