Similarities between various cavity emissivity theories have been examined. It is shown here that, for the special case of the (isothermal) diffuse spherical cavity, the closed form emissivity expressions obtainable from the work of Gouffé, DeVos, and Sparrow and Jonsson agree exactly. Further, this result is not limited by the size of the opening in the cavity.
M. L. Fecteau, "The Emissivity of the Diffuse Spherical Cavity," Appl. Opt. 7, 1363-1364 (1968)