Optical sensor protects US Charters of Freedom

17 Jun 2002

An infrared instrument from the US company Aerodyne will help protect the original US Constitution, Declaration of Independence and Bill of Rights from damage by water and oxygen.

The instrument, which is based on differential infrared absorption spectroscopy, will provide a single non-invasive optical sensor for monitoring the atmosphere inside display cases that are currently being designed to help to preserve the historical documents.

The light source in the device is a compact discharge lamp. The emission lines are magnetically tuned so that they coincide with the absorption lines of water and oxygen, and the concentrations are measured sequentially. The intensity of the transmitted light is compared with that from a second set of emission lines that are close to the first set in wavelength but are not absorbed by water or oxygen. The difference in intensity is proportional to the concentrations of the chemicals. The instrument measures water vapour down to a sensitivity of 25 parts per million. Oxygen can be measured to one part in a thousand.

The light beam is very low intensity and it passes above the documents rather than onto them, so the documents should not be at risk from photodamage.

"We have already used the water sensor to measure water on military aircraft, but the instrument has not been used commercially before," said Aerodyne's president, Charles Kolb. The conservation of these documents is a development project, so the US government is paying Aerodyne just over USD 100 000 for the design, fabrication and testing of the instrument. Once the device has been fully designed, commercial products should cost much less.

Kolb says that a magnetically tuned discharge lamp is less expensive than a laser source and easier to control.

The documents are on display in Washington DC. The US National Archives and Records Administration is replacing the deteriorating 50-year old display cases in collaboration with the National Institute of Standards and Technology. Aerodyne is scheduled to deliver the optical instruments for monitoring the cases in January 2000.

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How the device works

An earlier version of Aerodyne's instrument