Adaptive holographic illumination in comparative electronic speckle pattern interferometry

Development of Spatial Light Modulators (SLM) opened a new area in coherent optical metrology. These modulators are capable to optically reconstruct digital holograms; therefore they can be used as active holographic optical elements in Electronic Speckle Pattern Interferometry (ESPI) or in digital holographic interferometry. SLM is also capable to generate computer calculated wave fronts (not belonging to an existing object), and multiple projections can be performed during the measurement time. Using this feature active measuring system can be built. In our work adaptive comparative ESPI measurement is done, where an optically reconstructed image of a computer generated hologram is used for holographic illumination of another object, with which difference deformations can be calculated. These active interferometers can continuously adapt themselves to the change of measuring conditions, because the test displacement profile can be compared with a suitable arbitrary master displacement profile in a relatively simple optical setup. This approach is a straightforward digital implementation of the analogue comparative measuring technique, where regular hologram plates are used to store and reconstruct optical wavefronts.