Fourier-based automatic alignment for improved visual cryptography schemes

International audienceIn Visual Cryptography, several images, called "shadow images", that separately contain no information, are overlapped to reveal a shared secret message. We develop a method to digitally register one printed shadow image acquired by a camera with a purely digital shadow image, stored in memory. Using Fourier techniques derived from Fourier Optics concepts, the idea is to enhance and exploit the quasi periodicity of the shadow images, composed by a random distribution of black and white patterns on a periodic sampling grid. The advantage is to speed up the security control or the access time to the message, in particular in the cases of a small pixel size or of large numbers of pixels. Furthermore, the interest of visual cryptography can be increased by embedding the initial message in two shadow images that do not have identical mathematical supports, making manual registration impractical. Experimental results demonstrate the successful operation of the method, including the possibility to directly project the result onto the printed shadow image

Implementation of arbitrary real-valued correlation filters for the shadow-casting incoherent correlator

International audienceWe describe an incoherent correlator, based on the shadow-casting principle, that is able to implement any real-valued linear correlation filter. The correlation filter and the input image are displayed on commercial liquid-crystal television ~LCTV! panels. Although it cannot handle high-resolution images, the incoherent correlator is lensless, compact, low cost, and uses a white-light source. A bipolar technique is devised to represent any linear filter, computed from a single reference image or composite, in the correlator. We demonstrate experimentally the efficiency of the design in the case of optimal trade-off ~OT! filters and optimal trade-off synthetic discriminant function ~OT-SDF! filter

Entropy of partially polarized light and application to statistical processing techniques

International audienceWe have analyzed entropy properties of coherent and partially polarized light in an arbitrary number of spatial dimensions. We show that for Gaussian fields, the Shannon entropy is a simple function of the intensity and of the Barakat degree of polarization. In particular, we provide a probabilistic interpretation of this definition of the degree of polarization. Using information theory results, we also deduce some physical properties of partially polarized light such as additivity of the entropy and depolarization effects induced by mixing partially polarized states of light. Finally, we demonstrate that entropy measures can play an important role in segmentation and detection task

Shack-Hartmann multiple spots with diffractive lenses

International audienceIn this Letter we aim to bring an understanding to the apparition of multiple spots when using a Shack-Hartmann (SH) wavefront sensor behind diffractive lenses. In contrast to previous work, this phenomenon is described in terms of diffractive orders. It is illustrated with Zemax simulations, where three kinds of diffractive lenses (monofocal, bifocal, and trifocal) are set behind a microlens array. The presence of multiple spots is related to the phase jump of the diffractive profile and also to the number of steps seen through the microlens pupil. The possibility of assessing the optical quality of such lenses using SH measurements is discussed, in particular within the field of ophthalmology, where the need for precautions is underline

Probing multilayer stack reflectors by low coherence interferometry in extreme ultraviolet

International audienceWe use low coherence interferometry to investigate the depth structure of a complex multilayer stack reflector. The probing instrument is an interferometer based on a Fresnel's bi-mirror illuminated by relatively wide-band synchrotron undulator light near 13.5 nm. Simulations clearly confirm that our test object generates two back propagated signals that behave as if reflected on two effective planes. First results in this spectral range may open the way to a new physical approach to extreme ultraviolet sample characterization in the form of line-scan optical coherence tomography

Traitement optique en eclairage incoherent : possibilites theoriques et limitations experimentales.

On justifie les avantages de l'eclairage incoherent par rapport a l'eclairage coherent quant a l'influence des parasites qui affectent l'eclairement d'une image. Description d'experiences de traitement d'images incoherent, un chapitre special est consacre a l'holographie synthetiqu

La cohérence temporelle en optique classique

International audienceDéterminante pour l’observation d’interférences, la cohérence temporelle est souvent reliée à la comparaison entre différence de marche et longueur de « train d’onde ». Un examen plus détaillé fait ressortir les concepts les plus pertinents : l’autocorrélation du champ et le bruit classique de détection

CONCEPTION D’OPTIQUES DIFFRACTIVES ASSISTÉE PAR ORDINATEUR

National audienceCette communication présente l'état de l'art dans le domaine de la conception assistée par ordinateur de systèmes optiques incluant des éléments diffractifs. Les potentialités de l'optique diffractive seront tout d'abord illustrées par des exemples concrets. Les techniques de simulation informatique des phénomènes diffractifs seront ensuite abordées et leur rôle crucial dans les logiciels de calcul des combinaisons optiques évoqué. Enfin, les possibilités actuelles dans ce domaine de deux logiciels généralistes de conception optique seront présentées