2.5D virtual reality visualisation of shearography strain data from a canvas painting

摘要

Advanced measurement techniques for the structural diagnostics of artwork are increasingly providing more complex data that needs to be conveyed to conservators in a meaningful way. Holography and speckle interferometry based sensors are commonly used for this application and of these shearography is quite suitable for measurements outside the optics laboratory, due to the stability of using a common path interferometer configuration. Shearography provides non-contact full-field displacement gradient data on surface and sub-surface defects in the form of phase maps. The display of this data in the form of wrapped phase maps is only suitable for experienced users. A further image processing step generates unwrapped phase maps, which in an engineering environment are generally colour coded for display. For artwork measurement applications, the colour variation of the painting itself is important reference for the conservator to locate defect locations. In this manuscript the displacement gradient data is presented as false height on the flat painting surface. A virtual reality viewer, freely downloadable from the internet, is used to display the data and allow the user to interact with it by rotating the object in virtual space. The effect is rather similar to viewing a raked light photograph, however with the advantage of remote or online viewing.