In almost every practical scenario the light reflected from a surface is scattered in the atmosphere before it reaches a sensor. While this effect can be a little annoying for the amateur photograph trying to take a picture, it can have disastrous consequences for unmanned autonomous vehicle navigating through fog for instance.
By employing an innovative method based on a clever combination of spectral bands and polarization analysis, coupled with advanced image processing techniques, significant improvements have been achieved on fog obscurant using the existing passive full Stokes polarization imaging camera for visible light “SALSA” (developed by Bossa Nova Technologies).
For some applications, the need for fast polarization acquisition is essential (if the scene observed is moving or changing quickly). In this paper, we present a new acquisition method for Bossa Nova Technologies’ full Stokes passive polarization imaging camera, the SALSA. This polarization imaging camera is based on “Division of Time polarimetry” architecture. The use of this technique presents the advantage of preserving the full resolution of the image observed all the while reducing the speed acquisition time. The goal of this new acquisition method is to overcome the limitations associated with Division of Time acquisition technique as well as to obtain high-speed polarization imaging while maintaining the image resolution. The efficiency of this new method is demonstrated in this paper through different experiments.
Objective and background: We present a new method for the calibration of Bossa Nova Technologies’ full Stokes,
passive polarization imaging camera SALSA. The SALSA camera is a Division of Time Imaging Polarimeter. It uses
custom made Ferroelectric Liquid Crystals mounted directly in front of the camera’s CCD. Regular calibration process
based on Data Reduction Matrix calculation assumes a perfect spatial uniformity of the FLC. However, alignment of
FLC molecules can be disturbed by external constraints like mechanical stress from fixture, temperature variations and
humidity. This disarray of the FLC molecules alignment appears as spatial non-uniformity. With typical DRM condition
numbers of 2 to 5, the influence on DOLP and DOCP variations over the field of view can get up to 10%. Spatial nonuniformity
of commercially available FLC products is the limiting factor for achieving reliable performances over the
whole camera’s field of view. We developed a field calibration technique based on mapping the CCD into areas of
interest, then applying the DRM calculations on those individual areas.
Results: First, we provide general background of the SALSA camera’s technology, its performances and limitations.
Detailed analysis of commercially available FLCs is described. Particularly, the spatial non uniformity influence on the
Stokes parameters. Then, the new calibration technique is presented. Several configurations and parameters are tested:
even division of the CCD into square-shaped regions, the number of regions, adaptive regions. Finally, the spatial DRM
“stitching” process is described, especially for live calculation and display of Stokes parameters.
Objective and background: We present a new version of Bossa Nova Technologies' passive polarization imaging
camera. The previous version was performing live measurement of the Linear Stokes parameters (S0, S1, S2), and its
derivatives. This new version presented in this paper performs live measurement of Full Stokes parameters, i.e. including
the fourth parameter S3 related to the amount of circular polarization. Dedicated software was developed to provide live
images of any Stokes related parameters such as the Degree Of Linear Polarization (DOLP), the Degree Of Circular
Polarization (DOCP), the Angle Of Polarization (AOP).
Results: We first we give a brief description of the camera and its technology. It is a Division Of Time Polarimeter using
a custom ferroelectric liquid crystal cell. A description of the method used to calculate Data Reduction Matrix (DRM)5,9
linking intensity measurements and the Stokes parameters is given. The calibration was developed in order to maximize
the condition number of the DRM. It also allows very efficient post processing of the images acquired. Complete
evaluation of the precision of standard polarization parameters is described. We further present the standard features of
the dedicated software that was developed to operate the camera. It provides live images of the Stokes vector
components and the usual associated parameters. Finally some tests already conducted are presented. It includes indoor
laboratory and outdoor measurements. This new camera will be a useful tool for many applications such as biomedical,
remote sensing, metrology, material studies, and others.
We present a novel polarization based metrological method of 3D shape measurement for in-line control of optical
surfaces and control of highly aspheric optical surfaces. This technique is fast, non contact, high resolution, alignment
free and with unprecedented dynamic. It has the potential to reach tens of nanometers accuracy. In this paper we show
that a polarization imaging camera combined with an un-polarized illumination and 3D reconstruction algorithm lead to
the 3D reconstruction of optical element (regular lens and aspheric lens) and the measurement of their parameters. The
optical element to be measured is placed in a diffusive integrating sphere and illuminated by un-polarized light. The
reflection of the un-polarized light by the optical element gets partially polarized. A polarization camera captures the
image of the optical element and measures the polarization state of each pixel in real time. The Degree Of Light
Polarized and the Angle Of Polarization parameters are related to the geometry of the optical element. The 3D shape of
the optical element is reconstructed using dedicated software. The architecture of the hardware, calibration results and
sensitivity measurements is presented and experimental results and observations as well as possible further steps and new
applications are discussed.
We present a Stokes polarization camera prototype based on an electro-optic ceramic (PLZT) as the key
polarization component. Two pairs of electrodes are used to control the applied electric field and so the retardance and
orientation of the induced waveplate. The active area of the PLZT element is 120x120µm. To increase the effective
active area, a 2D array PLZT is used. Imaging through this 2D array with reduced fill factor is achieved by splitting the
focal plane. The focal plane is split by a microlenses array and interacts with each element of the ceramic array. A
modified focal plane is reconstructed by another microlenses array. Digital image processing is used to recover the prime
focal plane information. The technology used in this device (ceramic element, 2D array, imaging with split focal plane)
as well as characterization of the ceramic element and preliminary results will be presented.
We present a linear Stokes polarization camera working at visible wavelength. The camera is both compact and
robust for use in field experiments and outdoor conditions. It is based on fast polarization modulator. Four polarization
states images are acquired successively. Processing software allows live calculation, visualization and measurement of
polarization images deduced from the acquired images. The architecture of the hardware, calibration results and
sensitivity measurements is presented. Polarization image processing including polarization parameters computed are
proposed. These parameters include linear Stokes parameters (S0, S1 and S2), usual polarization parameters (intensity,
degree of linear polarization, and angle of polarization) and other polarization based parameters (polarized image,
depolarized image, virtual polarizer, polarization difference). Color data fusion and vector overlay algorithms are
presented. Finally experimental results and observations as well as possible applications are discussed.
Along with intensity and spectrum, the polarization of light carries abundant information. Polarization imaging has established strong interest for visual appearance measurement, based on its ability to analyze scattered light and for defense applications, thanks to its performances in term of object detection/identification. Image contrast enhancement and information on the objects (natural, man-made, detection of water bodies, 3D shape...) composing the scene can also be derived from the polarization analysis. In this paper, we will present an innovative polarization component based on ceramic PLZT and its integration in an imaging system. It will lead to a passive polarization camera that will measure the 4 Stokes parameters for each pixel of the image, in real-time, without any mechanical rotation and at high frame per second. Based on PLZT ceramic, we will present the design and the manufacturing of a rotatable and programmable waveplate. It will be the key component of a passive polarization imaging system. The component will be optimized, fabricated and integrated into a passive polarization camera. The performances of the polarization camera will be demonstrated in the laboratory. Measurement of Stokes vector for each pixel of the image will allow precise polarization measurement, leading to accurate analysis of the scattered light. Various parameters (gloss, color...) and images (polarization degree, surface scattering, volume scattering...) will be calculated from the Stokes parameters.
The potential application of multispectral polarization imaging for detection and recognition requires a good knowledge of the depolarizing behavior of targets. We measured the degree of polarization associated to several targets in a monostatic configuration as a function of wavelength and of the angle of incidence. Depolarization effects depending on the absorption of targets were observed and a phenomenological model based on Kubelka-Munk theory is proposed. It describes the behavior of paints and diffuse materials, taking into account both contributions of surface scattering and volume scattering. Target parameters such as roughness, refractive index, scattering coefficient are taken into account and enable to draw out a predictive model of the depolarizing behavior of targets. We found a good agreement between measurements and our predictions.
We present the modeling and the performance of a polarization
active imager at l5806 nm. The device operates in a monostatic
configuration, using a semiconductor laser to illuminate the target and a
telescope to create the image on a CCD matrix. Dual images (intensity
and polarization degree) of different scenes are obtained by a new
method (only two images acquired) and analyzed, showing the experimental
validation of this concept. An application of this active imager to
the detection of a target buried in the background (same reflectivity but
different polarization degree) is proposed. Field experiment results are
We present the concept of a Multispectral Polarization Active Imager operating in the visible range. The acquisition of three degree of polarization (0% < Dp < 100%) images and three intensity images at different wavelengths (RGB) enables to get information about the spectral signature of targets as well as their polarization properties. A theoretical analysis and an experimental validation of this technique are presented. The device is operating in a monostatic configuration, using a white light source to illuminate the target. The reception system consists of a liquid crystal achromatic polarization rotator followed by a liquid crystal color filter and a CCD camera, all driven by a computer. The three polarization images can be merged in one by using a RGB coding in order to visualize the whole polarization information. Results with different targets will be presented, emphasizing polarization effects about backscattering. In particular the degree of polarization is found to be highly correlated to the reflectance of diffuse materials like paints, papers and other coatings. A first interpretation is given to explain this phenomenon. Application to identification of targets with low sensitivity to irradiance can be envisaged thanks to multispectral polarization signature.
Liquid-crystal televisions are inexpensive display devices that can be used as arbitrary quasi-phase modulators to achieve arbitrary wavefront shapes, these are limited only by the available modulation depth and resolution. We discuss the properties of these devices and then demonstrate four applications of a particular liquid-crystal television: an active lens system, programmable optical image processing experiments, the resolution enhancement of an image sensor, and the measurement of the sensitivity of heterodyne detection to wavefront aberrations.
We present the modeling and the performances of a Polarization Active Imager at (lambda) equals 806 nm. The device is operating in a monostatic configuration, using a semiconductor laser to illuminate the target and a telescope to create the image on a CCD matrix. Dual images (intensity and polarization degree) of different scenes are obtained by a new method (only 2 images acquisition) and analyzed, showing the experimental validation of this concept. The application of this active imager to the detection of target buried in the background (same reflectivity but different polarization degree) is proposed.
We present the concept of a wide-field-of-view and atmospheric- distortion-insensitive coherent detection. We propose to mix a local oscillator plane wave and a focused backscattered signal on the detector. In first approximation, the field-of-view is only limited by the size of the detector. An original experimental set- up at (lambda) equals 633 nm is presented to validate this technique. We control the phase of the backscattered signal using a liquid-crystal spatial light modulator (SLM). This permits us to steer or to distort the wave front of the backscattered signal in order to measure and compare the field-of-view and the atmospheric perturbations sensitivity of the two set-ups. We will present the performances (wide field-of-view, distortion sensitivity, . . .) of our architecture.
A high speed optical correlator is presented in this paper. It is a joint transform correlator using a BSO photorefractive crystal in the Fourier plane. The performance of the system such a rotation and scale robustness are presented for fingerprint recognition. To demonstrate the interest of such an optical processor, a comparison with numerical systems is presented. Besides, we will also show that the evolution of correlators is quite compatible with the evolution of numerical processors.
We present the concept of a Polarization Diversity Active Imager operating at (lambda) equals 810 nm. Each pixel of the image is encoded by the polarization degree Pd (0% < Pd < 100%) given by its Mueller Matrix. The measurement of the Mueller matrices is obtained using the Dual Rotation Retarder Technique. A theoretical analysis and an experimental validation of this technique are presented. The device is operating in a monostatic configuration, using a semiconductor laser ((lambda) equals 810 nm) to illuminate the target and a telescope to create the image on a CCD matrix. The experiment is controlled by a computer that drives the rotation of the retarders, the digitalization and the encoding of the image. The measured intensity and polarization images are compared and the information contained in the polarization degree are analyzed. Dual images (intensity-polarization) of different targets are presented, showing the experimental validation of the technique. The application of this active imager to the detection and the decamouflage of target buried in the background (same albedo but different polarization degree) is proposed.
In this paper we present results we obtained in mine detection, in the course of a multi-national European research program. Trials were performed in the Joint Research Center in ISPRA, using polarimetric infrared imagers. Usually the 3 - 5 micrometer spectral band is used for this application, however we explain that the 8 - 12 micrometer band is physically a better choice. We thus obtained information on the polarization of the self emitted radiations of the objects so that our method should be more versatile regarding the environment. Images of the global intensity, the radiation global ellipticity and orientation are presented on several types of mines. The obvious increase of contrast between the observed mines and the clutter demonstrates the usefulness of this technique in mine and UXO detection.
A high-speed optical photorefractive correlator using a ferroelectric spatial light modulator and a new type of binary filters optimized for the crystal nonlinearity will be presented in terms of characteristics and performances.
The concept of a wide field-of-view coherent detection insensitive to the wavefront distortions of the signal to be detected is proposed. The principal of this detection is based on two-wave mixing in photorefractive materials. The experimental validation of this concept is demonstrated at (lambda) equals 0.5 micrometers using a photorefractive crystal of Bi12SiO20. The sensitivity of the detection is theoretically and experimentally analyzed. It led to the conception of a wide field-of-view coherent detection demonstrator operating in the eye-safe band using a photorefractive crystal of CdTe. The application of this device to vibrations analysis of a mechanical structure and the determination of its resonance frequency are demonstrated.