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IR Camera Peers into History

Photonics Spectra
Apr 1999
Daniel C. McCarthy

In 1814, through 25 hours of fierce naval bombardment by the British, American troops successfully defended Fort McHenry in Baltimore harbor. The Star-Spangled Banner's serene and resolute presence in the light of the following dawn inspired Francis Scott Key to immortalize it in what is now the US national anthem.


He used software techniques to resize and derotate the data and create a mosaic of the flag.

Today, after years of exposure to light, pollution and fluctuating temperatures, the historic banner is showing its age. To document the previous efforts to preserve the flag and prepare for further restoration, conservators at the Smithsonian Institution's National Museum of American History used a cooled infrared camera developed at NASA's Goddard Space Flight Center.
Previously, museum workers evaluated aspects of the banner's condition that were visible to the human eye, a task made especially difficult because the flag hung vertically on a 40-foot steel frame. John Hillman, lead scientist for a NASA team lending expertise and equipment in support of the project, explained that the camera helped to expose otherwise invisible stains as well as hard-to-detect holes in the banner's white areas where the linen backing shows through.
Hillman first demonstrated the camera's abilities to the conservator staff by detecting an organic oil stain on another piece of antique fabric that was hidden by grunge and invisible to the naked eye. "The camera made it jump off the sheet," he said.
Later, he used the camera to enhance documentation of hard-to-see holes in the flag's fabric. "If you take a raw set of images and apply a little processing edge enhancement, it brings out images of these holes very well," he said.
The camera is a cooled 256 x 256-pixel, mercury-cadmium-telluride array with an acousto-optic tunable filter. Hillman imaged the banner in square segments about 3 feet on a side and recorded the third dimension -- different wavelengths -- in each segment. He captured more than 100 images in a range of infrared wavelengths between 1.7 and 2.7 µm. If he ventured higher than that, the images started to display background noise.
Working from a 20-foot-high scaffold, Hillman couldn't move the camera to keep the flag's surface exactly on the camera's optical access, so he used an articulating mirror. "Distance could change from 17 meters to 23 meters -- meaning the plate scale varied from image to image," he said.



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