Segments of ellipsoids are often used in spectroscopy because they have two focal points. They are ideal elements for the transportation of radiation from one point in space (first focal point) to another (second focal point).

Ellispoids in SPRAY are defined in two steps: First the full ellipsoid is defined specifying the center and three vectors (perpendicular to each other) that point from the center into the three principal directions:

This way you describe the full ellipsoid:

With the Show box data command you open a second dialog which defines a rectangular box:

Location is the center of the box, Vector 1, 2 and 3 start at the center and point to the centers of  three adjacent rectangles, as already shown for volume light sources.

The ellipsoid segment is now defined by those parts of the ellipsoidal surface which are inside the rectangular box:

The definition of the interface covering ellispoids is the same as explained for rectangles.