Calibration and removal of lateral chromatic aberration in images

This paper addresses the problem of compensating for lateral chromatic aberration in digital images through colour plane realignment. Two main contributions are made: the derivation of a model for lateral chromatic aberration in images, and the subsequent calibration of this model from a single view of a chess pattern. These advances lead to a practical and accurate alternative for the compensation of lateral chromatic aberrations. Experimental results validate the proposed models and calibration algorithm. The effects of colour channel correlations resulting from the camera colour filter array interpolation is examined and found to have a negligible magnitude relative to the chromatic aberration. Results with real data show how the removal of lateral chromatic aberration significantly improves the colour quality of the image

Removing lateral chromatic aberration in bright field optical microscopy

"We present an efficient alternative to remove lateral chromatic aberration (LCA) in bright field light microscopy images. Our procedure is based on error calibration using time-sequential acquisition at different wavelengths, and error correction through digital image warping. Measurement of the displacements of fiducial marks in the red and green images relative to blue provide calibration factors that are subsequently used in test images to realign color channels digitally. We demonstrate quantitative improvement in the position and boundaries of objects in target slides and in the color content and morphology of specimens in stained biological samples. Our results show a reduction of LCA content below the 0.1% level.

Precise correction of lateral chromatic aberration in images

International audienceThis paper addresses the problem of lateral chromatic aberration correction in images through color planes warping. We aim at high precision (largely sub-pixel) realignment of color channels. This is achieved thanks to two ingredients: high precision keypoint detection, which in our case are disk centers, and more general correction model than what is commonly used in the literature, radial polynomial. Our setup is quite easy to implement, requiring a pattern of black disks on white paper and a single snapshot. We measure the errors in terms of geometry and of color and compare our method to three different software programs. Quantitative results on real images show that our method allows alignment of average 0.05 pixel of color channels and a residual color error divided by a factor 3 to 6

Full-field calibration and compensation of lateral chromatic aberration based on unwrapped phase

Lateral chromatic aberration (CA) of color cameras has great effects on the imaging quality. This paper presents a novel method to full-field calibrate lateral CA between color channels by using unwrapped phase data. Closed circle sinusoidal fringe patterns having the optimum three-fringe numbers are generated and displayed on a liquid crystal screen consecutively through red, green and blue channels. These closed fringe patterns are captured by a color camera. The wrapped phase and unwrapped phase of each pixel can be calculated by using four-step phase shifting algorithm and optimum three-fringe number method, respectively. The pixel deviations produced by lateral CA are computed by comparing the obtained absolute phase data between red, blue, and green channels in polar coordinate system and calibration is accomplished in Cartesian coordinate system. Lateral CA between color channels of the color camera can be compensated by using the calibrated data. Simulated and experimental results show the validity of the proposed calibration and compensation method

Correction of spherical single lens aberration using digital image processing for cellular phone camera

制度:新 ; 報告番号:甲3276号 ; 学位の種類:博士(工学) ; 授与年月日:2011/2/21 ; 早大学位記番号:新558

SSTRED: A data-processing and metadata-generating pipeline for CHROMIS and CRISP

We present a data pipeline for the newly installed SST/CHROMIS imaging spectrometer, as well as for the older SST/CRISP spectropolarimeter. The aim is to provide observers with a user-friendly data pipeline, that delivers science-ready data with the metadata needed for archival. We generalized the CRISPRED data pipeline for multiple instruments and added metadata according to recommendations worked out as part of the SOLARNET project. We made improvements to several steps in the pipeline, including the MOMFBD image restoration. A part of that is a new fork of the MOMFBD program called REDUX, with several new features that are needed in the new pipeline. The CRISPEX data viewer has been updated to accommodate data cubes stored in this format. The pipeline code, as well as REDUX and CRISPEX are all freely available through git repositories or web download. We derive expressions for combining statistics of individual frames into statistics for a set of frames. We define a new extension to the World Coordinate System, that allow us to specify cavity errors as distortions to the spectral coordinate.Comment: Draf

Full-field calibration of color camera chromatic aberration using absolute phase maps

The refractive index of a lens varies for different wavelengths of light, and thus the same incident light with different wavelengths has different outgoing light. This characteristic of lenses causes images captured by a color camera to display chromatic aberration (CA), which seriously reduces image quality. Based on an analysis of the distribution of CA, a full-field calibration method based on absolute phase maps is proposed in this paper. Red, green, and blue closed sinusoidal fringe patterns are generated, consecutively displayed on an LCD (liquid crystal display), and captured by a color camera from the front viewpoint. The phase information of each color fringe is obtained using a four-step phase-shifting algorithm and optimum fringe number selection method. CA causes the unwrapped phase of the three channels to differ. These pixel deviations can be computed by comparing the unwrapped phase data of the red, blue, and green channels in polar coordinates. CA calibration is accomplished in Cartesian coordinates. The systematic errors introduced by the LCD are analyzed and corrected. Simulated results show the validity of the proposed method and experimental results demonstrate that the proposed full-field calibration method based on absolute phase maps will be useful for practical software-based CA calibration

Novel Adaptive Optics Concepts : Wavefront Sensing with Sodium Laer Guide Stars at Extemely Large Telescopes and Simultaneous Differential Imaging

Durch den Einsatz Adaptiver Optik (AO) besitzt die astronomische Gemeinschaft seit fast 15 Jahren ein Werkzeug um mit modernen Großteleskopen beugungsbegrenzte Abbildungsqualität zu erreichen. Im Hinblick auf geplante Riesenteleskope wie das "Overwhelming Large Telescope" (OWL) der Europäischen Südsternwarte (ESO), mit Spiegeldurchmessern von bis zu 120m, wird der Einsatz von AO-Systemen der nächsten Generation, wie Natrium-Laser-Leitstern gestützte multi-konjugierte Adaptive Optik, unabdingbar sein, um das Leistungsvermögen (Auflösung und Lichtsammelvermögen) solcher Teleskope voll auszuschöpfen. Künstliche Natrium-Laser-Leitsterne führen aufgrund ihrer endlichen Entfernung und nicht vernachlässigbaren Ausdehnung in der Atmosphäre unter anderem zu perspektivischen Effekten, die die Messgenauigkeit herkömmlicher Wellenfrontsensoren stark beeinträchtigen. Um solchen Effekten entgegenzuwirken, werden im Moment verschiedene Konzepte diskutiert. Diese sind jedoch mit hohem technischen Aufwand und/oder stark begrenzter Photonenausbeute verknüpft. Deshalb wird im Rahmen dieser Arbeit ein neuartiger Wellenfrontsensor vorgestellt, der sich der vorgegebenen Geometrie künstlicher Laserleitsterne in einer natürlichen Art und Weise anpasst. Der Sensor besteht aus zwei voneinander unabhängig arbeitenden Komponenten; einem reflektierenden Glasstab sowie einer Maske mit konzentrischen Spalten. Beide repräsentieren neuartige Sensortypen basierend auf dem z-invariant- und dem invertierten Bessel-Strahlkonzept. Die Grundlagen und Funktionsweise beider Techniken werden (mit einem Schwerpunkt auf dem invertierten Bessel-Strahlkonzept) diskutiert und mit mehr technisch orientierten Konzepten wie dem "temporal gating" im Hinblick auf deren Lichtsammeleffizienz verglichen. Des weiteren wurde das Messkonzept des reflektierenden Stabes mit Hilfe eines Laboraufbaus, und in einer mehr realistischen Umgebung am William Herschel Teleskop (WHT) unter Einsatz eines Rayleigh-Laser-Leitsterns getestet und verifiziert. Ebenfalls eine neue Methode in der Adaptiven Optik, die an modernen 8m-Teleskopen bereits zum Einsatz kommt, ist das so genannte "Simultaneous Differential Imaging (SDI)" (Simultane Differenzielle Abbildung). SDI bietet die Möglichkeit zum direkten Nachweis extrasolarer Planeten, einem der neuesten Forschungsschwerpunkte in der galaktischen Astronomie. Mit Hilfe moderner AO-Systeme erreicht man an Grossteleskopen im Prinzip die notwendige Auflösung sowie ausreichend Kontrast, um extrasolare Riesenplaneten abzubilden. Das SDI-Modul, eine Erweiterung der AO-nahinfrarot-Kamera NACO am Very Large Telescope (VLT), ist optimiert extrasolare Gasriesen mehrerer Jupitermassen aufgrund deren Methanatmosphäre zu detektieren. Deshalb wurde im Rahmen dieser Arbeit für zukünftig geplante SDI-Projekte am Max-Planck-Institut für Astronomie ein Datenreduktionspaket entwickelt. Dieses wurde an Epsilon Eridani des im Augenblick vielversprechendsten Kandidaten für den direkten Nachweis eines Exoplaneten getestet und geeicht. Dazu wurde ein Datensatz von Epsilon Eridani aufgenommen und ausgewertet, der die bisher tiefsten jemals aufgenommenen Hochkonstrastaufnahmen dieser Quelle beinhaltet

Vers l’étalonnage interne de caméra à haute précision

This dissertation focuses on internal camera calibration and, especially, on its high-precision aspects. Two main threads are followed and examined: lens chromatic aberration correction and estimation of camera intrinsic parameters. For the chromatic aberration problem, we follow a path of digital post-processing of the image in order to get rid from the color artefacts caused by dispersion phenomena of the camera lens system, leading to a noticeable color channels misalignment. In this context, the main idea is to search for a more general correction model to realign color channels than what is commonly used - different variations of radial polynomial. The latter may not be general enough to ensure stable correction for all types of cameras. Combined with an accurate detection of pattern keypoints, the most precise chromatic aberration correction is achieved by using a polynomial model, which is able to capture physical nature of color channels misalignment. Our keypoint detection yields an accuracy up to 0.05 pixels, and our experiments show its high resistance to noise and blur. Our aberration correction method, as opposed to existing software, demonstrates a final geometrical residual of less than 0.1 pixels, which is at the limit of perception by human vision. When referring to camera intrinsics calculation, the question is how to avoid residual error compensation which is inherent for global calibration methods, the main principle of which is to estimate all camera parameters simultaneously - the bundle adjustment. Detachment of the lens distortion from camera intrinsics becomes possible when the former is compensated separately, in advance. This can be done by means of the recently developed calibration harp, which captures distortion field by using the straightness measure of tightened strings in different orientations. Another difficulty, given a distortion-compensated calibration image, is how to eliminate a perspective bias. The perspective bias occurs when using centers of circular targets as keypoints, and it gets more amplified with increase of view angle. In order to avoid modelling each circle by a conic function, we rather incorporate conic affine transformation function into the minimization procedure for homography estimation. Our experiments show that separate elimination of distortion and perspective bias is effective and more stable for camera's intrinsics estimation than global calibration methodCette thèse se concentre sur le sujet de la calibration de la camera interne et, en particulier, sur les aspects de haute précision. On suit et examine deux fils principaux: la correction d'une aberration chromatique de lentille et l'estimation des paramètres intrinsèques de la caméra. Pour la problème de l'aberration chromatique, on suit un chemin de post-traitement numérique de l'image, afin de se débarrasser des artefacts de couleur provoqués par le phénomène de dispersion du système d'objectif de la caméra, ce qui produit une désalignement perceptible des canaux couleur. Dans ce contexte, l'idée principale est de trouver un modèle de correction plus général pour réaligner les canaux de couleur que ce qui est couramment utilisé - différentes variantes du polynôme radial. Celui-ci ne peut pas être suffisamment général pour assurer la correction précise pour tous les types de caméras. En combinaison avec une détection précise des points clés, la correction la plus précise de l'aberration chromatique est obtenue en utilisant un modèle polynomial qui est capable de capter la nature physique du décalage des canaux couleur. Notre détection de points clés donne une précision allant jusqu'à 0,05 pixels, et nos expériences montrent sa grande résistance au bruit et au flou. Notre méthode de correction de l’aberration, par opposition aux logiciels existants, montre une géométrique résiduelle inférieure à 0,1 pixels, ce qui est la limite de la perception de la vision humaine. En ce qui concerne l'estimation des paramètres intrinsèques de la caméra, la question est de savoir comment éviter la compensation d'erreur résiduelle qui est inhérent aux méthodes globales d'étalonnage, dont le principe fondamental consiste à estimer tous les paramètres de la caméra ensemble - l'ajustement de faisceaux. Détacher les estimations de la distorsion de la caméra et des paramètres intrinsèques devient possible lorsque la distorsion est compensée séparément. Cela peut se faire au moyen de la harpe d'étalonnage, récemment développée, qui calcule le champ de distorsion en utilisant la mesure de la rectitude de cordes tendues dans différentes orientations. Une autre difficulté, étant donnée une image déjà corrigée de la distorsion, est de savoir comment éliminer un biais perspectif. Ce biais dû à la perspective est présent quand on utilise les centres de cibles circulaires comme points clés, et il s'amplifie avec l'augmentation de l'angle de vue. Afin d'éviter la modélisation de chaque cercle par une fonction conique, nous intégrons plutôt fonction de transformation affine conique dans la procédure de minimisation pour l'estimation de l'homographie. Nos expériences montrent que l'élimination séparée de la distorsion et la correction du biais perspectif sont efficaces et plus stables pour l'estimation des paramètres intrinsèques de la caméra que la méthode d'étalonnage global