Scattering Models
Scatter models are defined in terms of a probability distribution function. When OpticStudio scatters a ray, a new direction of propagation is chosen. The direction is chosen using a probability function and one or more random numbers. The net effect is that if many rays are traced, the resulting scattered ray distribution would approach the probability distribution function. There are seven scattering models available: none, Lambertian, Gaussian, ABg, ABg File, BSDF, and user defined. Each available scatter model is summarized below, but see Scattering for the detailed technical descriptions.
No Scattering The default scatter model is "No Scattering", which means no scattering will occur. The resulting ray is called the unscattered or the "specular" ray (even if the surface is not actually reflective). The Bi-Directional Scatter Distribution Function (BSDF), which is the scattered radiance per unit incident irradiance, is zero.
Lambertian In the Lambertian scattering model, the scattered ray projection vector has equal probability anywhere in the unit circle, and the BSDF is just 1/π. The scattered intensity is proportional to the cosine of the angle between the normal vector and the scattered ray angle. Note Lambertian scattering is independent of the ray incident angle. Most diffuse surfaces are nearly Lambertian. Although Lambertian is a valid option on the sequential surface dialog, rays which scatter using this model may go in any forward direction, which may cause rays to scatter at large enough angles so that they do not correctly propagate through the rest of the optical system.
Gaussian In the Gaussian scattering model, the scattering distribution is rotationally symmetric in direction cosine space, no matter what angle the specular ray makes with respect to the surface normal. The BSDF expression (see "Scattering") contains a dimensionless value σ, which determines the width of the Gaussian distribution on the projected plane. Values of σ greater than about 5.0 yield a BSDF that is nearly Lambertian. For this reason, the maximum allowed value of σ is 5.0.
ABg The ABg scattering model is a widely used method for defining the BSDF. This scattering model is generally a good model to use when the scattering is mainly due to random isotropic surface roughness, and the scale of the roughness is small compared to wavelength of light being scattered. These assumptions are generally valid for polished optical surfaces. See Scattering for a detailed technical description.
ABg File The ABg File scattering model allows a sum of ABg profiles to be used to define the scattering properties of a surface. The profiles to use are specified in a text file. The text file must have a .ABGF extension and must be located in the <data>\ABg_Data folder (see "Folders"). All of the profiles specified in the ABGF file must be defined in the currently loaded ABg Data File (see "ABg Data File"), and all profiles must be specified using upper case letters (regardless of capitalization of the profile name in the ABg Data File).
BSDF The BSDF scattering model allows the use of tabular BSDF data for defining the scattering properties of a surface. Data are provided via text files. Files must follow the Zemax Tabular BSDF Data file format described in the article entitled "Zemax Tabular BSDF data file format" (See: Zemax Tabular BSDF data file format – Knowledgebase), which may be found in the OpticStudio Knowledge Base. OpticStudio allows definition of separate BSDF data for reflection and refraction. If the specular ray reflects or refracts, the ray is subsequently scattered using data from the appropriate input file. For a file to be available for use in this scatter model, it must have a .BSDF extension (as indicated in the article describing the file format), and be located in the <data>\Scatterdata folder (see Folders). A full description of the model and its use is provided in the Knowledgebase article How to Use Tabular Data to Define the Surface Scattering Distribution also available on the OpticStudio web site.
User defined scattering Completely general surface scattering may be defined via an external program called a Dynamic Link Library (DLL). Sample DLLs are provided with OpticStudio with source code. New DLLs may be easily created with a suitable compiler. See also Comments about DLLs.
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