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:
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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.
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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:
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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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select Freeze as the Otherwise option.
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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.
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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.
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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.
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the Wall condition is set to any option that can reflect rays, including the following:
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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. If the
Specify different values for s- and p-polarization check box is selected, you can instead enter separate values for the absorption of p- and s-polarized light, called
αp and
αs, respectively.
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
Specify different values for s- and p-polarization check box is selected, you can instead enter separate values for the reflection of p- and s-polarized light, called
rp and
rs, respectively.
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.
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. 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.