What is Volume Scattering?

In order to use these particle scattering models to design real illumination systems with real scattering materials, you need specific parameter values that usually cannot be measured directly. At Synopsys, we have developed a technique that determines appropriate scattering parameter values (for both the Gegenbauer and Mie models) based on measured BSDF data of volumetric scattering material samples.

How do we measure BSDF? We perform measurements with a specialized optical bench: the Synopsys REFLET 180S. 

For volume scattering measurements, we measure the 2D BTDF of the same sample in four different thicknesses. Using these four BTDF measurements, we perform an optimization with the model we developed to find the parameter needed to simulate the material.  Then we verify that the calculated data provides the same simulation results as the measurements.

The parameters provided as a result of this optimization are: 

• Mean Free Path (MFP), which defines the average distance a ray will travel before experiencing scattering event that will change its propagation direction.
• Anisotropy Factors g and alpha, which describe the angular scattering distribution as defined by Gegenbauer phase function. At each scattering event, the angular change in propagation is randomly sampled from this distribution. The valid range of g is between -1 and 1, while alpha can have any value greater than -0.5.
• Transmittivity, which corresponds to the absolute transmission per scattering event. Transmittivity values greater than 1.0 will gain energy. 

These parameters can directly be used in LightTools illumination software to create a material (.mat) file that can be used for simulations.