Wall
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.
Wall Condition
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.
Table 3-2: Wall Condition Options
Options
Description
Freeze
Select to fix the ray position and wave vector at the instant a wall is struck. So, the ray position no longer changes after contact with the wall and the wave vector of the ray remains at the same value as when the ray struck the wall. This boundary condition is typically used to recover the ray intensity or phase at the instant contact was made with the wall.
Specular Reflection
Select to specularly reflect from the wall, so that the angle of incidence and the angle of reflection are equal.
Stick
Select to fix the ray position at the instant the wall is struck. The wave vector components are set to zero.
Disappear
This option means that the ray is not displayed once it has made contact with the wall. Use it if the ray location after contact with the wall is not of interest.
Pass through
This option allows rays to cross the boundary unimpeded. It does not change the ray direction at a boundary between different media; to model refraction, instead use the Material Discontinuity feature.
Diffuse scattering
Select to reflect rays at a wall according to Lambert’s cosine law. That is, the probability of a reflected ray propagating in a given direction within a solid angle dω is given by cos(θ)dω where θ is the angle between the direction of the ray and the wall normal.
Isotropic scattering
Select to reflect rays at a wall isotropically in random directions.
Mixed diffuse and specular reflection
Select to reflect rays at a wall either specularly or according to Lambert’s cosine law, based on a user-defined probability.
General reflection
Select to allow an arbitrary direction vector to be specified after a ray makes contact with the wall. The ray direction vector components can be functions of the incident ray direction, phase, or any other quantity.
General Reflection Settings
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.
Primary Ray Condition
Use this section to apply two different wall conditions on the same set of boundaries based on a probability or logical expression.
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.
Probability
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.
The value of γ should always be between 0 and 1.
For example, if the Wall condition is set to:
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Freeze and γ is set to 0.1, then for every 10 rays that strike the wall, on average one freezes and the remaining 9 rays behave according to the Otherwise setting.
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Stick and γ is set to 0.5 then on average half of the rays stick to the wall and the other half behave according to the Otherwise setting.
Expression
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.
Otherwise
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:
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Select Mixed diffuse and specular reflection as the Wall condition,
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set the Probability of specular reflection to 0.3/(1-0.5),
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set the Primary ray condition to Probability,
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set the Probability to 1-0.5, and
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select Freeze as the Otherwise option.
Frequency Reinitialization
This section is available when Polychromatic, specify frequency is selected from the Wavelength distribution of released rays list in the physics interface Ray Release and Propagation section, and when a Wall Condition (or Otherwise condition) other than Freeze, Stick, or Disappear is selected.
This section allows the ray frequency to be changed when rays are reflected by the boundary or pass through it. To do so, select the Reinitialize frequency check box; it is cleared by default. Then select one of the following from the Distribution function list:
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None (the default): enter a value or expression for the Ray frequency ν (SI unit: Hz) directly. The default is 3 × 1014 Hz.
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Normal: the new value of the ray frequency is sampled from a normal or Gaussian distribution. Enter a value or expression for the Mean ray frequency μ (SI unit: Hz). The default is 3 × 1014 Hz. Then enter a value or expression for the Ray frequency standard deviation σ (SI unit: Hz). The default is 3 × 1013 Hz.
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Lognormal: the new value of the ray frequency is sampled from a lognormal distribution; that is, its logarithm follows a normal or Gaussian distribution. A big advantage of the Lognormal distribution (compared to the Normal distribution) is that it statistical outliers will never cause the frequency to become negative. Enter a value or expression for the Mean ray frequency μ (SI unit: Hz). The default is 3 × 1014 Hz. Then enter a value or expression for the Ray frequency standard deviation σ (SI unit: Hz). The default is 3 × 1013 Hz.
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Uniform: the new value of the ray frequency will be sampled uniformly from an interval. Enter a value or expression for the Minimum ray frequency νmin (SI unit: Hz). The default is 3 × 1014 Hz. Then, enter a value or expression for the Maximum ray frequency νmax (SI unit: Hz). The default is 3.5 × 1014 Hz.
Wavelength Reinitialization
This section is available when Polychromatic, specify vacuum wavelength is selected from the Wavelength distribution of released rays list in the physics interface Ray Release and Propagation section, and when a Wall Condition (or Otherwise condition) other than Freeze, Stick, or Disappear is selected.
This section allows the wavelength to be changed when rays are reflected by the boundary or pass through it. To do so, select the Reinitialize wavelength check box; it is cleared by default. Then select one of the following from the Distribution function list:
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None (the default): enter a value or expression for the Vacuum wavelength λ0 (SI unit: Hz) directly. The default is 500 nm.
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Normal: the new value of the wavelength is sampled from a normal or Gaussian distribution. Enter a value or expression for the Mean vacuum wavelength μ (SI unit: m). The default is 500 nm. Then enter a value or expression for the Vacuum wavelength standard deviation σ (SI unit: m). The default is 50 nm.
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Lognormal: the new value of the wavelength is sampled from a lognormal distribution; that is, its logarithm follows a normal or Gaussian distribution. A big advantage of the Lognormal distribution (compared to the Normal distribution) is that it statistical outliers will never cause the wavelength to become negative. Enter a value or expression for the Mean vacuum wavelength μ (SI unit: m). The default is 500 nm. Then enter a value or expression for the Vacuum wavelength standard deviation σ (SI unit: m). The default is 50 nm.
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Uniform: the new value of the wavelength will be sampled uniformly from an interval. Enter a value or expression for the Minimum vacuum wavelength λ0,min (SI unit: m). The default is 500 nm. Then, enter a value or expression for the Maximum vacuum wavelength λ0,max (SI unit: m). The default is 550 nm.
Absorption Coefficient
This section is available when
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the ray intensity or power is solved for in the model, and
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the Wall condition is set to any option that can reflect rays, including the following:
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Specular Reflection,
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Diffuse scattering,
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Isotropic scattering,
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General reflection, or
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Mixed diffuse and specular reflection.
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.
New Value of Auxiliary Dependent Variables
This section is available if an Auxiliary Dependent Variable has been added to the model.
When a ray crosses or touches a boundary, the values of the user-defined auxiliary dependent variables can be changed. The value can be a function of any combinations of ray variables and variables defined on the boundary.
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.
Advanced Settings
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. Random numbers are also used if Reinitialization wavelength or Reinitialization frequency is activated.
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.