Domain Selection
It is possible for rays to pass through domains in the geometry and to propagate in the void region outside these domains. Boundary conditions can be specified at any boundary, including boundaries that are not adjacent to any domain in the geometry. This means that rays can be reflected or absorbed by a surface in 3D, or a line segment in 2D, even if it isn’t attached to any other object. The ray tracing algorithm can also detect boundary interactions in any order, without this order being specified.
In the physics interface Material Properties of Exterior and Unmeshed Domains section, the Optical dispersion model is specified. This determines the Refractive index of exterior domains, real part. 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 spatially uniform, scalar-valued quantity. The refractive index outside the domain selection cannot depend on field variables such as temperature or pressure and cannot be a graded-index medium. Domains with temperature-dependent or spatially nonuniform refractive indices should instead use the Medium Properties node.
Usually, the domain selection for the Geometrical Optics interface should include all objects that the rays might pass through. In a lens system, this would mean all lenses are included, but not necessarily the mount for these lenses.
Domains that are not included in the selection for the physics interface do not need to be meshed. However, these domains would still require a mesh if some other variables, like displacement and temperature, are solved for there. See Meshing Guidelines for Geometrical Optics Simulation for more details.
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, such as the dedicated Ray Heat Source multiphysics feature.