When the Emitted radiation direction is defined by Opacity controlled in surface-to-surface boundary features (Diffuse Mirror (Surface-to-Surface Radiation Interface), Diffuse Surface (Surface-to-Surface Radiation Interface), Prescribed Radiosity (Surface-to-Surface Radiation Interface), Opaque Surface (Surface-to-Surface Radiation Interface)), surface-to-surface radiation propagates in non-opaque domains. Alternatively the Emitted radiation direction can be defined using the normal orientation or on both sides of boundaries. In this case the Opacity node is ignored. Note that on boundaries where the Semitransparent Surface (Surface-to-Surface Radiation Interface) is applied, the Emitted radiation direction is Both sides, and the Opacity node is ignored.

By default, all the domains and the exterior of the Surface-to-Surface Radiation interface are considered as transparent. To set the exterior as opaque, change the Selection of the Opacity node to All voids in the Domain Selection section, and set it as Opaque. Depending on the geometry, an infinite void and several finite voids may be available for the selection. When the Heat Transfer with Surface-to-Surface Radiation multiphysics coupling is active, it controls the default opacity.

Depending on the Wavelength dependence of radiative properties defined in the Radiation Settings section of the physics interface settings, different sections display underneath:

Physics tab with Surface-to-Surface Radiation selected in the model tree: