Digital speckle photography and digital holography for the study of transparent media
Abstract: In the fields of combustion and fluid mechanics, temperature- and density fields are of interest to measure. Optical measurement methods may be used to study such transparent media. Flames, gases and liquids are examples of transparent media, often called phase objects in an optical context. Phase objects often have non-uniform refractive index distributions. Variations in density, temperature or pressure affect the refractive index. The phase of the optical wave is affected by the refractive index and therefore measurement of the phase of an optical wave gives useful information about the behaviour of the medium. Also the propagation direction of the optical wave is affected by a change in the refractive index. The aim with this thesis is to develop measurement methods, for the study of phase objects, which use only one observation direction and two recordings on a CCD, before and after a change in the phase object. In this thesis, the different optical measurement methods are based on digital speckle photography, interferometry and digital holography to visualize refractive index changes and to obtain phase object data. A phase object inserted in a laser speckle field introduces speckle displacement, from which information about the object may be extracted. The speckle displacements obtained from this defocused digital speckle photography technique are used to determine the phase gradients and the position of phase objects. For the study of transient events, like rapid changes of refractive index in gases, pulsed TV holography is a useful technique. Holograms are recorded, before and after a change in the phase object. The speckle patterns are reconstructed and numerically refocused to different imaging planes, where speckle displacement fields are calculated by digital speckle photography. A numerical refocusing technique that uses a numerical ideal lens in the Fourier domain is exploited. Interferometry is a useful method to measure phase changes between two object states. The phase change, which is affectedby the refractive index, may be used to create a wrapped phase map and the quality of the map may be poor if the phase object is out of focus during the measurement. The plane of maximum speckle correlation, which also is the position of the phase object, has to be found to get a phase map of best quality. A technique to improve the quality by first finding the right reconstruction distance and thereafter propagate the wave field to the determined distance is demonstrated. The results from the measurements show that the methods are suitable for the study of transparent media. The determination of phase gradients and positions from speckle displacement fields are applicable on phase objects that can be treated as thin sheets. Small measurement errors may occur because of this approximation. By use of the technique to find the right reconstruction distance before construction of the phase map, a reduction of the phase error was obtained
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