Using Ray Release Features
Rays that are released using the Geometrical Optics interface are either primary or secondary rays. A ray is considered a primary ray if its initial position and wave vector are defined explicitly. A ray is considered a secondary ray if its release is dependent on the behavior of another ray that is already present in the model, which may either be a primary ray or a secondary ray that had been previously released.
For example, when a ray interacts with a Material Discontinuity without undergoing total internal reflection, two rays propagate away from the boundary, including one reflected ray and one refracted ray. The degrees of freedom from the incident ray are re-used as the refracted ray. The additional (reflected) ray that is created during interaction with the Material Discontinuity is a secondary ray because its initial conditions are not specified directly, but instead result from an existing ray interacting with a boundary. The reflected ray may only be released if the total number of released secondary rays in the model is less than the value of the Maximum number of secondary rays in the settings window for the physics interface.
If the incident ray undergoes total internal reflection, there is no refracted ray, so the incident ray’s degrees of freedom are used to launch the reflected ray and no secondary ray degrees of freedom are used.
The following sections introduce the primary release features that are available in the Geometrical Optics interface.
Grid-Based Release of Rays
Use the Release from Grid feature to specify the initial positions of rays using a grid of points. It is useful to release rays from a grid when the initial ray position is known exactly. A grid-based release may be used, for example, when rays are released from the focus of a lens or when a system is excited by a laser. This is the easiest way to release rays from points in the void region outside the geometry.
Mesh-Based Release in Domains
Use the Release feature to release rays from within a set of domains. The initial ray positions are based on the mesh elements within the domain. It is possible to assign a nonuniform initial distribution of rays by entering a function of the spatial coordinates in the Density proportional to text field.
Release at Boundaries
The Inlet feature is used to release rays from boundaries. It is possible to release rays based on the boundary mesh, by specifying a function for the initial density of rays, or by projecting a grid of points onto the surface. The Inlet feature may be used when rays enter a system through an aperture of considerable size. It is also possible to initialize the principal radii of curvature of the rays using the curvature of the inlet surface.
When a mesh-based or density-based release of rays is used, the initial ray positions may not be the same in all COMSOL Multiphysics versions or when using different geometry kernels. However, when a density-based release is used, the total number of rays can always be specified exactly.
Release Using Imported Data
The Release from Data File node can be used to import the initial ray positions and directions from a text file. It is also possible to import the initial values of user-defined auxiliary dependent variables.
Illuminated Surfaces
Applications of geometrical optics frequently involve plane or spherical waves that reflect or refract off a large surface, then continue to interact with other nearby objects. In such cases, significant reduction in computation time, sometimes 50% or greater, can be achieved by using the Illuminated Surface feature. The Illuminated Surface feature releases the rays that would result from the reflection or refraction of a user-defined plane wave or spherical wave at a surface.
Figure 2-4: Comparison of the ray trajectories and intensity when sending rays at a wall and using a Bounce condition (left) and using the Illuminated Surface feature (right).
As shown in Figure 2-4, the Illuminated Surface feature can be used to compute the ray trajectories that result from launching a plane wave at a specularly reflecting surface. The Stokes parameters and the principal radii of curvature of the wavefront are also initialized based on the type of incident wave and the curvature of the surface. Therefore it is unnecessary to compute the trajectories of rays before they hit the surface, which may save a significant amount of time and make plots much easier to interpret.
Solar Radiation
The Solar Radiation feature is similar to the Release from Grid feature but includes dedicated settings for computing the direction of incident radiation using the time and location on Earth’s surface. Because rays are given initial radii of curvature of the same order of magnitude as the distance from the earth to the sun, they behave as plane waves in most applications.