In radiative heat transfer, a view factor, , is the proportion of the radiation which leaves surface that strikes surface . In a complex 'scene' there can be any number of different objects, which can be divided in turn into even more surfaces and surface segments. View factors are also sometimes known as configuration factors, form factors, angle factors or shape factors. Summation of view factors Because radiation leaving a surface is conserved, the sum of all view factors from a given surface, , is unity: For example, consider a case where two blobs with surfaces A and B are floating around in a cavity with surface C. All of the radiation that leaves A must either hit B or C, or if A is concave, it could hit A. 100% of the radiation leaving A is divided up among A, B, and C. Confusion often arises when considering the radiation that arrives at a target surface. In that case, it generally does not make sense to sum view factors as view factor from A and view factor from B (above) are essentially different units. C may see 10% of A 's radiation and 50% of B 's radiation and 20% of C 's radiation, but without knowing how much each radiates, it does not even make sense to say that C receives 80% of the total radiation. Self-viewing surfaces For a convex surface, no radiation can leave the surface and then hit later, because radiation travels in straight lines. Hence, for convex surfaces, For concave surfaces, this doesn't apply, and so for concave surfaces Superposition rule The superposition rule (or summation rule) is useful when a certain geometry is not available with given charts or graphs. The superposition rule allows us to express the geometry that is being sought using the sum or difference of geometries that are known. Reciprocity The reciprocity theorem for view factors allows one to calculate if one already knows . Using the areas of the two surfaces and , View factors of differential areas Taking the limit of a small flat surface gives differential areas, the view factor of two differential areas of areas and at a distance s is given by: where and are the angle between the surface normals and a ray between the two differential areas. The view factor from a general surface to another general surface is given by: The