Imaging technology originally designed to photograph planets from unmanned spacecraft will be used to help protect the United States' most valued documents as part of monitoring system developed by the Jet Propulsion Laboratory for the National Archives.
The Charters of Freedom Monitoring System, conceived by imaging scientists and engineers at JPL, will periodically assess the physical condition of the Constitution, Declaration of Independence and the Bill of Rights.
Designed and built for JPL by engineers at the Perkin-Elmer Corporation in Danbury, Connecticut, the system was recently installed at the National Archives in Washington, D.C.
Using sensor similar to those on modern planetary spacecraft, the system -- occupying two rooms -- creates and retains highly detailed electronic images of the three historic documents. Over time, conservators at the National Archives will be able to detect minute changes in contrast, shape and other indicators of degradation in the parchment pages by using the system's image processing computer to compare electronic images.
The system has been under development at the laboratory for more than five years with collaboration from Perkin-Elmer during the past two years, noted JPL task manager Dr. Christopher M. Stevens. The system is passive monitor and does not affect the condition of the documents.
At the heart of the system is charge-coupled device (CCD), highly sensitive detector used as the electronic "film" for the system's scanning camera. Similar CCDs are used in JPL projects for the National Aeronautics and Space Administration including the Jupiter spacecraft Galileo and the Wide-Field/Planetary Camera for the Hubble Space Telescope.
The system's CCD can only "see" one narrow line at time. Each line is divided into 1024 picture-element dots, or pixels, each 30-thousandths of millimeter square. The system moves the CCD camera over the document line by line until 1024 lines have been exposed. The resulting 1024-by- 1024-pixel images are then stored in computer data files. Each image records only one-inch-square portion of the document, divided into more than 1 million pixels.
The remainder of the massive system is designed to control precisely conditions under which the images are taken, so that analysis of images of the documents taken 50 years from now will reveal only changes in the documents, not in the system itself. Illumination, position, vibration and temperature are all monitored and controlled to ensure the repeatability of each image.
"The resulting sensitivity is five to 10 times better than what is possible with the human eye," Stevens said. "With the system it will be possible to detect changes in the documents long before the eye could."
The scanning portion of the system, which includes the CCD sensor, is housed in darkroom-like facility at the National Archives. An operator controls the system from computer located in an adjacent room.
In the past the documents have been subjected to variety of adverse environments. Concern over various aging effects prompted the National Archives' decision to have the system developed, according to Acting Archivist Frank G. Burke. Some of the documents are very difficult to read, he said, and conservators are anxious to prevent any further deterioration.
Until now, archivists have had no objective method for measuring the "readability" of document or work of art, or for measuring its degradation over time, said JPL cognizant engineer Edward A. Miller. The Charters Monitoring System provides conservators at the National Archives with tool for determining if and how the documents have deteriorated, he said, allowing the conservator to apply any preservation methods deemed necessary.
The Declaration of Independence has been exposed to much harsher conditions than have the Constitution and the Bill of Rights. After the Revolutionary War broke out, the Declaration was carried by wagon many times back and forth between eight cities before being moved in 1800 to Washington, D.C. When the U.S. and Great Britain were again at war in 1814, the Declaration and other documents were removed to Leesburg, Virginia, for several weeks.
Even when the charters were in more permanent quarters in Washington, the Declaration for years was displayed in various government buildings where it was exposed to prolonged sunlight. Since 1950 the documents have been permanently encased in glass cases filled with slightly humidified helium; this inert gas was chosen because it neither affects the documents nor supports organic life.
The charters were written on parchment produced from animal skin. Ink may sit on improperly prepared, greasy parchment, and thus has tendency to flake off if the parchment undergoes physical stress.
Other features of interest to conservators include the relative contrast of the ink and parchment, warping or "cockling" of the document pages, and any evidence of bacterial or chemical attack.
JPL scientists and engineers are currently working with conservators at the National Archives to refine new techniques for document analysis which make use of the monitoring system. Using combination of image processing techniques and specially controlled imaging procedures, conservators will be able to observe any document degradation features.
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