Laurent Bigué

Imaging linear polarimetry using a single ferroelectric liquid crystal modulator

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.

Contactless optical extensometer for textile materials

In this paper we present a contactless extensometer. For some flexible materials, with great displacements and deformations, contact during measurement is not acceptable. In fact, contact measurement can modify the tensile behavior, as is the case for fibrous materials. Contactless extensometers usually have to print or glue some marks on the sample, which may cause problems during measurement. These extensometers typically use digital image processingto obtain deformation data. The principle used in this study uses the natural periodicity or surface patterns inherent in most textile materials without any image processing. During deformation the distance between two periods or pattern elements changes, allowing this method to measure the real-time modification of this in-plane distance. The extensometer consists of two parts: an optical device and a signal processing unit performing a Fourier analysis. Some results obtained during a tensile test on woven fabrics and non-wovens are presented here.

Full Stokes polarimetric imaging using a single ferroelectric liquid crystal device

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-of-time 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.

Optimal multicriteria approach to the iterative Fourier transform algorithm

We propose a unified approach to the multicriteria design of diffractive optics. A multicriteria version of the direct binary search that allows the user to adjust the compromise between the diffraction efficiency and the signal-to-noise ratio already exists. This technique has proved to be extremely powerful but also very time consuming. We extend this multicriteria approach to the iterative Fourier transform algorithm, which helps to reduce the computation time dramatically, especially for multilevel domains. Simulations as well as experimental validations are provided.

High-speed degree-of-polarization imaging with a ferroelectric liquid-crystal modulator

Images describing polarimetric characteristics of objects are more and more used when studying complex scenes. In some cases, only the depolarization phenomenon, mostly due to scattering, proves of interest and estimating the degree of polarization (DOP) is sufficient. For a linearly polarized beam under normal incidence, when the materials in the scene exhibit neither birefringence nor dichroism, this comes down to an intensity measurement in two crossed directions. Nevertheless, the classical optical setup, consisting mainly of a rotating polarizer used as a polarization state analyzer, gives good-quality images, but cannot provide dynamic information. In this paper, we propose to use a ferroelectric liquid-crystal modulator in an imaging polarimeter. We demonstrate the use of this modulator as a polarization state analyzer for frame rates up to about 400 Hz. Provided a fast camera is used, we show that the polarimeter allows us to surpass the classical video frame rate. We propose a setup using a rapid CCD camera (up to 233 Hz). Images in DOP are of very high quality and hold their own against images obtained with a classical polarizer. With a faster camera, we were able to obtain images at up to 400 Hz.

Polarimetric measurements of fabric surfaces

We describe an optoelectronic setup designed to evaluate the surface parameters of fabrics that influence their tactile feel. The developed texturometer uses the periodic structure of a textile material and its ability to reflect light to evaluate its surface properties through its polarimetric properties. The device scans the surface with a laser line and performs a temporal Fourier analysis of the reflected light, which allows us to consider the periodical structure of the materials surface. Instead of using the overall reflected energy, the analysis is performed on the degree of polarization of light. Results obtained with this new texturometer are compared to those obtained with a nonpolarimetric device that uses overall reflected energy. Emerized and nonemerized twill fabrics are tested, as well as spun-bonded nonwovens. We show that discrimination between samples is enhanced with this polarimetric texturometer. For emerized fabrics, the results exhibit a decrease in depolarization as emerizing intensity increases. For nonwovens, a complementary study in polarimetric imaging has been performed to better understand the phenomena. Nonwoven thermobonded points exhibit lower depolarization of the lightwave than the rest of the structure. Moreover, their depolarization differentiates the tested nonwovens.

Filter implementation technique for multicriteria characterization of coding domains in the joint transform correlator

An improved method for implementing correlation filters in the joint transform correlator architecture is proposed. We derived the method from computer-generated holography techniques. It allows us to use any correlation filters, especially ones that provide an optimal trade-off between noise robustness, peak sharpness, and optical efficiency, with any spatial light modulator (SLM). This method also allows for an objective comparison of the performance of the coding domains of various SLMs.

Active contour segmentation by use of a multichannel incoherent optical correlator.

We describe an optoelectronic incoherent multichannel processor that is able to segment an object in a real image. The process is based on an active contour algorithm that has been transposed to optical signal processing to accelerate image processing. This implementation requires exact-valued correlations and thus opens attractive perspectives in terms of optical analog computation. Furthermore, this optical multichannel processor setup encourages incoherent processing with high-resolution images.

Implementation of optimal trade-off correlation filters with an optimized resolution technique

Several techniques can be used for the optical implementation of correlation filters onto spatial light modulators whose coding domain has a limited extent. Using the general framework of optimal trade-off filters, we compare them in the case of a 4f architecture and demonstrate that oversampling certain parts of the filters can improve performance.

Experimental implementation of a joint transform correlator using synthetic discriminant functions

Abstract A joint transform correlator (JTC) was implemented in the Optical Science Department at CERT. In order to ensure a rotational invariance, the reference image is replaced by a synthetic discriminant function (SDF) image displayed on an amplitude spatial light modulator (SLM). Numerical simulations are performed and then compared to experiments. We show that the pseudo-inverse SDP (PI-SDF) gives accurate experimental results. The training ability of the PI-SDF allows it to recognize the reference object unambiguously, even if the input view does not explicitly belong to the training set. Then, we propose to replace our amplitude device with a ternary phase and amplitude device, in order to display positive and negative images: this allows us to display any kind of SDF filters, e.g. OT-SDF or MINACE-SDF, which reduce the side lobes, with a good resistance to noise.