When producing photometrically accurate
EOIR synthetic scenes, it is important to understand how light reflects off of material surfaces. One piece of this is a mathematical concept known as a Bidirectional Reflectance Distribution Function (BRDF). This equation describes how radiant flux arriving from some input direction is reflected into multiple outgoing directions (scattering) according to the surface properties of a material. STK's
EOIR capability uses two different BRDFs depending on the materials specified: a modified Phong reflection model and a Lambertian model. In reality, the BRDF is a function of wavelength in addition to the incoming and outgoing directions. However, for many materials, especially natural materials, it is practical to treat BRDF as a function of direction alone. Both BRDFs normalize their outputs such that reflected flux is conserved; every arriving photon will be reflected in some direction.
Built-in materials, like aluminum MLI, use the modified Phong reflection model. This model simulates both backscatter and forward scatter using four parameters:
- k0 - Spectral vs. Lambertian nature of the surface
- k1 - Relative strength of forward scatter versus backscatter
- alpha - Sharpness of the forward (reflected) scatter peak
- beta - Sharpness of the backscatter peak
These parameters are fixed in STK and cannot be modified. You are welcome to reach out to AGI Support for more specific information on these parameters in STK. The modified Phong reflection model is governed by the following equation:
Custom materials and gray bodies in STK are modeled using the Lambertian reflectance model instead. Lambertian surfaces are used frequently to approximate rough, unfinished surfaces when its characteristics are unknown. This model defines a diffusely reflecting surface where the apparent brightness of the surface to an observer is the same regardless of the viewing direction (isotropic luminance). As such, unlike the modified Phong reflection model, there are no specific coefficients required.
