The Geometrical Optics (gop) interface (), found under the Optics>Ray Optics branch () when adding a physics interface, computes the paths of electromagnetic rays. In addition, it can compute the ray intensity, polarization, phase, and optical path length. The physics interface works on geometries, deformed geometries, and with models utilizing ALE.

When this physics interface is added, these default nodes are also added to the Model Builder—Medium Properties, Material Discontinuity, and Ray Properties. Then, from the Physics toolbar, add other nodes that implement, for example, boundary conditions. You can also right-click Geometrical Optics to select physics features from the context menu.

The Label is the default physics interface name.

The Name is used primarily as a scope prefix for variables defined by the physics interface. Refer to such physics interface variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the Name field. The first character must be a letter.

The default Name (for the first physics interface in the model) is gop.

Select the Allow frequency distributions at release features check box to allow radiation of multiple different frequency values to be modeled simultaneously. This option allocates an auxiliary dependent variable for the frequency of each ray, so the total number of degrees of freedom increases by 1 per ray. The ray frequency can be specified at release features by entering a value or expression, sampling the frequency from a distribution, or entering a list of values. By default, this check box is cleared so that all rays are assigned the same frequency, which is specified in the Ray Properties settings window.

Enter a value for the Refractive index of exterior domains (dimensionless). The default value is 1. This value of the refractive index is used when tracing rays outside of the domain selection of the Geometrical Optics physics interface. It is also used when rays propagate in the void region outside the geometry. Some limitations apply, as described in the Domain Selection section of the Ray Optics Modeling chapter.

The Maximum number of secondary rays prevents an inordinate number of rays from being generated by capping them at the number supplied in the text field. The default is 500. Rather than being produced directly by release features such as the Release from Grid node, secondary rays are released when an existing ray is subjected to certain boundary conditions. For example, when a ray undergoes refraction at a Material Discontinuity between different media, the incident ray is refracted and a reflected ray is created; the degrees of freedom for this reflected ray are taken from one of the available secondary rays, which are preallocated when the study begins.

If an insufficient number of secondary rays are preallocated, a reflected ray may not be released when an existing ray undergoes refraction, even if some radiation should be reflected at the material discontinuity. However, if a very large number of secondary rays are preallocated, then the number of degrees of freedom may become unnecessarily large. Thus, the Maximum number of secondary rays should be large enough that all reflected rays which significantly affect the solution can be released. Note that rays undergoing total internal reflection at material discontinuities are not considered secondary rays and do not require extra preallocated degrees of freedom.

Secondary rays are also required to release reflected rays of order zero and all higher diffraction orders when the Grating node and Diffraction Order subnodes are used to model the interaction of radiation with diffraction gratings.

Select an option from the Intensity computation list — None (the default), Compute intensity, Compute intensity and power, Compute intensity in graded media, or Compute intensity and power in graded media. For None the ray intensity is not computed.

The Compute phase check box is shown when the Intensity computation is set to Compute intensity, Compute intensity and power, Compute intensity in graded media, or Compute intensity and power in graded media. Select the check box to allocate an auxiliary dependent variable for the phase of each ray. When the phase of each ray is computed, it is possible to plot interference patterns and visualize the instantaneous electric field components of polarized rays in postprocessing. When this check box is selected, the total number of degrees of freedom increases by 1 per ray.

The Use corrections for strongly absorbing media check box is shown when the Intensity computation is set to Compute intensity, Compute intensity and power, Compute intensity in graded media, or Compute intensity and power in graded media. Select the check box to accurately model reflection and refraction of rays at boundaries between strongly absorbing media, in which the imaginary part of the refractive index is very large. This option allocates two or three auxiliary dependent variables per ray based on space dimension. For more information about the way this option affects the intensity calculation, see Refraction in Strongly Absorbing Media in Theory for the Geometrical Optics Interface.

When the Intensity computation is set to Compute intensity, Compute intensity and power, Compute intensity in graded media, or Compute intensity and power in graded media, enter a value or expression for the Reference intensity (SI unit: W/m2). The reference intensity represents the approximate order of magnitude of the intensity of a typical ray in the model. The default value is 1000 W/m2. It is recommended to change the reference intensity only when modeling systems of rays in which the intensity is many orders of magnitude greater than or less than 1000 W/m2. The reference intensity is used to define numerical tolerances that are used internally by some release features and boundary conditions.

When the Intensity computation is set to Compute intensity in graded media or Compute intensity and power in graded media enter a Tolerance for curvature tensor computation (dimensionless). This tolerance is used internally when computing the principal radii of curvature of propagating wavefronts in a graded medium. A larger tolerance makes the solution less accurate but more stable.

Select the Compute optical path length check box to allocate an auxiliary dependent variable for the optical path length of each ray. It is possible to reset the optical path length to 0 when rays interact with boundaries.

Select the Store ray status data check box to add new variables for quantities that cannot necessarily be recovered from the ray trajectory data alone. This is especially true if automatic remeshing is used in a model. The following variables are created:

The global variable names in Table 3-1 all take the unreleased secondary rays into account. For example, suppose an instance of the Geometrical Optics interface includes 100 primary rays and 100 allocated secondary rays. At the last time step, suppose that 80 of the primary rays have disappeared at boundaries and that 40 secondary rays have been emitted, all of which are still active. Then the variable gop.fac, the fraction of active rays at the final time step, would have the value (20 + 40)/(100 + 100) or 0.3.

This section is only shown when Advanced Physics Options are enabled (click the Show button on the Model Builder).

The Wall accuracy order sets the accuracy order of the time stepping used for time steps during which a ray-wall interaction happens. Select an order of 1 to use a forward Euler step and compute the motion both before and after the wall interaction. Select an order of 2 (the default) to use a second-order Taylor method to compute the trajectory before the wall interaction. After the ray-wall interaction a second-order Runge-Kutta method is used.

Select an option from the Arguments for random number generation list — Generate unique arguments (the default), Generate random arguments, or User defined. This setting determines how the additional argument to random functions is defined in features such as the Wall boundary condition with the Diffuse scattering wall condition. Typically the random numbers are functions of the ray index, position, time, and another argument i, defined as follows:

By default the Allow propagation outside selected domains check box is selected. When this check box is selected, rays can propagate in domains that are not included in the selection for the physics interface. These exterior domains do not need to be meshed. Rays can even propagate through the void region outside the geometry. However, all boundaries that the rays interact with must be meshed.

The Allow multiple release times check box, which is cleared by default, allows an array of release times for the rays to be specified in any of the ray release features. If the check box is cleared, all rays are released at time t = 0.

Enter a value for the Maximum number of wall interactions per time step. The default value is 1000. If a ray undergoes more than the specified number of boundary interactions in a single time step taken by the solver, the ray will disappear. This is included as a safeguard to prevent rays from getting stuck in infinite loops if the time between successive ray-wall interactions becomes infinitesimally small.

The dependent variables (field variables) are the components of the Ray position and Wave vector. The name can be changed but the names of fields and dependent variables must be unique within a model.