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 w all and thus threaten to fall off. Commonly, walls are inspected by the aco ustic response to a gentle finger-tapping (percussion method). Since this i s a costly and cumbersome technique, means for a more automatic inspection are searched for. A TV-holography system of increased sensitivity in combin ation with acoustic excitation of the object is shown to be a new and power ful tool for monitoring of loose areas. It has the advantage of non-contact and remote operation which, for example, is extremely useful in large buil dings. Principles of the method, experimental results obtained at an artifi cial wall in the laboratory, and a thorough comparison of results from hist orical sites gained by the traditional percussion method and the new techni que are presented. The latter shows very good agreement in the assessment o f wall quality and thus is evidence of the suitability of the optical equip ment for tasks in conservation. (C) 2000 Elsevier Science Ltd. All rights r eserved.