Charge-Coupled Device (CCD) Imaging Arrays
In recent years, the silicon charge-coupled device (CCD) has revolutionized both home and studio video recording. The CCD is a solid-state chip that turns light into electric signals. CCD video cameras are light-weight, require little power (so the batteries are light as well), are inexpensive, and are more sensitive to light than the large, bulky, and power-hungry vacuum tubes previously used for television cameras. Earlier space missions, such as the Viking Orbiter mission to Mars and the Voyager spacecraft that flew by Jupiter, Saturn, Uranus, and Neptune, used versions of the old television vacuum tubes called vidicons. Since 1974, the NASA Instrument and Sensing Technology program has been investing in the new CCD technology. This has made it available for flight on missions such as the Galileo mission to Jupiter, the Hubble Space Telescope, the Yohkoh Soft X-ray Telescope, and the Space Shuttle Electronic Still Camera.
Charge coupled device technology was first demonstrated in 1969 at the Bell Laboratories. In 1974, under the sponsorship of the NASA Instrument and Sensing Technology program, the Jet Propulsion Laboratory began a program to increase the size of CCD arrays (then less than 100-by-100 picture elements, or pixels) and to lower their readout noise levels. Shortly thereafter, the Office of Space Science (OSS) added funding, and by 1978 CCD arrays of 500-by-500 pixels had been produced, achieving noise levels of 10 electrons rms (root mean square). After this, OSS continued the development of the 800-by-800 arrays that are currently being used by Galileo (1989 launch) and the Hubble Space Telescope (1990 launch).
In 1982, the NASA Instrument and Sensing Technology program substantially increased its funding and, combined with OSS Advanced Development program, funded the development of the second generation of CCD detectors. These have yet larger formats, lower noise, improved manufacturing yield, and for the first time excellent x-ray response. This work continued successfully and led directly to the Yohkoh Soft X-ray Telescope that is successfully operating in orbit. Many future missions plan to use this technology, including the Cassini Imaging Science Subsystem. These second generation CCD's surpass their predecessors in almost every characteristic. They have larger formats (1024-by-1024 verses 800-by-800), smaller pixels (12 microns verses 15 microns), lower noise (2 electrons verses 10 electrons rms), lower cost, and higher reliability, that the first generation CCD's. Such an improved CCD flew on the Space Shuttle as part of the Johnson Space Center's Electronic Still Camera. Our most recent CCD technology efforts have been focused on understanding radiometric accuracy for very sensitive measurements (such as searching for planets around other stars) and on improving CCD response in the ultraviolet region of the electromagnetic spectrum.
The NASA Instrument and Sensing Technology program is also developing active pixel sensor (APS) technology, which has several important advantages over CCD's. The APS " camera-on-a-chip" approach will enable low cost, low power miniature cameras with on-chip timing, control and drive electronics. This reduces system size, cost, and complexity. The cost of fabricating an APS wafer is one-third the cost of fabricating a similar wafer using a specialized CCD process. An article on this promising new technology is in the March 6, 1995 issue (pages 54 and 55) of Business Week.
For more information contact Gordon Johnston:
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