Low-coherence videoholography for sub-surfacxe deformation measurements in layered objects

摘要

TV-holography or electronic speckle pattern interferometry (ESPI) is used for the detection of micro-deformations on a variety of optically rough surfaces. These deformations are often a result of processes in the whole volume. If the investigated object is, for example, composed of different layers the interaction between these layers can result in a deformation of the surface. Such effects are often seen on antique paintings and result in a net-like structure of cracks, the so-called craquelee. To understand these processes it is necessary to measure the movement of each layer separately. ESPI can be used for sub-surface deformation measurements on layered objects, when it is operated with a low-coherence light source like a superluminescense diode as used in optical coherence tomography. Interferometric information is only registered if the difference between the path length of object and reference beam is less than the coherence length. By changing the path length of one of the beams it is possible to select the region where deformations will be measured even if it is located below the surface, provided the light penetrates the material sufficiently. A serious problem is the separation of the coherent light and the incoherent background. This is performed by a combination of spatial phase shift (SPS) and Fourier filtering.