Modelling of an active burst illumination imaging system

Abstract

ABSTRACT Onera, The French Aerospace Lab, has developed a new active burst illumination imaging system with a short time gating. This imaging device is used to obtain a passive or an active image of a small volume of the illuminated scene. To better understand and evaluate the relevant physical phenom ena (scintillation, speckle ) impacting on the performance on burst illumination imaging system, Onera has implemented a code (PIAF). The aim of this paper is to describe the model and to present some first results. Efforts have done on three principal points, the laser source model, the propagation through the turbulence concerning the illuminati on of the scene, and the interaction with the target. The model can take into account any lase r sources. To evaluate th e propagation through the atmosphere, electromagnetic models are implemented but we specially focused our attention on simplified methods to reduce computing time. Considering the 3D target and its elemen tary plane surfaces, we analyze each cont ribution like the incoherent solar field or the incident laser field. We adapt classical and physical models for light reflection. Speckle contributions are also treated using data bases generated by an Onera tool. In the last section of this paper, we attempt to model artefacts associated with such imaging devices, including photon noise, gain issues and electronic noise. We also present experimental results and comparisons with Pi af simulations in an associated paper. Keywords: Active imaging, Numerical model, Atmospheric propagation, Light interaction, Sensing device 1. INTRODUCTION 1.1 Main objective Onera has developed an active laser illumination system with time gating camera: GIBI (Gated Imager with Burst Ilumination). It is a new active burst illumination imaging system generating short pulses to illuminate a target and makes available long-range night vision in complete darkness as well as in degraded weather conditions such as rain, fog or haze. This concept becomes more and more important in defence and security applications. We decided to model ones of the most important physical phenomena witch can influe nce our system. PIAF is the French acronym for a Physical Model in Active Imaging. The aim is to compare and validate the concept. 1.2 General architecture of the model The general architecture is shown on figure 1. Piaf code is structured around several modules in order to estimate separately the influence of various parameters as the shape of the laser beam, the turbulence effect, the interaction with a simple target. As shown in figure 1, input parameters are extracted from a general data base. Data are adapted from measurements or generated by physical models. Piaf can be used to better understand the influence of each parameter introduced. In fact, it is possible to directly access to several parameters during a run. Moreover, the modular concept allows us to modify or to adapt the structure of our code to another experimental device instead of Gibi.