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1.3 PHYSICAL SCIENCES
The aim of the work reported in this thesis is to develop a simple electronic speckle pattern interferometry (ESPI) system by combining holographic optical element technology with speckly interferometry. A holographic optical element is used in an ESPI system instead of the lenses, mirrors, beam splitters and beam combiners which are usually required in a conventional system. The final ESPI system consists only of a single holographic element, laser and CCD camera. Many currently available systems are complicated and consist of expensive optics that can be difficult to align. Even the simplest require several conventional optical elements to manipulate the laser light and provide the necessary object and reference beams. In this thesis holographic optical element technology is combined with speckle interferometry in order to make a simple, compact and low cost ESPI system. A compact ESPI system is built by incorporating a single holographic optical element, which generates a specked reference beam and combines this with the object wavefront as it approaches the camera. In addition, a dual interferometer was constructed with a facility to test an object using both ESPI and holographic interferometry. In this way the advantages of high quality interference fringes in holographic interferometry, speed and convencience of ESPI can be exploited. Firstly, the use of a self developing holographic recording material allowed live holographic interferometry to be carried out, and viewed with a CCD camera. It was then shown that the recorded hologram could be used, with no further adjustment, to provide a speckle reference beam to the camera, so that ESPI could be performed in the same optical set up. The out of plane deformation of the object was studied with the two techniques in one interferometer and results were presented. The self developing acrylamide based photopolymer used was a holographic recording material formulated and prepared at the Centre for Industrial and Engineering Optics (IEO). Initially, silver halide emulsions (including the HP series silver halide emulsions produced in the Central Laboratory for Optical Storage and Processing of Information, Bulgarian Academy of Sciences, Sofia, Bulgaria) were used to fabricate some of the holographic optical elements because they allowed the recording of reflection format HOEs essential to the compact final ESPI system design. However, as improved formulations of IEO’s own photopolymer became available, reflection holographic optical elements were also recorded in the photopolymer. Full field displacement maps of object deformations were obtained by implementing phase shifting techniques using a laser diode in which the drive current was modulated to produce a path length change by varying the wavelength. The final compact ESPI system using a reflection holographic optical element was also used to study vibration mode patterns. Amplitude and phase of the modes were mapped using phase shifting techniques and the results were presented. For the first time, reflection holographic optical elements recorded in acrylamide based photopolymer material were used in a single optical element interferometer. This system was also used to study the vibration mode pattern of the object.
Reddy, G. (2005). A simple electronic speckle pattern interferometry system using holographic optical elements. Doctoral thesis. Dublin Institute of Technology. doi:10.21427/D7NC8X