Use the Wall node to determine what happens to the rays when contact with a boundary is made. Rays automatically detect walls that intersect their path, without the order of the interactions being specified. At boundaries, rays can be reflected or absorbed. You can also select two different conditions and assign a criterion or probability to determine which one is applied.

The Accumulator (Boundary) subnode is available from the context menu (right-click the parent node) or from the Physics toolbar, Attributes menu. If the ray intensity or power is computed, the Thin Dielectric Film subnode is also available. If the ray power is computed, the Deposited Ray Power (Boundary) subnode is also available.

Select a Wall condition: Freeze (the default), Specular Reflection, Stick, Disappear, Pass through, Diffuse scattering, Isotropic scattering, Mixed diffuse and specular reflection, or General reflection.

This section is available when General reflection (see Table 3-2) is selected as the Wall condition.

Enter values for the Reflected ray direction vector Lp (dimensionless) either in Cartesian coordinates (x, y, z) (the default) or select the Specify tangential and normal direction vector components check box to enter coordinates in the tangent-normal coordinate system (t1, t2, n). In this case the normal direction is selected so that an incident ray is reflected back into the domain it previously occupied if the specified normal direction vector component is positive. The tangential directions are oriented so that they form a right-handed coordinate system, together with the normal direction.

Select a Primary ray condition: None (the default), Probability, or Expression. When the default, None, is kept, it means that the Wall condition is always respected by the incident rays.

If Probability is selected, the Wall condition is applied with a certain probability. Enter a value for the Probability, γ (dimensionless). If the Wall condition is not used, the ray instead behaves according to the Otherwise setting.

For example, if the Wall condition is set to:

If Expression is selected, the Evaluation expression e (dimensionless) is evaluated whenever a ray strikes the wall. The default expression is 1. If the Evaluation expression is nonzero, the ray behaves according to the Wall condition, otherwise the ray behaves according to the Otherwise setting.

The options available for the Otherwise setting are the same as for the Wall Condition, except that General reflection and Mixed diffuse and specular reflection are not available. The Otherwise setting can be used to make rays interact with a wall differently with a certain probability or when a certain condition is satisfied. For example, to model reflection at a partially specular surface in which 50% of the ray intensity is absorbed and 30% is reflected specularly:

Select one of the following from the Compute reflected intensity using list: Absorption coefficients or Reflection coefficients.

For Absorption coefficients enter the Absorption coefficient α (dimensionless). The default is 0. The intensity or power of the reflected ray will be proportional to 1 − α.

For Reflection coefficients enter the Reflection coefficient r (dimensionless). The default value is 1. The intensity or power of the reflected ray will be proportional to r2.

If the Wall condition is set to Mixed diffuse and specular reflection, this section is instead called Absorption Coefficients and the absorption coefficients for the diffusely and specularly reflected rays are specified separately. All of the text fields are given subscripts s and d for specularly and diffusely reflected rays, respectively.

This section is available if an Auxiliary Dependent Variable has been added to the model.

Select the Assign new value to auxiliary variable check box or boxes based on the number of auxiliary variables in the model. Then enter the new value or expression in the field. For example, if there is an auxiliary variable, psi, then enter a value for psinew in the field. So, to increment the value of psi by 1 when a ray touches or crosses a boundary, enter psi+1 in the text field for psinew.

If the Primary ray condition is set to Probability, or if the Diffuse scattering, Isotropic scattering, or Mixed diffuse and specular reflection wall condition is used, then the Wall feature generates random numbers.

If, in addition, the Arguments for random number generation setting is set to User defined in the physics interface Advanced Settings section, the Advanced Settings section is available.

Enter the Additional input argument to random number generator i (dimensionless). The default value is 1.

The Advanced Settings section is also shown if the Compute optical path length check box is selected under the physics interface Additional Variables section. Select the Reset optical path length check box to set the optical path length to 0 when a ray touches the wall.