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The Nusselt Analog.

Differentiation of the basic form factor equation is difficult even for
simple surfaces.  Nusselt developed a geometric analog which allows
the simple and accurate calculation of the form factor between a
surface and a point on a second surface.  

The 'Nusselt analog' involves placing a hemispherical projection
body, with unit radius, at a point on a surface.  The second surface is
spherically projected onto the projection body, then cylindrically
projected onto the base of the hemisphere.  The form factor is, then,
the area projected on the base of the hemisphere divided by the area
of the base of the hemisphere.