Frequently Asked Questions - Materials and Radiometry

The material description sub-model is very flexible. The top most classification of materials is broken into "surface" and "volume" descriptions. The only difference between these two classes is the type of optical descriptions associated with them.

Most "hard target" or "background" materials fall into the "surface" materials. A surface material can have a reflectance model, an emissivity model or both associated with it. Both the reflectance and emissivity models are responsible for providing spectral coefficients for supplied geometries. There are several reflectance models including directional hemispherical reflectance (DHR) and Bidirectional Reflectance Distribution Function (BRDF) models. There are also a handful of emissivity models. In the event that the reflectance is known but the emissivity is not, the later will be computed from the former for the requested geometry. If the emissivity is known and the reflectance is not, the computed reflectance is assumed to be diffuse (Lambertian).

Materials like gas plumes, clouds, water, etc. fall into the "volume" category. These materials have absorption, scattering and/or extinction properties associated with them. Like with surface materials, if two of the three properties are know, the third will be computed. For example, if the scattering and extinction are known, the absorption will be computed. If the scattering is the unknown quantity, then the computed scattering is assumed to be isotropic.