The default boundary condition is the Material Discontinuity condition on all interior and exterior boundaries. For detailed descriptions of the available wall conditions, see Material Discontinuity and Wall.

When several different media are arranged in layers, it is possible for a very large number of secondary rays to be released as the incident ray undergoes a series of reflections between these layers. To prevent the creation of an arbitrarily large number of secondary rays of extremely low intensity, enter a value or expression in the Threshold intensity text field in the Material Discontinuity settings window. The release of a secondary ray is suppressed if its initial intensity would have been less than the threshold intensity.

The Material Discontinuity node reinitializes ray trajectories whenever a ray crosses the boundary, even if the media on both sides of the boundary are equal. Although the resulting ray trajectories are correct and the unnecessary release of secondary rays can be suppressed, the reinitialization of ray trajectories requires more time than a typical time step in which all rays merely propagate within domains. To minimize solution time it is recommended that unnecessary interior boundaries should be removed from the geometry.