Digital holography replaces the permanent recording material of analog holography with an electronic light sensitive matrix detector, but besides the many unique advantages, this brings serious limitations with it as well. The limited resolution of matrix detectors restricts the field of view, and their limited size restricts the resolution in the reconstructed holographic image. Scanning the larger aerial hologram (the interference light field of the object and reference waves in the hologram plane) with the small matrix detector or using magnification for the coarse matrix detector at the readout of the fine-structured aerial hologram, these are straightforward solutions but have been exploited only partially until now. We have systematically applied both of these approaches and have driven them to their present extremes, over half a magnitude in extensions.
In the paper a brief survey of research at the Department is given that had a twofold aim: to support the project DISCO in indirect and direct ways as well. In this respect the physical state of the art of the analogue basis of DISCO is discussed: principle of the comparative technique named difference holographic interferometry (DHI) is analysed and applications for comparative deformation and shape control in analogue holography are presented. Peculiarities of holographic illumination as well as requirements for repositioning of the master and test objects at comparative measurements are treated. Investigations of the measuring range extension in holographic interferometry are described. Detailed analysis is devoted to the puzzle-read-out extension technique. Finally, application of the principles of DHI in comparative deformation control by digital holography and the developed portable laboratory device for comparative deformation control are presented.
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