Radiated Energy as a Function of Energy DensityIf we consider energy radiated perpendicular to a small increment of area, then it must be noted that half of the energy density in the waves is going toward the walls and half is coming out if the system is in thermal equilibrium. Evaluating the power seen at a given observation point requires a consideration of the geometry:
Other elements of area on the surface send radiation through the specified surface area, and that radiation has a component perpendicular to the surface. To evaluate that contribution, consider a surface element at an arbitrary angle as shown below.
Rayleigh- Jeans references
Blackbody radiation concepts