Christophe Stolz

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.

Single tree species classification from Terrestrial Laser Scanning data for forest inventory

Due to the increasing use of Terrestrial Laser Scanning (TLS) systems in the forestry domain for forest inventory, the development of software tools for the automatic measurement of forest inventory attributes from TLS data has become a major research field. Numerous research work on the measurement of attributes such as the localization of the trees, the Diameter at Breast Height (DBH), the height of the trees, and the volume of wood has been reported in the literature. However, to the best of our knowledge the problem of tree species recognition from TLS data has received very little attention from the scientific community. Most of the research work uses Airborne Laser Scanning (ALS) data and measures tree species attributes on large scales. In this paper we propose a method for individual tree species classification of five different species based on the analysis of the 3D geometric texture of the bark. The texture features are computed using a combination of the Complex Wavelet Transforms (CWT) and the Contourlet Transform (CT), and classification is done using the Random Forest (RF) classifier. The method has been tested using a dataset composed of 230 samples. The results obtained are very encouraging and promising.

Shape from polarization: a method for solving zenithal angle ambiguity

We report a multispectral based method that permits the evolution of shape from polarization setup applied to 3D shape estimation of transparent objects. The setup is based on a polarization imaging technique which is a recent imaging method based on the analysis of the polarization state of the light in the observed scene. The technique has rapidly evolved with the development of electro-optic components and some polarization cameras are now available on the market. Shape from polarization consists in measuring the azimuthal and zenithal angles characterizing the normal of each point of the observed surface. We focus on the ambiguity in the measurement of the zenithal angle and propose a general multispectral approach for estimating the appropriate value of this angle.

Optimization of transparent objects digitization from visible fluorescence ultraviolet induced

A novel reconstruction scheme using ultraviolet (UV) structured point for transparent objects three-dimensional (3D) measurement was reported in our previous works. We address two main approaches behind a low-cost system within two suggested configurations (according to the structured light generation source: UV spot and UV line) in order to improve the results accuracy, which endows our system with its adaptability for industrial applications. The first approach consists of determining the optimal configuration of the practical setup while controlling the modeling error (related to the 3D reconstruction approach) with the implementation of an additional validation method in the reconstruction process. The second approach deals with a method which tracks the fluorescence points in the presence of noises inherent to our acquisition system. The specificity of the implemented tracking method relies on spectroscopic analysis. Indeed, experimental investigation has been also carried out to characterize the application. Some digitized objects are presented with an accuracy never reached by any previously reported works dealing with the digitization of transparent objects without any prior preparation.

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.

Characterizing weld pool surfaces from polarization state of thermal emissions

In this Letter, a vision-based remote sensing methodology is proposed to measure the topography of weld pool surfaces from one single view. Thermal radiations emitted by the hot liquid metal at a wavelength within the arc plasma blind spectral window are acquired by a wavefront division polarimetric system. The refractive index of the liquid metal and the topography of the weld pool surface are inferred from the polarimetric state of the thermal radiations.

Review and comparison of non-conventional imaging systems for three-dimensional digitization of transparent objects

Fashion and design greatly influence the conception of manufactured products which now exhibit complex forms and shapes. Two-dimensional quality control procedures (e.g., shape, textures, colors, and 2D geometry) are progressively being replaced by 3D inspection methods (e.g., 3D geometry, colors, and texture on the 3D shape) therefore requiring a digitization of the object surface. Three dimensional surface acquisition is a topic which has been stu- died to a large extent, and a significant number of techniques for acquiring 3D shapes has been proposed, leading to a wide range of commercial solutions available on the market. These systems cover a wide range from micro-scale objects such as shape from focus and shape from defocus techniques, to several meter sized objects (time of flight technique). Nevertheless, the use of such sys- tems still encounters difficulties when dealing with non-diffuse (non Lambertian) surfaces as is the case for transparent, semi-transparent, or highly reflective materials (e.g., glass, crystals, plastics, and shiny metals). We review and compare various systems and approaches which were recently developed for 3D digitization of transparent objects.

3D reconstruction by polarimetric imaging method based on perspective model

Classical 3D inspection systems require users to coat transparent objects before measurement. Experimental techniques via non contact measurement, suggested in literature, do not treat inter reflections. The aim of our work is to develop a non contact 3D measurement system for transparent objects by using a polarimetric imaging method in far infrared range. The classical approach relies on the use of orthographic model generated by a telecentric lens in practical setup. However telecentric lenses working in far infrared range are not available. Therefore, we have to adapt pinhole model corresponding to non-telecentric lenses for shape from polarization. In this paper we introduce a 3D reconstruction method to exploit polarimetric imaging with perspective model. We also propose two mathematical approaches in order to reduce reconstruction error: data analysis method to better estimate Stokes parameters and a validation method after Stokes parameters estimation. These techniques are applicable irrespective of the nature of the selected model and any linear system resolution.

Active Lighting Applied to Shape from Polarization

We present a new way of solving the ambiguity that appears in the shape from polarization method. This ambiguity, concerning the normals orientation of the surface, was previously solved thanks to a propagation algorithm. In this paper, the ambiguity is solved thanks to a special active lighting. This method is less time computing and is more robust to noise acquisition. We first demonstrate how to compute the right normals on the whole surface. Then, we show an application on the 3D reconstruction of a specular metallic object made by stamping and polishing.

Non Contact 3D Measurement Scheme for Transparent Objects Using UV Structured light

This paper introduces a novel 3D measurement scheme based on UV laser triangulation to ascertain the shape of transparent objects. Transparent objects are extremely difficult to scan with traditional 3D scanners because of the refraction problem observed in the visible range. Therefore, the object surface needs to be preliminary powdered before being digitized with commercial scanners. Our approach consists of using non contact measurement scheme while dealing with the refraction problem in visible environment. The object shape is computed by classical triangulation method based on stereovision constraint. The proposed acquisition system is composed of two classical visible range cameras and a UV laser source. The exploitation of the UV laser for triangulation system characterizes the novelty of the proposed approach. The fluorescence generated by the UV radiation enables to acquire 3D data of transparent surface with a classical stereovision scheme.