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Title: Use of photometric stereo for the accurate modelling of three-dimensional skin microrelief
Author: Sohaib, A.
Awarding Body: University of the West of England, Bristol
Current Institution: University of the West of England, Bristol
Date of Award: 2013
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The largest organ of human body - "skin" is a multilayered organ with complex reflectance properties that not only vary with the direction of illumination but also with the wavelength of light. The complex Three Dimensional (3D) structure and optical properties of human skin makes it very difficult for techniques like Photometric stereo to accurately recover its 3D shape. One problem in particular concerns the presence of interreflections at concave regions of the skin surface topography. Common features such as wrinkles, moles, lesions, burns and surgical scars can appear as elevated skin area or as indentations in the skin surface, and usually have a different colour when compared to the surrounding skin. These differences in colour and the degree of concavity determine the amount of interreflection present and hence significantly affect the overall recovery of the 3D topography of the skin. This thesis explores the use of varying incident light wavelength in the visible spectrum to improve the recovered topography of human skin. New algorithms were developed and implemented to minimise the effects of interreflections and an accuracy assessment of using wavelengths in the visible spectrum was carried out for Caucasian, Asian and African American skin types. The results demonstrate that white light is not ideal for imaging skin relief and also illustrate the differences in recovered skin topography due to a non-diffuse Bidirectional Reflectance Distribution Function (BRDF) for each colour illumination used. In addition to the complication caused by interreflections, the translucency of skin allows light to scatter within the skin. This confutes the assumption associated with photometric stereo that - light incident on a particular patch of the skin is reflected at the same point. This issue is taken care of by incorporating polarisation information with photometric stereo to improve the 3D skin shape recovery. The experiments conducted show improvement in the recovered skin topography when specular reflected light was used instead of the multiply subsurface scattered diffuse light emanating from within the skin. The results also show an increase in polarised reflected light as the albedo of the skin decreases with wavelength in the visible spectrum, while darker skin tend to reflect more polarised light than lighter skin at similar wavelengths in the visible spectrum. These characteristics of human skin have not been investigated before and are important additions to the current state of the art in skin optics.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available