Partially Immersed Surfaces

In FRED, a surface is defined as an interface between two materials. If you need to define a model where a surface is partially immersed then the surface must be split into constituent surfaces to correctly handle the various interfaces.

This is shown via the following example of an N-BK7 sphere partially submerged in a block of F2 glass, all surrounded by air.

In this case the sphere must be split into two surfaces - one that is immersed (interface of N-BK7 and F2) and one that is not (interface of N-BK7 and Air). And the block, whilst all surfaces simply define an interface between F2 and Air, must be trimmed to remove the part of the block surface that overlaps the sphere.

Example

First create the base shapes with the materials of N-BK7 and F2. A starting point is provided in the following file: PartiallyImmersedSurface_start.frd. If you ray trace this file you will see material resolution errors reported in the output window due to material mismatches.

Next, right-click on the sphere object and choose to convert it to a custom element. Rename it to Outer Sphere, and expand to see the surface. Double click the surface and in the aperture tab set the trim to be fractionally larger than the radius of the sphere:

Due to the tightness of the trim you may see some gaps in the rendered sphere - these are just rendering artifacts and can be resolved by increasing the tessellation (from the Visualization Attributes r-click menu)

Then copy and paste the sphere to give us two spheres, and rename the other to be the Inner Sphere:

Next, drag and drop F2 from the material folder onto the Inner Sphere, and choose to replace Air with F2 to give the correct material boundary at this interface:

Now we can start trimming away the overlaps. Double click on "Outer Sphere.Surface" and add the surface trimming specification as shown below, this trims away the part of the sphere inside the block:

Next double-click on the "Inner Sphere.Surface" and add the surface trimming specification as shown below, this trims away the part of the sphere outside the block:

Finally, double click on "Block.Front Face (+Z)" and add the surface trimming specification as shown below, this trims away the part of the surface that is inside the sphere:

Noting that the Sphere's trim volume was set to be tight to the surface in order to be able to trim with this TRIM VOLUME option without leaving a gap between the sphere and the surface.

This final result will trace without any material resolution errors because we have set the materials correctly and ensured the proper surface boundaries exist with no overlaps via trimming.

The completed FRED file is available here: PartiallyImmersedSurface_end.frd