View factor

Summary

The view factor $F_{i \to j}$ from a surface $A_i$ to a surface $A_j$ is the fraction of the radiation leaving $A_i$ that reaches $A_j$ directly. This reading assumes that $A_i$ emits and reflects diffusely. It depends only on the geometry: the sizes, distance and orientation of the two surfaces. View factors satisfy the reciprocity rule $A_i F_{i \to j} = A_j F_{j \to i}$ and, in a closed enclosure, sum to $1$ over all target surfaces. They are the geometric input of every enclosure calculation in thermal radiation.