In
geometry, a focaloid is a shell bounded by two
concentric,
confocal ellipses (in 2D) or
ellipsoids (in 3D). When the thickness of the shell becomes negligible, it is called a thin focaloid.
Mathematical definition (3D)
If one boundary surface is given by
with semiaxes a, b, c the second surface is given by
A focaloid can be used as a construction element of a matter or charge distribution. The particular importance of focaloids lies in the fact that two different but confocal focaloids of the same mass or charge produce the same action on a test mass or charge in the exterior region.