CHORALE (simulated Optronic Acoustic Radar battlefield) is used by the French DGA/DET (Directorate for Evaluation of the French Ministry of Defense) to perform multi-sensors simulations. CHORALE enables the user to create virtual and realistic multi spectral 3D scenes, and generate the physical signal received by a sensor, typically an IR sensor. Some assessments concern the study of the duality between a threat (a missile for example) and a target (a battle tank for example) in the battlefield. In these cases, obscurants are special counter measures (clouds), classically used to hide armored vehicles and/or to deceive threatens. To evaluate their efficiency in visible and infrared wavelength, simulations tools, that give a good representation of physical phenomena, are used. The first part of this article describes the elements used to prepare data for the simulation. The second part explains the physical model used in CHORALE for the resolution of the Radiative Transfer Equation when obscurants are set in the scene. Obscurants are modeled by a set of voxels (elementary volume elements). Each voxel contains the spectral absorption and scattering coefficients, phase function coefficient and temperature information. The shape is changing with time to take into account the dynamic evolution of the obscurant. A "photon map" method is used in the ray tracing process to take into account global illumination within the cloud and solve the Radiative Transfer Equation.