STK's EOIR capability utilizes many built-in materials for objects and the Earth's surface when generating a synthetic scene. EOIR uses two main functions: the Bidirectional Reflectance Distribution Function (BRDF) and the Spectral Reflectance Function (SRF). EOIR uses a modified Phong reflection model for its BRDF, and this model describes how light is scattered off an object. The modified Phong reflection model is governed by four constants:
- k0: Spectral versus Lambertian nature of the surface
- k1: Relative strength of forward scatter versus back scatter
- alpha: Sharpness of the forward (reflected) scatter peak
- beta: Sharpness of the back scatter peak
The SRF describes the material's reflectance as a function of incident wavelength; different materials will reflect light at different wavelengths. In the SRF, the reflectance value is between 0 and 1, where 0 reflectance means that incoming light at that wavelength is completely absorbed, while a reflectance of 1 means that light is completely reflected. STK EOIR considers all materials to be complete opaque (transmission = 0). Therefore, the SRF will directly give the material's reflectance at a specific wavelength, and the emissivity of the material can be calculated as 1 - reflectance.
The attached Excel files show all surface materials available for selection on STK objects as well as the material properties used to model Earth's surface.
