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Reference: Stephen O. Otim, (2007). Simplified fixed pattern noise correction and image display for high dynamic range CMOS logarithmic imagers. DPhil. University of Oxford.Citable link to this page:


Simplified fixed pattern noise correction and image display for high dynamic range CMOS logarithmic imagers

Abstract: Biologically inspired logarithmic CMOS sensors offer high dynamic range imaging capabilities without the difficulties faced by linear imagers. By compressing dynamic range while encoding contrast information, they mimic the human visual system’s response to photo stimuli in fewer bits than those used in linear sensors. Despite this prospect, logarithmic sensors suffer poor image quality due to illumination dependent fixed pattern noise (FPN), making individual pixels appear up to 100 times brighter or darker.This thesis is primarily concerned with alleviating FPN in logarithmic imagers in a simple and convenient way while undertaking a system approach to its origin, distribution and effect on the quality of monochrome and colour images, after FPN correction. Using the properties of the Human visual system, I propose to characterise the errors arising from FPN in a perceptually significant manner by proposing an error measure, never used before.Logarithmic operation over a wide dynamic range is first characterised using a new model; yi j =aj +bj ln(exp sqrt(cj +djxi)−1), where yi j is the response of the sensor to a light stimulus xi and aj, bj, cj and dj are pixel dependent parameters. Using a proposed correction procedure, pixel data from a monochromatic sensor array is FPN corrected to approximately 4% error over 5 decades of illumination even after digitisation - accuracy equivalent to four times the human eyes ability to just notice an illumination difference against a uniform background.By evaluating how error affects colour, the possibility of indiscernible residual colour error after FPN correction, is analytically explored using a standard set of munsell colours. After simulating the simple FPN correction procedure, colour quality is analysed using a Delta E76 perceptual metric, to check for perceptual discrepancies in image colour. It is shown that, after quantisation, the FPN correction process yields 1−2 Delta E76 error units over approximately 5 decades of illumination; colour quality being imperceptibly uniform in this range.Finally, tone-mapping techniques, required to compress high dynamic range images onto the low range of standard screens, have a predominantly logarithmic operation during brightness compression. A new Logr'Gb' colour representation is presented in this thesis, significantly reducing computational complexity, while encoding contrast information. Using a well-known tone mapping technique, images represented in this new format are shown to maintain colour accuracy when the green colour channel is compressed to the standard display range, instead of the traditional luminance channel. The trade offbetween colour accuracy and computation in this tone mapping approach is also demonstrated, offering a low cost alternative for applications with low display specifications.

Digital Origin:Born digital Type of Award:DPhil Level of Award:Doctoral Awarding Institution: University of Oxford


Dr Steve CollinsMore by this contributor


 Bibliographic Details

Issue Date: 2007

Copyright Date: 2007 Identifiers

Urn: uuid:6a8cbdbf-ef5c-473f-a22e-76e1f8a2603b Item Description

Type: thesis;

Language: en Keywords: high dynamic range CMOS sensors imaging logarithmic sensors EKV fixed pattern noise FPN modelling colour quality image sensors tone mapping contrast sensitivity CIELAB colour errorSubjects: Physical Sciences Engineering & allied sciences Electrical engineering Image understanding Sensors Tiny URL: ora:2443


Autor: Dr Stephen O. Otim - institutionUniversity of Oxford facultyMathematical,Physical and Life Sciences Division - Engineering Scienc



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