Triangle 1 subtends a spherical triangle from a vertex of triangle 2. If the spherical triangle is sufficiently small to be treated as a point source, the configuration factor is a simple function of the angular size and the angle of incidence at the vertex.
In lighting calculations, an area source may be treated as a point if the ratio of its maximum dimension to its distance to the illuminated surface is less than an arbitrary value e. The value of e is frequently taken to be 1/5.
The form factor which relates the illuminance of triangle 2 to the emittance of triangle 1 is found by determining the mean configuration factors over regular points on the receiving triangle. If the receiving triangle is small, the form factor can be found from the vertex values, as follows: