3D Laser Imaging

This paper addresses 3D laser imaging. 3D active imaging or 3D laser imaging has been greatly improved with the development of highly sensitive, high pixel density cameras. In this paper, we develop a 3D laser imaging method using tomographic algorithm. We obtain the three-dimensional reconstruction by a cone-beam algorithm (1), which is a convolution back- projection algorithm deduced from the Radon transform. This algorithm uses a set of two- dimensional projections which contain the data collected by the pixels of a focal plane area. These data are related to the intensity backscattered by the scene illuminated by a laser pulse. The scattered intensity contains the information for the 3D reconstruction. This process allows the reconstruction of a 3D scene from a series of images parameterized by an angle of axial rotation. In this paper, we study the robustness of this algorithm. To analyze the 3D reconstruction algorithm, we give experimental proof of the theory. We develop a 3D imaging experiment based upon a two-dimensional imaging device with high pixel density in the visible band. 1. INTRODUCTION In this paper, we present 3D laser imaging experimental data. There is a considerable interest in the development of new optical imaging systems that are able to give three-dimensional images. Potential applications range across the fleld of defense and security for the recognition of targets, the medical fleld for the detection of subcutaneous and cutaneous tumors or the fleld of micro- electronics for the observation of hardware components during their manufacture. Identifying targets or objects concealed by foliage or camou∞age is a critical requirement for operations in public safety, law enforcement and defense. The most promising techniques for these tasks are 3D laser imaging techniques. Active imaging systems use a near-infrared or visible laser and a suitable detector. We propose a tomographic method to enable a super-resolution 3D reconstruction from several intensity images of hidden objects. The laser illuminates the scene and the re∞ected radiation is collected by a detector to give a two-dimensional image. We obtain a three-dimensional image through a tomographic algorithm taking into account 2D images with difierent angles of view. 2. THREE-DIMENSIONAL REFLECTIVE TOMOGRAPHIC RECONSTRUCTION An optronic system creates a 3D image of an object present in a complex optronic scene. This image must for example make it possible to pick out camou∞aged targets behind camou∞age netting, under trees, etc. This operational condition constitutes the major problem. Our technique is based upon a 3D tomographic reconstruction (2,3). In this fleld of 3D imaging, it is necessary to obtain a set of measured data of the considered object to be reconstructed dependent on a variable parameter (angle of view). This data set makes it possible to reconstruct the volume by applying inverse reconstruction techniques. From a mathematical point of view, the technique breaks down into two steps: direct measurement, optionally processed using a model of the physical phenomena which are measured or in our case measured data, and then reconstruction by inversion on the basis of these direct measures. The flrst problem consists in providing in a reasonable time a set of direct data that can be utilized for the inversion process. This problem naturally covers all the technological di-culties (high rate illumination laser, short pulse, detector block, pointing.). The second problem relates to the inversion procedure used and its mathematical implementation. For the inversion method, we use a cone-beam algorithm, which is a convolution back-projection algorithm deduced from the Radon transform. In classical tomography, a thin beam (X-ray, laser .) propagates through the studied volume and reaches the detector which collects the data given by the transmitted electromagnetic waves. The obtained projection represents the attenuation by the penetrated volume along a straight line of the ray. As shown in Fig. 1, the projection data Pµ obtained for a given angle of observation µ is the line integral of the 2D-object f(x;y) in the (t;s) axis.