Claus, Daniel (2010) Resolution improvement methods applied to digital holography. PhD thesis, University of Warwick.

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Abstract

This thesis discusses the creation, acquisition and processing of digital holograms. Several techniques to improve the optical resolution have been investigated and developed. The optical resolution of numerically reconstructed digital holograms is restricted by both the sampling frequency and the overall sensor-size of the digital camera chip used. This thesis explores the limitations on the optical resolution of the holograms obtained. A typical sensor-size and sampling frequency for digital holograms is 10 mm and 100 lp/mm, respectively, whereas holographic plates used for optical holography can be more than a meter in size and have a sampling frequency of 3000 lp/mm. In order to take full advantage of the benefits digital holography offers, such as fast image acquisition and direct phase accessibility, the problem of reduced resolution needs to be overcome. Three resolution improvement methods have been developed in the scope of this PhD thesis. Prior to implementing the resolution improvement methods, different holographic setups have been analyzed, using the Space-bandwidth product (SBP) to calculate the information distribution both in the recording and reconstruction process. The first resolution improvement method is based on the synthetic aperture method. In this manner an increased sensor area can be obtained resulting in a larger numerical aperture (NA). A larger NA enables a more detailed reconstruction. The problem encountered in doing this is that an increased optical resolution results in a smaller depth of field. This has been overcome in this thesis by applying the extended depth of field method. As a result a high resolution in focus reconstruction of all longitudinal object regions is obtained. Moreover, the extended depth of field method allows a topological mapping of the object. The second resolution improvement method is based on sampling the interference pattern with sub-pixel accuracy. This was carried out on a CMOS-sensor and implemented by moving the light sensitive pixel-area into the dead zone in a 4x4 grid to cover whole the pixel-area. As a result the sensor's sampling frequency is doubled. The increased sampling frequency permits a reduction of the recording distance which results in an increased optical resolution of the reconstructed hologram. The third and novel approach described in this thesis has been to increase the optical resolution stored in a digital hologram by the combination of the synthetic aperture and the sub-pixel sampling methodBy analogy with the Fresnel-. The resolution improvement methods have been demonstrated both for lens-less digital holography and digital holographic microscopy.

Item Type:	Thesis or Dissertation (PhD)
Subjects:	Q Science > QC Physics T Technology > TA Engineering (General). Civil engineering (General)
Library of Congress Subject Headings (LCSH):	Holography, Resolution (Optics), Digital images, Image processing -- Digital techniques
Date:	2010
Institution:	University of Warwick
Theses Department:	School of Engineering
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Bryanston-Cross, P. ; Timmerman, Brenda
Extent:	xxxii, 265 leaves : ill., charts
Language:	eng
URI:	http://wrap.warwick.ac.uk/id/eprint/47211

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