Creating a Mesh for Geometrical Optics Modeling
A mesh must be present on any boundary that the rays may interact with. In addition, any domains in the selection for the Geometrical Optics interface must be meshed. Domains outside the selection can be left unmeshed. They are assumed to have the same Refractive index of exterior domains that is used to model propagation in the exterior void domain; see the section Domain Selection.
In most cases, the main requirement for a suitable mesh is the resolution on curved surfaces. While the mesh within on flat surfaces may often be very coarse, the mesh on boundaries with small principal radii of curvature must be fine enough so that the direction of the surface normal can be determined accurately; this in turn improves the accuracy of the reinitialized wave vectors of the reflected and refracted rays. If virtual operations are included in the geometry sequence or if any form of mesh deformation is present in the model, then the accuracy of the reinitialized wavefront radii of curvature also depends on the resolution of the mesh on curved boundaries.
To reduce the mesh size on curved surfaces without creating an unnecessarily fine mesh elsewhere, reduce the Curvature factor in the Size settings window in the mesh sequence. A smaller curvature factor gives a finer mesh along curved boundaries. If the curved surfaces are very small relative to the rest of the geometry, it may be necessary to reduce the Minimum element size.
Figure 2-10: Ray trajectories passing through a lens (left). A high-quality mesh (right) should be finer near the curved surface of the lens.
The following features are used to define dependent variables on domains or boundaries: Accumulator (Boundary), Accumulator (Domain), and Deposited Ray Power (Boundary). Because the dependent variables are defined on domain or boundary elements whereas the rays are treated as occupying infinitesimally small points in space, the values of the dependent variables that are created by these features are inherently mesh-dependent. If a very fine mesh is used when one of these features is present, it may be necessary to increase the total number of rays so that the number of rays passing through each mesh element is sufficiently large.
An alternative to using an extremely fine mesh is to manually increase the Geometry shape order in the settings for the model component. By default, most boundary element surfaces are approximated as quadratic polynomials. Increasing the shape order to Cubic or Quartic can dramatically increase the accuracy of surface normals calculated by the Geometrical Optics interface. However, this may increase the computational cost of other, finite element-based physics interfaces.