Holger Joost

Mural inspection by vibration measurements with TV-holography

A commonly encountered problem in the conservation of historical murals is the identification of sections in the plaster that have detached from the wall and thus threaten to fall off. Commonly, walls are inspected by the acoustic response to a gentle finger-tapping (percussion method). Since this is a costly and cumbersome technique, means for a more automatic inspection are searched for. A TV-holography system of increased sensitivity in combination with acoustic excitation of the object is shown to be a new and powerful tool for monitoring of loose areas. It has the advantage of non-contact and remote operation which, for example, is extremely useful in large buildings. Principles of the method, experimental results obtained at an artificial wall in the laboratory, and a thorough comparison of results from historical sites gained by the traditional percussion method and the new technique are presented. The latter shows very good agreement in the assessment of wall quality and thus is evidence of the suitability of the optical equipment for tasks in conservation.

Large-scale investigation of plaster detachments in historical murals by acoustic stimulation and video-holographic detection

In the conservation of historical murals an important issue is the detection of plaster or paint layers that detach from the supporting material and thus threaten to fall off. Commonly, walls are inspected by the acoustic response to a gentle finger-tapping (percussion method). Since this is a costly and cumbersome technique there is need for a metrological instrument serving the same purpose. In the last few years we have shown, that a time-average version of electronic speckle pattern interferometry (ESPI) with increased sensitivity in combination with acoustic excitation of the object can be a powerful tool for monitoring of loose areas. It offers full-field, video real time capability and has the advantage of non-contact and remote operation which, for example, is extremely useful in large buildings. Recently, a fully computer-based evaluation and control system was added to the system to assist in the introduction of the method as a generally approved tool in artwork monitoring. Principles of the method and instrumental features of the equipment are presented and some results and their interpretation obtained with the computerized system in the church and chapel at St. Johns convent at Mnstair, Switzerland are demonstrated.

Artwork Monitoring by Digital Image Correlation

The mechanical response of artwork to external loads from varying climate is monitored by time-records of displacement maps. These are obtained by digital correlation of series of images from either ordinary photographs in white-light illumination or speckle images in laser light. Special features and the advantages of either version are discussed. Both methods are applied in the study of degradation in historical leather tapestry — right at the site (some representative halls of Jever castle in Northern Germany) as well as for basic model investigations on leather behavior in artificial climate. Ideas are presented to derive the stress situation in the material from the original displacement data.

Vibration Monitoring by TV-Holography ­ a Diagnostic Tool in the Conservation of Historical Murals

A TV-holography system is designed to monitor detached plaster areas in historical murals. Defects in the wall respond to sound irradiation by vibrations. These are detected optically. A feature of the system is the enhanced sensitivity obtained by reference wave modulation. Experimental results obtained at a historical site show very good agreement with data gained by the traditional percussion method.

Faintly Falling Plaster

Shocks and vibrations endanger historical frescoes. A new acoustic-optical measuring technique enables the remote detection of loose plaster

Leise rieselt der Putz

Erschutterungen gefahrden historische Wandmalereien. Ein neues akustisch-optisches Messverfahren kann losen Putz aus der Ferne aufspuren

Tomographic mapping of airborne sound fields by TV-holography

Tomographic ESPI is a powerful technique for the non-intrusive mapping of a 3D sound field. It has been shown that sophisticated processing of the measurement data from a state-of-the-art ESPI instrument in time-average mode allows to measure the spatial distribution of sound pressure and phase in airborne ultrasound fields. The optical data have been used as input for an acoustic optimization of a parametric array for the nonlinear generation of audio sound that could not have been achieved otherwise.

TV-holographic mapping of airborne sound fields for the design of parametric arrays

Parametric acoustic arrays are built to generate highly directional audio sound by nonlinear interaction of ultrasound. Arrays especially built for applications at high audio sound pressure use the most effective ultrasonic transducer for airborne sound, i.e., a piezoelectric (PZT) bimorph. Since the individual transducer elements are very small (<16 mm in diameter) several hundred of them have to be combined to reach the desired audio sound pressure level. For high performance it is a prerequisite that all transducers radiate in phase. However, fluctuations in their properties result in according fluctuations in their phases. The construction of such a device therefore requires a non-intrusive technique for monitoring amplitude and phase of a three-dimensional sound field without creating any nonlinearity. TV-holography or Electronic Speckle Pattern Interferometry (ESPI) in its time-averaging mode combined with reference wave modulation has been applied for this purpose. The recordings represent a two-dimensional projection of the sound field integrated along the viewing direction. The three-dimensional field is obtained from many such projections through the sound field at different angles in a tomographic setup. Inversion by filtered backprojection yields the three-dimensional sound amplitude and phase that can be utilized to optimize transducer operation. The performance of such a system is demonstrated in the development of an ultrasonic array at 38.5 kHz. It is shown how the generation of highly directive audio sound has been improved by guidance from the optical results. The highly directional source of audio sound finally produced is needed for an application in monument research where loose areas in historical murals have to be identified.

Damage monitoring in historical murals by speckle interferometry

In the conservation of historical murals it is important to identify loose plaster sections that threaten to fall off. Electronic speckle interferometry in combination with acoustic excitation of the object has been employed to monitor loose areas. To avoid disadvantages of high sound irradiation of the complete building a novel directional audio-sound source based on nonlinear mixing of ultrasound has been introduced. The optical system was revised for optimum performance in the new environment. Emphasis is placed on noise suppression to increase sensitivity. Furthermore, amplitude and phase data of object response over the frequency-range inspected are employed to gain additional information on the state of the plaster or paint. Laboratory studies on sample specimen supplement field campaigns at historical sites.