Since the DIRSIG model supports simulating the thermal infrared regions of the spectrum, the contributions from surface self-emission are important. In order to predict this self-emission component, the surface temperature must be known. The prediction of surface temperatures is handled by thermal models that compute surface temperatures based on material thermodynamic properties and environmental conditions (air temperature, wind speed, etc.)
In a real world situation, an object is absorbing and emitting energy simultaneously with surrounding surfaces, as wells as convecting and conducting heat energy. Thermal models have been created to solve for the temperature of surfaces resulting from these different mechanisms at work. DIRSIG currently incorporates a modified version of the Air Force Infrared Simulated Image Model (AIRSIM) known as THERM. The details of this thermal model are described later in this chapter.