Design and experimental validation of a snapshot polarization contrast imager

International audienceWe present a degree of polarization imaging system based on a Wollaston prism and a single CCD camera. This architecture eliminates technical inaccuracies and noise sources that are present in experimental setups containing a polarization switching element. After the acquisition of two images corresponding to two orthogonal states of polarization, one can compute the orthogonal state contrast image (OSCI), which is an estimate of the local degree of polarization of the backscattered light when the observed materials are purely depolarizing. The instrument design coupled to an efficient calibration enables the estimation of the OSCI from a single image acquisition and significant reduction of technical noise present in other polarization imaging systems. The setup was tested in realistic conditions where it represents a real asset

Target detection in active polarization images perturbed with additive noise and illumination nonuniformity.

International audienceActive imaging systems that illuminate a scene with polarized light and acquire two images in two orthogonal polarizations yield information about the intensity contrast and the orthogonal state contrast (OSC) in the scene. Both contrasts are relevant for target detection. However, in real systems, the illumination is often spatially or temporally nonuniform. This creates artificial intensity contrasts that can lead to false alarms. We derive generalized likelihood ratio test (GLRT) detectors, for which intensity information is taken into account or not and determine the relevant expressions of the contrast in these two situations. These results are used to determine in which cases considering intensity information in addition to polarimetric information is relevant or not

Comparative study of the best achievable contrast in scalar, Stokes and Mueller images

International audienceWe compare the relative performance of different active polarimetric imaging architectures for target detection applications. We show that if the noise that affects the measurements is additive and if the only relevant parameter is the contrast between an object of interest and a background with different Mueller matrices, the most efficient imaging architecture consists in acquiring a single intensity image while optimizing the illumination and analysis states of polarization

Detection in polarimetric images in the presence of additive noise and non-uniform illumination

International audienceActive polarimetric imaging systems yield information about the intensity contrast and the Orthogonal State Contrast (OSC) in the scene. However, in real systems, the illumination is often spatially or temporally non uniform which creates artificial intensity contrasts that can lead to false alarms. We derive the Generalized Likelihood Ratio Test (GLRT) detectors when intensity information is taken into account or not. These results are used to determine in which cases considering intensity information in addition to polarimetric information is relevant or no

Sources of possible artefacts in the contrast evaluation for the backscattering polarimetric images of different targets in turbid medium

International audienceIt is known that polarization-sensitive backscattering images of different objects in turbid media may show better contrasts than usual intensity images. Polarimetric image contrast depends on both target and background polarization properties and typically involves averaging over groups of pixels, corresponding to given areas of the image. By means of numerical modelling we show that the experimental arrangement, namely, the shape of turbid medium container, the optical properties of the container walls, the relative positioning of the absorbing, scattering and reflecting targets with respect to each other and to the container walls, as well as the choice of the image areas for the contrast calculations, can strongly affect the final results for both linearly and circularly polarized light

Near-infrared active polarimetric and multispectral laboratory demonstrator for target detection

International audienceWe report on the design and exploitation of a real-field laboratory demonstrator combining active polarimetric and multispectral functions. Its building blocks, including a multiwavelength pulsed optical parametric oscillator at the emission side and a hyperspectral imager with polarimetric capability at the reception side, are described. The results obtained with this demonstrator are illustrated on some examples and discussed. In particular it is found that good detection performances rely on joint use of intensity and polarimetric images, with these images exhibiting complementary signatures in most cases

Acquisition et extraction d'information des images polarimétriques actives.

OSC ("Orthogonal State Contrast"imaging is an active polarimetric imaging mode which consists in illuminating the scene with a totally polarized light and analyzing the backscattered light along two orthogonal states of polarization. It reveals contrasts which are not present in conventional intensity images. It arouses a great interest in several domains such as remote sensing, biomedical imaging or vision through diffusive medium. The objective of this thesis was to design an OSC imaging system and to characterize its performance in terms of acquisition capabilities and signal processing. We designed and tested a simple robust setup which is able to acquire simultaneously both orthogonal states of polarization. This architecture reaches the fundamental limits set by detector noise, photon shot noise or spatial and temporal non-uniformity of th illuminaiton. We demonstrated experimentally its benefit when imaging moving targets or imaging through turbulent medium. In order to characterize the performance of this imaging mode, we determine estimation algorithm depending on the predominant type of noise and compare their precision to the Cramer- Rao lower bound. We then studied the performance for target detection as a function of the non-uniformity of the illumination. Finally, we studied the maximization of contrast in scalar polarimetric images. We proposed a method which simultaneously optimizes the states of polarization of the illumination and the reception. This is an interesting perspective for Mueller imagers improvement.L'imagerie d'OSC ("Orthogonal State Contrast") est un mode d'imagerie polarimé- trique active qui consiste à illuminer la scène avec une lumière polarisée et à analyser le signal rétro-diffusé dans deux états de polarisation orthogonaux. Elle permet de mettre en évidence des contrastes qui ne sont pas présents dans les images d'intensité classiques. Les applications sont très nombreuses en télédétection, dans le domaine biomédical ou pour la vision à travers les milieux diffusants. L'objectif de cette thèse était de concevoir un instrument et de caractériser ses capacités en terme d'acquisition et de traitement de l'information. Nous avons réalisé et validé un système d'acquisition simple et robuste permettant de faire l'image simultanée des deux états de polarisation. Cette architecture permet d'atteindre des limites fondamentales fixées par le bruit de détecteur, le bruit de photons ou la non-uniformité spatiale et temporelle de l'illumination. Nous avons montré expérimentalement son apport pour l'imagerie de cibles mobiles ou à travers un milieu turbulent. Pour évaluer les capacités de ce mode d'imagerie, nous avons déterminé les algorithmes d'estimation et caractérisé leurs précisions en fonction du type de bruit prédominant grâce à la borne de Cramer-Rao. Dans un deuxième temps, nous avons étudié les performances de détection de cible en fonction de la non-uniformité de l'illumination. Enfin, d'une manière plus générale, nous nous sommes intéressés à la maximisation du contraste dans une image polarimétrique scalaire. Nous avons proposé une méthode d'optimisation conjointe des états de polarisation d'illumination et d'analyse qui présente des perspectives prometteuse pour l'amélioration des imageurs de Mueller

Acquisition et extraction d'information des images polarimétriques actives.

OSC ("Orthogonal State Contrast"imaging is an active polarimetric imaging mode which consists in illuminating the scene with a totally polarized light and analyzing the backscattered light along two orthogonal states of polarization. It reveals contrasts which are not present in conventional intensity images. It arouses a great interest in several domains such as remote sensing, biomedical imaging or vision through diffusive medium. The objective of this thesis was to design an OSC imaging system and to characterize its performance in terms of acquisition capabilities and signal processing. We designed and tested a simple robust setup which is able to acquire simultaneously both orthogonal states of polarization. This architecture reaches the fundamental limits set by detector noise, photon shot noise or spatial and temporal non-uniformity of th illuminaiton. We demonstrated experimentally its benefit when imaging moving targets or imaging through turbulent medium. In order to characterize the performance of this imaging mode, we determine estimation algorithm depending on the predominant type of noise and compare their precision to the Cramer- Rao lower bound. We then studied the performance for target detection as a function of the non-uniformity of the illumination. Finally, we studied the maximization of contrast in scalar polarimetric images. We proposed a method which simultaneously optimizes the states of polarization of the illumination and the reception. This is an interesting perspective for Mueller imagers improvement.L'imagerie d'OSC ("Orthogonal State Contrast") est un mode d'imagerie polarimé- trique active qui consiste à illuminer la scène avec une lumière polarisée et à analyser le signal rétro-diffusé dans deux états de polarisation orthogonaux. Elle permet de mettre en évidence des contrastes qui ne sont pas présents dans les images d'intensité classiques. Les applications sont très nombreuses en télédétection, dans le domaine biomédical ou pour la vision à travers les milieux diffusants. L'objectif de cette thèse était de concevoir un instrument et de caractériser ses capacités en terme d'acquisition et de traitement de l'information. Nous avons réalisé et validé un système d'acquisition simple et robuste permettant de faire l'image simultanée des deux états de polarisation. Cette architecture permet d'atteindre des limites fondamentales fixées par le bruit de détecteur, le bruit de photons ou la non-uniformité spatiale et temporelle de l'illumination. Nous avons montré expérimentalement son apport pour l'imagerie de cibles mobiles ou à travers un milieu turbulent. Pour évaluer les capacités de ce mode d'imagerie, nous avons déterminé les algorithmes d'estimation et caractérisé leurs précisions en fonction du type de bruit prédominant grâce à la borne de Cramer-Rao. Dans un deuxième temps, nous avons étudié les performances de détection de cible en fonction de la non-uniformité de l'illumination. Enfin, d'une manière plus générale, nous nous sommes intéressés à la maximisation du contraste dans une image polarimétrique scalaire. Nous avons proposé une méthode d'optimisation conjointe des états de polarisation d'illumination et d'analyse qui présente des perspectives prometteuse pour l'amélioration des imageurs de Mueller

Optimization of the contrast in polarimetric scalar images

We consider active polarimetric imaging systems that illuminate a scene with an incident polarization state and project the backscattered light on another polarization state in order to produce a scalar intensity image. We present and analyze a method for determining the configuration of illumination and analysis polarization states that maximizes the observed contrast between a target and the background when the scene is partially depolarizing and in the presence of additive Gaussian detection noise. © 2009 Optical Society o