Laurent Bigue

Target detection with a liquid crystal-based passive Stokes polarimeter

We present an imaging system that measures the polarimetric state of the light coming from each point of a scene. This system, which determines the four components of the Stokes vector at each spatial location, is based on a liquid-crystal polarization modulator, which makes it possible to acquire four-dimensional Stokes parameter images at a standard video rate. We show that using such polarimetric images instead of simple intensity images can improve target detection and segmentation performance.

Implementation of high-resolution diffractive optical elements on coupled phase and amplitude spatial light modulators

We propose the optical implementation of diffractive optical elements onto electrically addressed liquid-crystal spatial light modulators. We compare the classic implementations onto amplitude-only or phase-only domains with the implementations onto coupled phase and amplitude (spiral) domains. We demonstrate that the coupling between amplitude and phase provides a trade-off between diffraction efficiency and the signal-to-noise ratio in the reconstruction. Furthermore, when investigating the influence of the maximum dephasing on phase domains and spiral domains through the use of optimal trade-off design, we show that phase-only domains with limited maximum dephasing can provide satisfactory performance. Finally, optical implementations are provided.

POLARIZATION IMAGING FOR INDUSTRIAL INSPECTION

This paper aims at reviewing the recent published works dealing with industrial applications which rely on polarization imaging. A general introduction presents the basics of polarimetry and then 2D and 3D machine vision application are presented as well as the latest evolution in term of high speed polarimetric imaging.

High-speed portable polarimeter using a ferroelectric liquid crystal modulator

The implementation of an imaging polarimeter able to capture dynamic scenes is presented. Our prototype is designed to work at visible wavelengths and to operate at high-speed, ie above 200 Hz, contrary to commercial or laboratory liquid crystal polarimeters previously reported in the literature. It has been used in the laboratory with controlled illumination conditions (wavelength, coherence or incoherence, incidence, ...) as well as in a natural environment with sunlight or any lamp or light source. The device consists of commercial components whose cost is moderate. The polarizing element is based on a ferroelectric liquid crystal modulator which acts as a half-wave plate at its design wavelength. The device has been fully characterized and verified to work up to 1 kHz. It produces a 90° polarization rotation. In this Orthogonal State Configuration Imagery (OSCI), images in degree of polarization (DOP) can be obtained with simple post-processing of two consecutive images. With a commercial CCD camera, operation up to 360 Hz is demonstrated, resulting in images whose quality is equivalent to that of classical low speed polarimeters.

Dynamic diffractive optical elements displayed on spatial light modulators

We present the optical implementation of dynamic diffractive optical elements on different types of commercially available spatial light modulators; one using twisted-nematic liquid crystals, one using analog ferro-electric liquid crystals and one using a matrix of micromirrors. Experimental results are shown and the various implementations are compared.

Implementation of a high-speed imaging polarimeter using a liquid crystal ferroelectric modulator

Imaging polarimetry through evaluation of the degree of polarization of light can give much information about complex scenes. A classic optical set-up usually consists in using a rotating polarizer or a tunable phase modulator as a polarization state analyzer. In this study, we implement an imaging polarimeter using a ferroelectric liquid crystal spatial light modulator as a polarization state analyzer able to run at 1000 Hz. This way, our polarimeter can overcome the classic limit of 50 Hz, provided that the camera is fast enough. Our polarimeter is compared with classical systems which use rotating polarizers.

Spatial Light Modulators For Information Processing: Applications And Overview

We present the characterization of a ferro-electric liquid crystal spatial light modulator and of a nematic liquid crystal spatial light modulator. It is shown that the nematic device can be compensated for its phase distortions and a method is proposed for increasing its frame rate while maintaining a maximum phase modulation of 2π. Several applications in information processing are presented with experimental results.

Polarimetric measurements for fabric surface state characterization

This paper describes an optoelectronic setup which has been designed in order to evaluate some finishing process parameters of textile fabrics such as emerizing or raising. This evaluation is usually performed by trained people and our goal is to perform it in an automatic and objective way without any human operation. Our setup evaluates the periodical structure of fabrics through a temporal Fourier analysis of the degree of polarization of the light reflected by a fabric surface. The measurement sensitivity to surface hairiness is shown to be greatly improved when polarization is taken into account.

Optical implementation of a snake-based segmentation with an incoherent correlator

This paper presents an incoherent optoelectronic processor which is able to segment an object in a real image. The process, based on active contours (snakes), consists in correlating adaptive binary references with the scene image or with a preprocessed version of the scene image. The proposed optical implementation of algorithms which are already operational numerically opens attractive perspectives as far as speed is concerned. Furthermore, this experiment is a new application for optical processors.

Characterization of an analog ferroelectric spatial light modulator: application to dynamic diffractive optical elements and optical information processing

Spatial light Modulators (SLM) are key devices for the development of optical information processors. Low cost twisted nematic liquid crystal (TN-LC) SLMs are widely available and their characteristics have been extensively studied. Beside the fact that they exhibit a coupled amplitude and phase modulation, their speed is limited to approximately the video frame rate. An alternative to TN-LC devices can be the use of analog ferroelectric liquid crystal (FLC) devices. These devices now commercially available produce a gray level pure amplitude modulation at typical frame rates of 1 kHz. In order to determine its coding capabilities and its limitations, the characterization of such a device, manufactured by Boulder Nonlinear Systems Inc., is presented in this paper. This SLM has a resolution of 512 by 512 pixels with a pitch of 15 μm and has a reflective VLSI backplane. The study of its potential applications for the display of dynamic diffractive optical elements and also as a component of an optical processor for pattern recognition will be followed by experimental results and comparisons with TNLC devices.