Ray Tracing Study Step
The Ray Tracing and Time Dependent study steps are very similar, and either one could be used for the vast majority of geometrical optics models. The Ray Tracing study step has some additional features that make it more convenient to use, such as more reasonable default values and built-in stop conditions.
A Note on Geometric Nonlinearity
If a physics interface solves for the displacement field, such as the Solid Mechanics interface, then the check box Include geometric nonlinearity appears in the Study Settings section. It is very important to select this check box when tracing rays in a system that is deformed due to external forces or thermal stress. If the check box is cleared, then rays are instead traced in the undeformed geometry.
Time Steps and Optical Path Length Intervals
By default, the Ray Tracing study step computes ray trajectories from t = 0 to t = 1 ns with a time step size of 0.01 ns. However, it is often useful to think of ray tracing in terms of the maximum distance of ray propagation instead of the maximum time. To express the duration of the study in terms of a maximum optical path length, change the Time step specification setting from the default, Specify time steps, to Specify maximum path length. Then select a Length unit (default m), enter a set of Lengths (default range(0,0.01,1)), and enter a Characteristic group velocity (default c_const, a built-in constant for the speed of light in a vacuum). With the default solver settings, the time-dependent solver must take at least one time step whenever the optical path length of a ray moving at the Characteristic group velocity would have reached one of the values in the list of Lengths.
Built-in Stop Conditions
The Ray Tracing study step includes options to create a Stop condition node in the default solver sequence. The Stop condition node can terminate the study before the full range of specified times or optical path lengths has been simulated, if a condition is met before then.
To use one of the built-in stop conditions, select one of the following options from the Stop condition list in the Study Settings:
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None: The study ends at the specified maximum time or maximum optical path length.
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No active rays remaining: The study terminates if all rays have been stopped or absorbed. A ray can be absorbed by a boundary, or it can be removed by the Ray Termination domain feature.
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Active rays have intensity below threshold: This option is only available when ray intensity is computed. The study terminates if every ray in the model is either stopped, absorbed, or has sufficiently low intensity.
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Active rays exceed maximum number of reflections: This option is only usable when the Count reflections check box is selected in the physics interface Additional Variables section. The study terminates if every ray in the model is either stopped, absorbed, or has been reflected the specified number of times.
Stop Condition in the COMSOL Multiphysics Reference Manual.
Coupled Physics Interfaces
If other physics interfaces are also solved for in the Ray Tracing study step, it is assumed that these other fields vary over the same time scale as the ray propagation. This is seldom true. If instead the coupled physics do not change over the time scale for ray propagation, consider a study with two steps: Stationary for all other fields and Ray Tracing just for the rays.
In the COMSOL Multiphysics Reference Manual:
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Ray Tracing
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Studies and Solvers