Lateral light scattering in fibrous media is investigated by computing the modulation transfer function (MTF) of 22 paper samples using a Monte Carlo model. The simulation tool uses phase functions from infinitely long homogenous cylinders and the directional inhomogeneity of paper is achieved by aligning the cylinders in the plane. The inverse frequency at half maximum of the MTF is compared to both measurements and previous simulations with isotropic and strongly forward single scattering phase functions. It is found that the conical scattering by cylinders enhances the lateral scattering and therefore predicts a larger extent of lateral light scattering than models using rotationally invariant single scattering phase functions. However, it does not fully reach the levels of lateral scattering observed in measurements. It is argued that the hollow lumen of a wood fiber or dependent scattering effects must be considered for a complete description of lateral light scattering in paper. / PaperOpt

In this dissertation we provide a comprehensive theory for the formation of a moire pattern in a sampled halftone image. We explore techniques for restoring a sampled halftone image with a moire pattern and techniques for preventing a moire pattern when a halftone picture is scanned. Specifically, we study the frequency, phase, and spatial geometry of a moire pattern. We observe and explain the half period phase reversal phenomenon that a moire pattern may exhibit. As a case study, we examine the moire patterns generated by a commercial scanner. We propose three restoration methods, including a notch filtering method, a simulation method, and a relaxation method. We also describe a moire prevention method, the partial inverse Fourier transform method. Finally, we propose a research agenda for further investigation. / Ph. D.

Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1982. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING / Bibliography: leaves 166-174. / by Yao-Ming Chao. / Sc.D.

To reproduce color images in print, the continuous tone image is first transformed into a binary halftone image, producing various colors by discrete dots with varying area coverage. In halftone prints on paper, physical and optical dot gains generally occur, making the print look darker than expected, and making the modeling of halftone color reproduction a challenge. Most available models are based on macroscopic color measurements, averaging the reflectance over an area that is large in relation to the halftone dots. The aim of this study is to go beyond the macroscopic approach, and study halftone color reproduction on a micro-scale level, using high resolution images of halftone prints. An experimental imaging system, combining the accuracy of color measurement instruments with a high spatial resolution, opens up new possibilities to study and analyze halftone color prints. The experimental image acquisition offers a great flexibility in the image acquisition setup. Besides trichromatic RGB filters, the system is also equipped with a set of 7 narrowband filters, for multi-channel images. A thorough calibration and characterization of all the components in the imaging system is described. The spectral sensitivity of the CCD camera, which can not be derived by direct measurements, is estimated using least squares regression. To reconstruct spectral reflectance and colorimetric values from the device response, two conceptually different approaches are used. In the model-based characterization, the physical model describing the image acquisition process is inverted, to reconstruct spectral reflectance from the recorded device response. In the empirical characterization, the characteristics of the individual components are ignored, and the functions are derived by relating the device response for a set of test colors to the corresponding colorimetric and spectral measurements, using linear and polynomial least squares regression techniques. Micro-scale images, referring to images whose resolution is high in relation to the resolution of the halftone, allow for measurements of the individual halftone dots, as well as the paper between them. To capture the characteristics of large populations of halftone dots, reflectance histograms are computed as well as 3D histograms in CIEXYZ color space. The micro-scale measurements reveal that the reflectance for the halftone dots, as well as the paper between the dots, is not constant, but varies with the dot area coverage. By incorporating the varying micro-reflectance in an expanded Murray-Davies model, the nonlinearity caused by optical dot gain can be accounted for without applying the nonphysical exponentiation of the reflectance values, as in the commonly used Yule-Nielsen model. Due to their different intrinsic nature, physical and optical dot gains need to be treated separately when modeling the outcome of halftone prints. However, in measurements of reflection colors, physical and optical dot gains always co-exist, making the separation a difficult task. Different methods to separate the physical and optical dot gain are evaluated, using spectral reflectance measurements, transmission scans and micro-scale images. Further, the relation between the physical dot gain and the halftone dot size is investigated, demonstrated with FM halftones of various print resolutions. The physical dot gain exhibits a clear correlation with the dot size and the dot gain increase is proportional to the increase in print resolution. The experimental observations are followed by discussions and a theoretical explanation.

This degree work places itself in the field of textile design within printing and surface design. The primary motive is to explore methods of designing textural expressions of a non-woven polyester fabric which combine relief and printing techniques in order to design textiles with three-dimensional properties. The work explores possibilities on how to bring an aesthetic expression into a synthetic non-woven inlay fabric by using screen print, sublimation print and relief moulding towards an interior context. The purpose is to take advantage of the technical properties such as expansion, softness and stiffness of a non-woven polyester fabric into the design work. The design method consisted of a material-based pre-study to gain knowledge about non-woven materials and their reactions to heat, moulding possibilities and printing options. Further developments were done through workshops that explored frottage as design inspiration for final designs and gradations with halftones as a colouring method. The outcome of this degree work resulted in a collection of three textile pieces; a wall covering, a room divider and a sound absorber that are seen as prototypes for further development on how to give synthetic non-woven textiles an alternative aesthetic expression.

Les solutions pour lutter contre la contrefaçon permettant une sécurisation des produits dépendent des capacités de mesure à l’échelle microscopique de points imprimés. Les recherches explicitées dans ce manuscrit ont donc été consacrées au développement d’un équipement et de méthodes permettant de mesurer les imprimés à l’échelle microscopique. Un microscope en réflexion avec lumière polarisée a été associé avec un appareil photographique numérique. La matrice de Bayer a été retirée de la surface du capteur photographique et des images raw ont été enregistrées. La platine du microscope, l’appareil photographique, un photomètre et un thermomètre ont été contrôlés directement dans une interface logicielle développée en Python. Une méthode de mesure permettant d’élargir la gamme dynamique de reflectances mesurées a été proposée. L’appareil et les méthodes de mesures ont permis d’améliorer la précision et d’automatiser la mesure des points de trame à l’échelle microscopique. Les élargissements physique et optique des points de trame ont alors été séparés et analysés. Une méthode d’ajustement des pics de l’histogramme, correspondant à l’encre et au papier, avec une fonction Gaussienne a été proposée. Des algorithmes de seuillage ont été employés pour séparer l’élargissement optique et physique des points de trame. Une méthode objective d’évaluation des algorithmes de seuillage a été développée pour déterminer leurs performances sur les images de tramés. Cette méthode d’évaluation procède à une simulation des effets de la diffusion de la lumière et des défauts générés par l’imagerie afin de générer des images tests et images de référence. 30 algorithmes de seuillage de la littérature ont été évalués et ont présenté une dépendance avec le pourcentage de couverture de l’encre. Deux nouveaux algorithmes de seuillage ont alors été développés spécialement pour traiter les imprimés tramés. Le premier algorithme détermine le déplacement du pic correspondant à l’encre sur l’histogramme. Le deuxième algorithme proposé se base sur une pseudo-déconvolution permettant de prétraiter les images et se basant sur une séparation des effets de l’élargissement optique. Une caractérisation de l’élargissement optique et physique a alors été menée sur 2708 images d’imprimés tramés. Enfin, un modèle de l’élargissement physique et un modèle de l’élargissement optique des points de trame ont été proposés. Le modèle physique se base sur une génération de particules d’encre placées selon une fonction de probabilité et sur une fusion des particules d’encre. Le modèle a été évalué en considérant 43269 points de trame différents, mesurés automatiquement sur le microscope. Un nouveau modèle prédisant la réflectance des tramés a été développé, basé sur une double convolution avec deux fonctions différentes d’étalement du point. Ce modèle a permis de simuler de manière précise les effets principaux de la diffusion de la lumière dans le tramé, tout en simulant les effets de piégeage de la lumière à proximité des bords des points de trame. / Applications in the field of product security and authentication to prevent counterfeiting rely on abilities of microscale measurements of printed dots. Thus, researches described in this manuscript have been directed toward the development of measurement methods and apparatus to characterize halftone dot at the microscale. A polarized reflection optical microscope has been adapted with a commercial digital camera. The Bayer matrix was removed from the surface of the camera and raw images were retrieved. The microscope stage, the camera, the photometer and the thermometer were controlled directly in a Python graphic user interface specifically developed. A high dynamic range capture method was proposed and tuned specifically to obtain richer information on the ink and paper regions. The measurement apparatus and methods helped improve the accuracy and automate the measurements of the halftone dots at the microscale. The physical and optical dot gains were then separated and analyzed. A Gaussian fitting of the ink and paper histogram peaks was proposed to measure automatically the ink and paper region reflectance as a function of the ink coverage. Thresholding algorithms were applied to separate optical and physical dot gain. An objective threshold evaluation method was developed in order to define the best threshold algorithms for halftone images. The method was based on a simulation of the optical dot gain effects and of the microscope distortions to obtain test images and ground truth images. 30 threshold algorithms from literature were evaluated and demonstrated dependency on the ink surface coverage of the halftones. Two novel threshold algorithms were then developed specifically to process halftones. The first threshold algorithm was based on the determination of the amount of ink peak shift. The second threshold algorithm proposed a pretreatment of the images by applying a pseudo-deconvolution strategy, removing the optical dot gain from the halftones. Characterizations of the optical and physical dot gains were then conducted analyzing 2708 different halftones. Finally, a physical dot gain model and an optical dot gain model were proposed in order to predict the halftone reflectances from raster to print. The physical dot gain model was based on the generation of single ink particles placed according to a probability mask and on a fusion of the ink particles. The model was evaluated with 43269 dot morphologies that were captured automatically on the microscope. A novel halftone reflectance model was proposed based on a double convolution with two different paper point spread functions. It allowed an accurate reproduction of the main effects of the light diffusion with, at the same time, an accurate reproduction of the light entrapment near the edges of the dots.

La protection des documents fiduciaires et identitaires contre la fraude exige le développement d’outils de contrôle fondés sur des effets visuels sans cesse renouvelés, difficiles à contrefaire (même pour un expert ... de la contrefaçon !). Ce projet de recherche s’inscrit dans cette problématique et vise à apporter des solutions originales via l’impression de supports diffusants d’une part, et le développement de modèles de rendu visuel d’autre part. Les effets visuels recherchés sont des ajustements de couleurs entre les deux faces d’un imprimé lorsque celui-ci est observé par transparence devant une source lumineuse. Pour obtenir facilement des ajustements de couleurs quelles que soient les couleurs visées, il est capital d’avoir un modèle à disposition, permettant de calculer les quantités d’encre à déposer. Un modèle doit être capable de prédire les facteurs spectraux de réflexion et de transmission du support imprimé en décrivant les phénomènes de diffusion optique présents en pratique dans les couches d’encre et le support. Nous nous intéressons plus particulièrement aux imprimés translucides contenant des couleurs en demi-ton des deux côtés de la surface avec pour objectif de prédire le rendu visuel pour diverses configurations d’observation. Pour cela, nous proposons une nouvelle approche basée sur l’utilisation de matrices de transfert de flux pour prédire les facteurs spectraux de réflexion et de transmission des imprimés lorsqu’ils sont éclairés simultanément des deux côtés. En représentant le comportement optique des différents composants d’un imprimé par des matrices de transfert, la description des transferts de flux entre ces composantes s’en trouve simplifiée. Ce cadre mathématique mène à la construction de modèles de prédiction de couleurs imprimées en demi-ton sur des supports diffusants. Nous montrons par ailleurs que certains modèles existants, comme le modèle de Kubelka-Munk ou encore le modèle de Clapper-Yule, peuvent également être formulés en termes de matrices de transfert. Les résultats obtenus avec les modèles proposés dans ce travail mettent en évidence des qualités de prédiction équivalentes, voire supérieures, à celles qu’on retrouve dans l’état de l’art, tout en proposant une simplification de la formulation mathématique et de la description physique des échanges de flux. Cette simplification fait de ces modèles des outils de calcul qui s’utilisent très facilement, notamment pour la détermination des quantités d’encre à déposer sur les deux faces de l’imprimé afin d’obtenir des ajustements de couleurs / The protection of banknotes or identity documents against counterfeiting demands the development of control tools based on visual effects that are continuously renewed. These visual effects become thus difficult to counterfeit even by an expert forger ! This research tries to deal with that issue. Its objective is to bring new solutions using on the one side, the printing of diffusing materials, and on the other side the development of visual rendering models that can be observed. The visual effects that are sought-after are the color matching on both sides of a printed document when observed against thelight. To easily obtain a color matching, whatever the colors that are aimed for, it is essential to have a model that helps in calculating the quantity of ink to be left on the document. A model must be used to predict the spectral reflectance and the transmittance factors of the printed document by describing the phenomena of optical diffusion really present in the ink layers and in the document. We shall focus our interest especially on translucent printed documents that have halftone colors on both sides. Our goal here is to predict the visual rendering in different configurations of observation. To that end, we are offering a new approach based on the use of flux transfer matrices to predict the spectral reflectance and transmittance factors of prints when they are simultaneously lit up on both sides. By representing with transfer matrices the optical behavior of the different components present in a printed document, we see that the description of flux transfer between these elements is thus simplified. This mathematical framework leads to the construction of prediction models of halftone printed colors on diffusing materials. We also show that some existing models, such as the Kubelka-Munk or the Clapper-Yule models, can also be formulated in transfer matrices terms. The results that we get with the models used in this work make apparent identical prediction quality and in some cases even better ones to the ones found in the state of the art, while offering a simplification of the mathematical formulation and the physical description of the flux transfer. This simplification thus transforms these models into calculation tools that can easily be used especially for the choice of quantities of ink that must be left on both sides of the document in order to obtain color matching