Interest of polarimetric refocused images calibrated in depth for control by vision
Unconventional Optical Imaging, 2018
This work shows the interest of combining polarimetric and light-field imaging. Polarimetric imag... more This work shows the interest of combining polarimetric and light-field imaging. Polarimetric imaging is known for its capabilities to highlight and reveal contrasts or surfaces that are not visible in standard intensity images. This imaging mode requires to capture multiple images with a set of different polarimetric filters. The images can either be captured by a temporal or spatial multiplexing, depending on the polarimeter model used. On the other hand, light-field imaging, which is categorized in the field of computational imaging, is also based on a combination of images that allows to extract 3D information about the scene. In this case, images are either acquired with a camera array, or with a multi-view camera such as a plenoptic camera. One of the major interests of a light-field camera is its capability to produce different kind of images, such as sub-aperture images used to compute depth images, full focus images or images refocused at a specific distance used to detect defects for instance. In this paper, we show that refocused images of a light-field camera can also be computed in the context of polarimetric imaging. The 3D information contained in the refocused images can be combined with the linear degree of polarization and can be obtained with an unique device in one acquisition. An example illustrates how these two coupled imaging modes are promising, especially for the industrial control and inspection by vision.
Utilisation d’une technique de flot optique pour corriger les artéfacts d’un polarimètre imageant à division temporelle
En imagerie de polarisation le problème des artéfacts apparaissant sur l’estimation de la mesure ... more En imagerie de polarisation le problème des artéfacts apparaissant sur l’estimation de la mesure est un problème récurrent, et ce quelle que soit l’architecture de polarimètre employée. Dans le cas d’une architecture à division temporelle, les artéfacts appa- raissent dès lors que l’on est en présence de mouvement dans la scène, faussant ainsi toute interprétation physique des estimations de mesure. Malgré cela, il existe très peu de publications traitant de la résolution de cette problématique, ce qui peut s’expliquer, a priori, par la complexité de la tâche. En effet, dans le cas d’une architecture à division temporelle, l’estimation d’une mesure nécessite la combinaison d’au minimum 4 images (appelées ici “images d’états de polarisation”) acquises successivement selon des polarisations différentes. Les 4 images ainsi acquises peuvent avoir des dynamiques de niveaux de gris totalement différentes, pouvant rendre difficile le recalage direct d’une séquence d’images polarimétriques ...
Polarimètre imageant rapide utilisant un unique modulateur à cristaux liquides ferroélectriques
Le laboratoire MIPS développe un prototype de polarimètre de Stokes imageant basé sur un modulate... more Le laboratoire MIPS développe un prototype de polarimètre de Stokes imageant basé sur un modulateur composé d'une cellule à cristaux liquides ferroélectriques et d'un polariseur linéaire (http://dx.doi.org/10.1117/12.733693). Ce type de modulateur est connu pour sa rapidité de commutation. Cependant, la majorité des applications ne l'utilise que pour des commutations entre deux états uniquement, ce qui limite le champ d'application. Au travers de travaux récents, le MIPS a montré qu'il est possible d'utiliser un modulateur cristaux liquides ferroélectriques pour analyser la polarisation de la lumière afin de reconstruire la partie linéaire du vecteur de Stokes (http://dx.doi.org/10.1117/12.826403) pour en déduire l'orientation et le degré de polarisation linéaire.
We have implemented a portable high-speed imaging polarimeter using a single ferroelectric liquid... more We have implemented a portable high-speed imaging polarimeter using a single ferroelectric liquid crystal cell. The originality of this work lies in the fact that we do not work at the nominal wavelength of the cell. Consequently we obtain the full polarimetric information (i.e. the Stokes vector) with only one modulator. Our device can produce polarimetric information at 200 fps. This type of polarimeter, based on the division-of-time principle, produces images needing to be registered. To our knowledge, this issue has rarely been addressed in such a context. We propose an efficient post- processing algorithm based on optical flow techniques adapted to multi-component Stokes images. We show the benefits of this off-line post- processing on real sequences and present resulting images as well as quantitative evaluation of the correction.
Correction of temporal division polarimeter artifacts with an optical flow technique
For any kind of imaging polarimeter, at least four intensity images, named polarization state ima... more For any kind of imaging polarimeter, at least four intensity images, named polarization state images, are needed to compute one full Stokes vector. When the polarimeter is designed according to the division-of-time principle, polarization state images are acquired sequentially. Consequently, the main issue is the systematic occurrence of artifacts as the scene is not static. Even though this is well known, little research has been done on this subject. Here a two-step motion-compensation-based method is proposed to fix it. The first step consists in estimating the motion between each image acquired according to the same polarization state. Then each image is warped according to a fraction of the previously estimated motion. Due to their dense and accurately estimated motion field we have shown optical-flow techniques to be the most efficient for motion-estimation in this case. Compensating the motion using optical flow to estimate it actually leads to a strong correlation criterion ...
Correction of erroneous degree of polarization of moving objects in a video sequence
SPIE Proceedings, 2008
The polarimeter we developed is composed of a single commercial CCD camera with a bistable ferroe... more The polarimeter we developed is composed of a single commercial CCD camera with a bistable ferroelectric liquid crystal modulator as a polarizing element; therefore the degree of polarization (DOP) is evaluated from two successive acquisitions. Thus, when an object moves during the acquisition, issues occur in the polarization information reconstructed from the two state pictures: edges of objects have neither
We have developed an imaging polarimeter working in the visible range. It is composed of a CCD ca... more We have developed an imaging polarimeter working in the visible range. It is composed of a CCD camera with a bistable ferroelectric liquid crystal modulator as a polarizing rotator; therefore the degree of polarization (DOP) is evaluated from two successive acquisitions. Apparent motion in the scene induces a false evaluation of the DOP, especially on the edges of the objects. We present our acquisition system and two methods to correct defects in DOP images of moving objects. First, we propose a post-processing temporal median filtering, correcting the DOP once computed. The second method consists in performing a motion estimation to correct the object's displacement between the two polarization state pictures. Comparison based on qualitative and quantitative results with real data is provided.
Cancellation of motion artifacts caused by a division-of-time polarimeter
Polarization Science and Remote Sensing V, 2011
ABSTRACT For any kind of imaging polarimeter, at least four intensity images, named polarization ... more ABSTRACT For any kind of imaging polarimeter, at least four intensity images, named polarization state images, are needed to compute one full Stokes vector. When the polarimeter is designed according to the division-of-time principle, polarization state images are acquired sequentially. Consequently, the main issue is the systematic occurrence of artifacts as the scene is not static. Even though this is well known, little research has been done on this subject. Here a two-step motion-compensation-based method is proposed to fix it. The first step consists in estimating the motion between each image acquired according to the same polarization state. Then each image is warped according to a fraction of the previously estimated motion. Due to their dense and accurately estimated motion field we have shown optical-flow techniques to be the most efficient for motion-estimation in this case. Compensating the motion using optical flow to estimate it actually leads to a strong correlation criterion between corrected and reference polarization images. Our method allows the estimation of the polarization by post-processing the polarization state image sequence. It leads to a good estimation quality whether the scene is static or not, thus fixing the main issue of a divisionof- time polarimeter.
Small format optical sensors for measuring vegetation indices in remote sensing applications: A comparative approach
TENCON 2012 IEEE Region 10 Conference, 2012
ABSTRACT In this paper we report the comparative study of two types of digital cameras to be used... more ABSTRACT In this paper we report the comparative study of two types of digital cameras to be used for vegetation index measurement. Using either a dedicated multispectral JAI AD-80-GE camera or a custom dual Canon S3IS camera system, we obtained RGB and NIR images and processed them to obtain vegetation indices (NDVI, EVI and two-band EVI). Since no radiometric calibration is available for both systems, normalized reflectance images are not available and we have to determine relative gain factors for the various bands in order to get likely vegetation indices. Moreover, the two camera system requires image registration. Experimental results are provided.
Implementation of liquid crystal-based polarimeters: trade-off between speed and performance
SPIE Proceedings, 2012
This work considers the implementation of polarimeters with liquid crystal (LC) cells as polarizi... more This work considers the implementation of polarimeters with liquid crystal (LC) cells as polarizing elements. Most works generally try to implement architectures with one or two pure retarding modulators such as nematic devices. In this case, rather thick LC devices able to provide a 2π retardation are generally used. Unfortunately, LC device switching speed is known to evolve as the inverse square of their thickness, which leads to practical implementations limited to a few tens of Hertz in the visible region. The alternative consisting in using much faster devices made of ferroelectric liquid crystals is not that obvious since these devices often operate in bistable mode. We show that using thinner, therefore faster nematic devices is possible with a minimal penalty in terms of performance. Therefore, several solutions can be considered. Performance evaluation will be performed through studying the system matrix condition number.
This paper reports the design and the implementation of a Stokes imaging polarimeter able to prov... more This paper reports the design and the implementation of a Stokes imaging polarimeter able to provide full polarimetric information at 200 fps. This portable implementation is based on a division-oftime architecture and uses a single ferroelectric liquid crystal device as the polarization modulating element. Our system is designed to work at 532 nm with natural light or with controlled illumination, without temperature control. We propose an optimized driving scheme of the modulator such that the liquid crystal device can produce four polarization states which makes it possible to retrieve the full polarimetric information. The modulator characterization is reported and experimental results are provided. C 2011 Society of Photo-Optical Instrumentation Engineers (SPIE).
In the field of polarimetry, ferroelectric liquid crystal cells are mostly used as bistable polar... more In the field of polarimetry, ferroelectric liquid crystal cells are mostly used as bistable polarization rotators suitable to analyze crossed polarizations. This paper shows that, provided such a cell is used at its nominal wavelength and correctly driven, its behavior is close to that of a tunable half-wave plate, and it can be used with much benefit in lightweight imaging polarimetric setups. A partial Stokes polarimeter using a single digital video camera and a single ferroelectric liquid crystal modulator is designed and implemented for linear polarization analysis. Polarization azimuthal angle and degree of linear polarization are available at 150 frames per second with a good accuracy.
We present a high-speed ferroelectric liquid crystal based imaging polarimeter. It can evaluate t... more We present a high-speed ferroelectric liquid crystal based imaging polarimeter. It can evaluate the first three Stokes parameters. Contrary to previous high-speed systems, it only uses a single liquid crystal cell, driven in an optimized way in order to produce a tunable rotation of polarization. Its characterization is presented, as well as its integration in a portable implementation working at 633 nm. Preliminary results are provided.
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