Promotion of Tarek Abu Haila
Congratulations! Tarek Abu Haila, employee of the department "Cultural Heritage Digitization", successfully defended his dissertation "The Impact of Cross-Polarization on RGB Imaging, Color Calibration and Accuracy" at TU Darmstadt on January 22, 2024. The first examiner and doctoral advisor was Prof. Arjan Kuijper (Fraunhofer IGD/TU Darmstadt), the second examiner was Prof. Dieter W. Fellner (Fraunhofer IGD/TU Darmstadt), and the third examiner was Prof. Thomas S. A. Wallis (TU Darmstadt).
Polarization filters are very useful and come very handy in various applications ranging from artifacts digitization to medical imaging where shiny reflective surfaces hinder clear imaging otherwise, and consequently would cause false reading of the data. Usually polarization filters are used so to eliminate any undesirable reflections, specularities and highlights a surface would depict given the surface’s material and the lighting setup. In most applications that uses polarization filters, more information of the actual imaged surface are retrieved which wouldbe obscured behind highlights otherwise, so the imaged surface looks rather continuous and complete. However, how polarization filters actually affect and influence the imaged surface color receive too little attention despiteits importance in applications such as cultural heritage digitization, faithful product photography or medical skinand tissues imaging among others.
In this thesis we address fundamental issues polarization filters cause to color registration, reproduction and consequently color calibration, so to make people and other researchers aware of the consequences of using polarization filters in their applications especially people whose concern is to have a faithful and accurate color reproduction such in the cultural heritage digitization sector. We propose, as well, a new method of automated mosaic scanning that is, first, overcome the problem of undesirable surface reflections despite the use of cross-polarization due to the highly reflective material’s surface finish. Second, paving the way towards colorcalibration for 3D textures.
The following work demonstrates in details how polarization filters affect color reproduciblity and gray scalelinearity, it shows as well how polarization filters can be a potential cause of shift in the white-point of a lightsource which has consequences on how surface colors under such polarized light source would be rendered. Finally, we demonstrate, while using cross-polarization, a new proposed method of color calibration based ona mosaic approach that can be the backbone of standardizing 3D digitization and texture color correction while keeping color accuracy in mind.
With the help of this work, it is possible now to understand the changes that happen to the imaged colors while using cross-polarization and how these changes make color faithful reproduction deviate from the real measurements that are usually measured using a spectrophotometer (the ground-truth; actual perception). This body of research also explains and show how grayscale colors are affected so that grayscale linearity cannot be guaranteed anymore depending on the imaged material, and also why some colors may look washed-out when imaged using cross-polarization. The reader will also be able to understand how cross-polarization contributes to a shift in the light source’s white-point and a change in the color-correlated temperature (CCT) which will affect consequently how surface colors would interact with the light source and being mathematically calculated. Finally, a new approach to color calibration with the help of cross-polarization and a moving camera/light source combination for large color target scans and/or highly reflective surfaces is introduced, the new approach is being assessed against the ISO standards for color calibration and reproduction in cultural heritage showing that it is viable and passes the highest possible ISO level for color accuracy and fidelity.