For the purpose of calculating the view factor for radiation heat transfer, surfaces may be divided.

For the purpose of calculating the view factor for radiation
heat transfer, surfaces may be divided into three categories of convex, flat
(plane), or concave. As shown in the figure, surfaces a, b, c, and d are
examples of convex, and surfaces f, g, h, and i are examples of concave
surface. If a surface can view itself then F11 > 0 otherwise, F11
= 0. Consider surfaces a through i, and specify the surfaces that have non-zero
view factor.