Fluorescence lidar multispectral imaging for historical monuments, the Lateran Baptistery interdisciplinary project

Introduction
For the future conservation, restoration and daily maintenance the need for developing non-destructive and in-situ investigations is vital. At Lund University in Sweden an interdisciplinary research project is being held aiming at, with the help of a mobile system of remote fluorescence lidar, survey and study the facades of historic monuments, and to develop the method and its application for the cultural heritage sector. Earlier studies using the fluorescence lidar technique has been undertaken at, for example, Parma, Lund and Pisa Cathedral. During the winter 2005 two field studies were performed in Rome, one on parts of the Flavian Amphitheatre Coliseum, and the other on parts of the Lateran Baptistery. The studies were performed from remote and with no need for taking samples. The aim was to identify different materials, different biodeteriogens and other objectives not yet visible for the human eye.
Objectives for the project
The primary aim of the research project is to analyse the potential of the fluorescence lidar technique in building investigation. Furthermore, to establish a scientific remote-sensing method for surveying and analysing the historical facade that is non-destructive and takes place in-situ. The aim of the fluorescence lidar method is to use point-monitoring and scanning of areas which can provide multispectral images in order to identify different types of stones, their origin, different biodeteriogens and conservation chemicals. A specific aim of the project is to develop the method in comparison with existing documentation methods, to improve the understanding of the built heritage and to provide a basis for new interventions, documentation and maintenance.

Methods
Fluorescence techniques make it possible to extend the application of fluorescence spectroscopy to the outdoor environment (remote sensing), with sometimes large distances and uncontrollable background light. The ultra-violet laser pulse used in the experiments (usually 355 nm and 308 nm) is directed to the point of interest, exciting atoms and molecules in the material, which then relax emitting fluorescence light. The unique fluorescence spectra from each point taken can then be analysed and identified. By applying a function to the different spectra, each point is assigned a value, and from this a false-colour coded image can be produced, indicating points with different properties, revealing e.g. different materials and earlier treatments.

Lateran Baptistery field study
The study of the Lateran Baptistery in Rome is a co-operation between Lund University, the Swedish Institute in Rome, CNR-IFAC in Florence, and the research project led by Dr. O. Brandt and Dr. G. Eriksdotter, from the Swedish Institute, the Vatican Museums and the Pontifical Institute of Christian Archaeology, also involving Italian and Swedish archaeology students who has performed a stratigraphic analysis during 2005. The Baptistery, part of the San Giovanni in Lateran church complex, dates back to the fourth hundred-century AC. The analysis from the lidar campaign and the stratigraphic studies are now being cross-examined and the analysis aims to identify brick material and if possible distinguish different building and restoration phases of the baptistery.