Scientists Evaluate Statues, Cathedrals with Laser Radar

Michael D. Wheeler

A research group here has collaborated with an Italian team to evaluate the integrity of ancient monuments and historical structures using laser radar that was originally designed for atmospheric sensing.
The problem of decaying artifacts is nothing new to Europe. In many cities -- Venice, Rome and Athens -- years of pollution and neglect have taken their toll on marble statues, cathedrals and temples. Restorative work is often time-consuming.
To help restoration planning, Professor Sune Svanberg and collaborators at the Lund Institute of Technology teamed up with an Italian research group led by Giovanna Cecchi and Luca Pantani, senior scientists in the remote sensing department at Consiglio Nazionale Delle Ricerche in Florence. The group employed laser radar featuring a frequency-tripled Nd:YAG emitting in the UV at 355 nm with a 20-Hz pulse rate.
When the laser pulses hit a stone wall or statue, the structure fluoresces. A 40-cm telescope equipped with a spectrometer and a charge-coupled device detector captures the fluorescence. A gated image intensifier helps make signals stronger.

Inspecting the walls
To test how well the laser radar scanner would work on historical structures, the scientists scanned parts of an ancient cathedral in Lund. While results are preliminary, the laser scanner provided the scientists with a host of important data. They found some types of algae and other vegetation on the walls' surface. The hues of the fluorescence provided them with clues to the age of the stone.
Some parts of the cathedral have ancient stones, and others have much newer stones, Svanberg said. "We can see the difference in these stones that to the naked eye look quite similar," said Svanberg. The team hopes to refine the technique so they could even identify the quarry each stone came from.
Although the results are encouraging, Svanberg said many questions about the technique remain. "Really, this was exploratory. It's not exactly clear what we can use all of this information for," he said. "It is important to collaborate with specialists on monuments." Svanberg will present his findings in September at CLEO Europe, held in Glasgow, Scotland.
In the short term, a quick scan with the laser radar could help determine whether a section of a structure requires restoration.