Ray Termination
Use the Ray Termination feature to remove rays from the model as they exit the geometry, or to annihilate rays of sufficiently low intensity or power. Only one Ray Termination node can be added to the model, but it is possible to combine position-based and intensity-based criteria for removing rays if the ray intensity or power is computed by the model.
Termination Criteria
Select an option from the Spatial extents of ray propagation list: Bounding box, from geometry (the default), Bounding box, user defined, or None. This list controls whether rays are terminated based on their positions.
•
For Bounding box, from geometry, a ray is immediately terminated if it reaches the surface of the bounding box of the geometry, defined as the smallest rectangle or rectangular prism with edges parallel to the axes of the global coordinate system that encloses all of the geometric entities. This bounding box is extended by 5% in each direction so that rays can still interact with boundaries that coincide with the perimeter of the bounding box. Therefore rays can propagate a short distance away from the geometry before disappearing.
•
For Bounding box, user defined, a ray is immediately terminated if it reaches the surface of a box of user-specified dimensions. To define the box, enter maximum and minimum values of each spatial coordinate.
•
For None, the rays that propagate outward from the geometry will continue to propagate until the study is complete. If the maximum path length for the Ray Tracing study step is too large, this may result in trajectories that are extremely long relative to the characteristic geometry size.
If rays are released from outside the bounding box, they will not immediately disappear; instead, they will only disappear if they enter the bounding box and then exit it again.
Intensity- and Power-Based Termination Criteria
If the ray intensity or power is being computed in the model, you can then terminate rays based on either or both of these quantities.
Select an option from the Additional termination criteria list. None (the default) is always available. If ray intensity is solved for, then Intensity is available. If ray power is solved for, then Power is available. If both ray intensity and power are solved for, the Intensity and Power option is also shown.
•
For Intensity or Intensity and power, enter the Threshold intensity Ith (SI unit: W/m2). The default is 1 mW/m2. Rays will be removed from the model if their intensity decreases below the specified value.
•
For Power or Intensity and power, enter the Threshold power Qth (SI unit: W). The default is 10-3 mW.
 
The Ray Termination feature can terminate rays at the exact time at which their intensity or power reaches the specified threshold (to within numerical precision), if there exists a closed-form analytic expression for this time. Such a closed-form analytic solution exists if:
•
The rays represent planar wavefronts in an absorbing medium, or
•
The rays represent spherical or ellipsoidal wavefronts in a nonabsorbing medium.
Otherwise, the rays are stopped at the first discrete time step taken by the solver for which the intensity or power is less than the threshold value.
Ray Termination Theory