Domain Selection
It is possible for rays to pass through domains in the geometry and to propagate in the void region outside these domains. In addition, boundary conditions can be specified at any boundary, even at boundaries that are not adjacent to any domain in the geometry.
In the physics interface node’s Ray Release and Propagation section, the Refractive index of exterior domains is specified. This refractive index is used in any domains outside of the selection for the Geometrical Optics interface, as well as the void domain outside the geometry. It is a constant, scalar-valued quantity; thus, the refractive index outside the domain selection cannot depend on field variables such as temperature and cannot be a graded-index medium.
Typically, the most efficient way to specify the domain selection is to select all dielectric media except the medium that occupies the greatest volume. For typical lens systems, this means that all lenses should be selected and that the air or vacuum surrounding the lenses can either be deselected or left as part of the exterior void domain.
A major advantage to excluding some domains from the selection for the physics interface is that these domains do not need to be meshed; see Creating a Mesh for Geometrical Optics Modeling for more details.
Note that some physics features require a domain mesh and will not function on domains outside the physics interface selection. This includes all types of Accumulator (Domain) feature, including the dedicated Ray Heat Source multiphysics feature.