Lidar maps Swedish cathedral

17 Jun 2002

A cathedral in Lund is the first historical building to be imaged with fluorescence lidar.

Scientists from Sweden and Italy have used lidar to image the different types of stone used to build a Swedish cathedral. Petter Weibring and colleagues at the Lund Institute of Technology could also tell which of the walls had biological growth on them (Appl. Opt. 40 6111).

The team says that the technique could be used to detect organisms such as algae and lichens, which discolor the walls, before they can be seen with the naked eye or other detection techniques.

Identifying the different types of stone that were used at different times could also help historians trying to evaluate the construction history of buildings.

Weibring and colleagues used a mobile lidar station with an Nd:YAG laser emitting 8 ns-long pulses at a 20 Hz repetition rate to image the 12th century Lund Cathedral - the largest Romanesque building in northern Europe.

The main constituent of the older parts of the cathedral is quartzitic sandstone, which is quarried nearby. The sandstone is weather-resistant, but tends to get a black stain when it is exposed to the atmosphere.

In the 19th century, a harder, denser sandstone was used to renovate the cathedral, and granite was used to reconstruct some parts. All of these types of stone and biological organisms have different fluorescence signatures. Two detection systems were used in parallel: transmission filters at 600 nm and 448 nm were used in one system, and the full spectral range was captured in the second setup.

The investigation was carried out in full daylight, at a distance of 60 m. In their paper, the researchers say that the technique should work at a distance of at least 100 m in full sunlight, and from twice as far as this in the low-ambient-light levels at night.