Design and implementation of a digital holographic microscope with fast autofocusing
İlhan, Aytekin Hazar (2014) Design and implementation of a digital holographic microscope with fast autofocusing. [Thesis]
Holography is a method for three-dimensional (3D) imaging of objects by applying interferometric analysis. A recorded hologram is required to be reconstructed in order to image an object. However one needs to know the appropriate reconstruction distance prior to the hologram reconstruction, otherwise the reconstruction is out-of-focus. If the focus distance of the object is not known priori, then it must be estimated using an autofocusing technique. Traditional autofocusing techniques used in image processing literature can also be applied to digital holography. In this thesis, eleven common sharpness functions developed for standard photography and microscopy are applied to digital holograms, and the estimation of the focus distances of holograms is investigated. The magnitude of a recorded hologram is quantitatively evaluated for its sharpness while it is reconstructed on an interval, and the reconstruction distance which yields the best quantitative result is chosen as the true focus distance of the hologram. However autofocusing of highresolution digital holograms is very demanding in means of computational power. In this thesis, a scaling technique is proposed for increasing the speed of autofocusing in digital holographic applications, where the speed of a reconstruction is improved on the order of square of the scale-ratio. Experimental results show that this technique offers a noticeable improvement in the speed of autofocusing while preserving accuracy greatly. However estimation of the true focus point with very high amounts of scaling becomes unreliable because the scaling method detriments the sharpness curves produced by the sharpness functions. In order to measure the reliability of autofocusing with the scaling technique, fifty computer generated holograms of gray-scale human portrait, landscape and micro-structure images are created. Afterwards, autofocusing is applied to the scaleddown versions of these holograms as the scale-ratio is increased, and the autofocusing performance is statistically measured as a function of the scale-ratio. The simulation results are in agreement with the experimental results, and they show that it is possible to apply the scaling technique without losing significant reliability in autofocusing.
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