Laser-optical vibration measurement method
Laser-optical vibration measurement method
Laser-optical vibration measurement method
(VIB-ESPI) for the detection of loose plaster and paint layers
on historical wall paintings
Non-destructive diagnostic tool for heritage conservation
Historic wall paintings are often endangered by the painted plaster layer peeling away from the substrate. When endeavouring to preserve them and when monitoring repair measures, it is often necessary to check the adhesion of the paint-bearing plaster to the underlying wall. Loose areas must be localised as gently as possible so that conservation measures can be planned and carried out in good time. This is usually carried out by the conservator by carefully tapping the wall (percussion method), in which he deduces the adhesion from the sound. This method is very time-consuming and labour-intensive, it is not contactless, scaffolding is required to reach higher paintings, it is cost-intensive and the result depends on the restorer and is therefore subjective.
The laser-optical system presented here, on the other hand, enables a non-contact and simultaneous examination of an object surface in the square metre range, even from a great distance, and provides objective result data. In this method, the surface to be analysed is irradiated with sound from a loudspeaker, which excites loose areas to tiny vibrations that are then measured optically. The high sensitivity of the optical measurement technology allows very careful handling of the historical substance, as vibration amplitudes of just a few millionths of a millimetre can be detected. The expansion, shape and mechanical characteristics of the plaster layer determine the reaction of loose areas to sound excitation, so that the excitation frequency of the loudspeaker is tuned during an examination to ensure that all relevant detachments are detected.
To detect these very tiny amplitudes, an optical vibration measurement system is used whose sensitivity has been adapted accordingly. Its principle is based on video holography, in which the light scattered back from the object under investigation by a laser is superimposed on a reference light wave and recorded by a camera as a video hologram. Both the measurement and the analysis are fully computer-controlled. During the measurement, the process provides an immediate visualisation of vibrating plaster or paint layers on a monitor in the form of clearly visible pulsating changes in brightness. For the evaluation, the video images of each examined frequency are recorded with a PC during the measurement and analysed after the measurements have been completed. The evaluation provides a map of the areas detected as loose for each examination frequency, from which an overall result is created by summarising them. In the results image (see Fig. 1), which is underlaid with a black and white image of the area under investigation in order to enable precise allocation of the respective areas within the area under investigation, loose areas are identified by corresponding colour coding; in the results image shown, areas in yellow and red tones are loose.
Fig. 1: Result image of the vibration examination, underlaid with a black and white photograph of the painting (size approx. 110 x 90 cm²). Loose areas are shown in yellow-red colour tones.
This result serves as a basis for the restorer's further work. For larger areas to be examined, the total area must be divided into several smaller fields because the area that can be examined at once is limited. It depends, among other things, on the laser power, the reflectivity of the examination area and the distance between the measuring head and the examination area. In terms of laser power, areas of around one to three square metres can be examined at once with the current system.
The development and basic investigations were scientifically supported by the Dresden University of Fine Arts, Department of Restoration, and funded by the German Federal Environmental Foundation (DBU). In the current further development, funded by the German Research Foundation (DFG), a sound directional radiator is integrated into the system, which enables targeted sonication of the respective area under investigation and a reduction of disturbing sound in the rest of the building.
Application example of an on-site measurement: Adhesion of the Romanesque wall painting in the monastery church of St. Johann, Müstair, Switzerland
The laser-optical vibration measurement method for the detection of plaster and paint layer delamination has now proven its worth on a large number of historical objects. The results of an extensive measurement carried out in 2000 at the St. Johann Monastery (UNESCO World Heritage Site) in Müstair, Switzerland, are shown here as an example. The study was financed by the Pro Kloster St. Johann Müstair foundation and carried out in collaboration with the Institute for Monument Preservation at ETH Zurich.
Carolingian and Romanesque frescoes can be found in the monastery church, whereby the Carolingian frescoes in the apses are partially covered by the Romanesque ones. Large sections of the Romanesque fresco have become detached over the centuries, parts have fallen off and others have been removed to secure them. The remaining areas of this fresco were examined for detachment using the laser-optical vibration measurement method and the results mapped as the current state. A repeat measurement is intended to clarify whether the condition is stable or whether the detachments have increased further in the meantime. Fig. 2 shows part of the measuring system used in front of the painting in the north apse of the monastery church.
Fig. 2: Optical measuring head on a tripod in front of the wall painting in the north apse of the monastery church (still partially in the picture on the left: the loudspeaker).
Fig. 3 shows the result of the investigation for an area of about 4x4m². The image has been compiled from the results of several individual investigation areas. The areas detected as loose are shown in yellow-red colours.
Fig. 3: Result of the laser-optical vibration investigation in the north apse of the monastery church of St. Johann, Müstair, on an outline image of the investigation area. Size of the investigation area approx. 4x4m², overall result is composed of the results of the individual investigation fields. The geometric distortions of the individual images were corrected by K. Zehnder (Institute for Monument Preservation ETHZ). The plan basis for the depiction of the Romanesque wall painting is a 1:1 line tracing by Oskar Emmenegger & Söhne AG.
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