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The Form Factor.
The 'form factor' describes the fraction of energy which leaves one
surface and arrives at a second surface. It takes into account the
distance between the surfaces, computed as the distance between
the centers of the surfaces, and their orientation in space relative to
each other, computed as the angle between each surface's normal
vector and a vector drawn from the center of one surface to the center
of the other surface. A form factor is a dimensionless quantity.
The form factor, as initially shown, describes the form factor between
two differential areas. This is a point-to-point form factor. To use this
form factor with surfaces which have a positive area, the equation
must be integrated over one or both surface areas. However, the
form factor between a point on one surface and another surface with
positive area can be used if the assumption is made that the single
point is representative of all of the points on the surface.