Wall Conditions
For most boundary conditions other than refraction, the Wall feature can be used. It includes a wide variety of boundary conditions including the following:
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Specular reflection
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Diffuse reflection
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Isotropic reflection
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Combination of diffuse and specular reflection
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User-defined reflection
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Pass through
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Several varieties of absorption condition (see below)
In this context, diffuse reflection means Lambertian scattering, following the cosine law; as opposed to isotropic scattering, in which reflected light has an equal probability of being reflected in any direction to one side of the boundary.
If you choose Mixed diffuse and specular reflection, you can assign some probability that the ray is reflected specularly; otherwise, it is reflected diffusely.
The Diffuse scattering and Mixed diffuse and specular reflection rely on pseudorandom number generation, so they are not guaranteed to give exactly the same numerical results in different software versions or on different architectures.
Types of Ray Absorption
There are several different types of absorption condition, classified by the type of information that they retain about the ray.
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The Disappear condition annihilates the ray completely. After the ray disappears, its position and other degrees of freedom evaluate to not-a-number (NaN).
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The Freeze condition retains the ray position and direction after the ray hits the boundary, although these quantities no longer change over time.
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The Stick condition retains the ray position, but all other degrees of freedom are set to zero.
If the model includes several different types of boundary condition and you want to know what type of condition was applied to each ray, select the Store ray status data check box. This will store a variable that indicates the final status of each ray: whether it has not been released yet, is still propagating, or has been absorbed.
Conditional Ray-Wall Interactions
The Wall feature also support conditional ray-wall interactions. For example, you could cause rays to Freeze if a logical expression is satisfied — for example, having optical path length greater than a specified threshold — and subject them to Specular reflection otherwise. Together with the Mixed diffuse and specular reflection condition, it is possible to prescribe up to 3 different types of wall interaction at a single boundary.