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Definition
Volume scattering is a phenomenon that occurs in an optical medium (e.g., glass or plastic), where the presence of small particles within the volume of the material causes light to scatter. Particulate flaws in the medium may occur due to small bubbles, inclusions, or contamination. Scatter from these sources is similar in nature to that from surface particulates except that it cannot be eliminated by cleaning. The particles within such an optical medium generally scatter light in both forward and backward directions. The scattered intensity and polarization are not easily related to defect characteristics, and there is no minimum scatter angle associated with the illuminated spot size, as is found with surface scatter.
How is volume scattering important to an optical design?
In an optical design, the geometry alone does not determine the light distribution; it’s the optical properties that determine how the energy and direction of light rays change, and accurate optical properties are essential for accurate simulation results. For this reason, it’s important to know as precisely as possible the optical characteristics of the materials used in your design. The best way to obtain precise characteristics is to measure the material directly and export the data to use in optical design software.
Requirements:
- Optical designers need accurate optical properties for ray tracing simulations.
- R&D department needs to design the right material with given optical properties.
- The quality check in the manufacturing process must be perfectly controlled.
Solutions:
- Angular optical scattering with bidirectional scattering distribution function: classic BSDF or total internal reflection (TIR) BSDF
- Amount of light propagation from total internal scatter (TIS): reflectance, transmittance, absorbance ratio
- Volume scattering properties
What solution does Synopsys offer for measuring volume scattering?
Synopsys LightTools software has the capability to model volume scattering that enables a user to analyze how light behaves in media such as diffusing plastics, a plate of glass with imperfections (bubbles), or a dusty atmosphere. When rays are traced through a volumetric scattering material, generally one ray is generated per incoming ray for each particle scattering event. The distribution of scattering events within the material is statistically calculated based on the density of the scattering particles, which is generally assumed to be uniform within the material. The direction of the outgoing ray from each scattering event is computed using a statistical particle scattering model, such as Gegenbauer or Mie.
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.
Synopsys REFLET 180S
The Synopsys REFLET 180S optical bench is easy to use for spot inspection or quick analysis.
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.
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