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Graphic element Research > Growth > Research projects > Measurements & testing projects > Lasers highlight the cracks in antique art
Graphic element Lasers highlight the cracks in antique art
    13-06-2000
 

Restorers are constantly seeking new tools to help them save and protect the world's great art treasures. In the LASERART project, European researchers have investigated the potential of various laser techniques for identifying cracks and other faults in frescoes and icons, and demonstrated that lasers offer significant advantages over traditional diagnostic techniques.


Orvieto cathedral

Frescoes and icons are particularly fragile works of art because they consist of layers of water-based paint applied to either plaster or wood. At present, diagnosing the structural condition of such works of art relies on the expertise of the restorer. Visual inspection is one of the main diagnostic methods, but skilled restorers can also identify defects by tapping the surface of a painting, feeling the resultant vibrations through their fingertips and listening to the sound. Clearly the latter technique is neither objective nor repeatable - and on a large fresco, it can be extremely time-consuming.

Now, thanks to a research project funded by the European Commission, art restorers have a number of high-tech alternatives. LASERART, a three-and-a-half-year project that finished in March 2000, was set up to develop non-intrusive measurement techniques using lasers to diagnose the state of frescoes and icons.
Laser-based techniques offer a number of advantages over the tap-the-surface method. For a start, it is not necessary for the equipment to be in contact with the artwork, which not only eliminates the possibility of accidental damage also avoids the need for restorers to work on scaffolding. The techniques are also cheaper and quicker than the manual approach, plus the measurements are completely automated and can be stored in a computerised database.

A choice of technologies

The project examined the potential of two different types of investigative technique: laser vibrometry and coherent optical imaging. Laser vibrometry is most useful for locating and measuring structural detachments where paint has come loose from the substrate) and delaminations, where layers of paint have become separated from each other. Vibrometry lends itself to locating cracks at the surface and in the outer and inner layers of artworks.

Vibration measurements are made using a laser Doppler vibrometer (LDV) which uses an interferometer to measure the Doppler frequency shift of a laser beam scattered by a moving target. Most people are familiar with the Doppler effect from the changing sound of a police siren it moves past them. In this case, the Doppler effect is an apparent change of frequency caused by the motion of the light source in relation to the measuring device. By combining two moving mirrors with the interferometer, it becomes possible to direct the laser beam at the desired measurement point, turning the LDV into a scanning laser Doppler vibrometer (SLDV).

A number of optical imaging techniques were explored in order to find one that had the required technical specifications, i.e. a measuring distance of about 5 meters and a spatial resolution of 10 µm.

A consortium of art experts

The work was carried out by a consortium of five European partners, led by the University of Ancona in Italy. Two well-known institutions in the field of artwork restoration - the French Cultural Ministry's Laboratoire de Recherche des Monuments Historiques (LRMH) and the Greek Benaki Museum- provided fundamental guidance to the project. LRMH has particular expertise in fresco restoration while the Benaki Museum provided experience with icons.

The two other partners were ARTT, a non-profit making research association based in Greece, and the UK SME (small and medium size enterprise) Ometron, which manufactures optical instruments. For Ometron, a direct benefit of the project was the development of Duoscan, a specialised SLDV designed to facilitate data analysis and which has proven useful for a number of other applications.

  The importance of field measurements

Restorers naturally want to see concrete results from new technologies rather than laboratory demonstrations, so an important part of the LASERART project was field measurements. In total, five measurement sessions were performed at various sites in Europe, including in Orvieto cathedral in Italy. Here the company in charge of restoration, Tecnireco, wanted some fourteenth century frescoes to be examined prior to restoration work.

A LASERART team therefore performed measurements using two vibrometers and acoustic excitation under the supervision of a professional restorer. The defects found using the lasers were compared to those identified by Tecnireco's restorers. Good agreement was found between the two, but the laser measurements also showed up several damaged areas that the restorers had not identified or had considered insignificant, mainly due to their small size.

  Complementing human expertise

There was less correlation with the identification of large defects. Some incidents were recorded in which the restorers found defects that the lasers did not, as well as vice versa. However, where the lasers failed to identify defects, this was explained by the fact that the vibrometry technique relies on behavioural differences across a scanned area, and the defects could be identified by comparing adjacent scanned areas. Given the evidence, the research partners are keen to emphasise that the laser techniques should be viewed as a complement to the skills of professional restorers rather than a total replacement.

   
A choice of technologies
A consortium of art experts
The importance of field measurements
Complementing human expertise
   

Key data

Research under the Measurements and testing generic action has demonstrated ways in which lasers can help art restorers assess the condition of works of art.

The LASERART project studied the potential of different types of laser-based investigative techniques and demonstrated their use in the field.

Project: LASERART - Non-intrusive laser measurement techniques for diagnostics of the state of conservation of frescoes, paintings and wooden icons (SMT4962062)

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