Consider a set of opaque diffuse gray surfaces. The net radiative heat rate from the surface of index i is expressed in function of its radiosity and irradiation:

where the surface radiative resistance Ri is defined as:

with Ai the surface area (SI unit: m²) and εi the surface emissivity (dimensionless). See Theory for Surface-to-Surface Radiation for more details.

This allows a network representation of the radiative heat rate with Ebi and Ji as nodes. The surface resistance tends toward 0 for a large surface or a surface with large emissivity, for which the blackbody approximation holds.

When considering the surface of index i as part of an enclosure, the net radiative heat rate is expressed as:

where Fij is the view factor for surfaces i and j.

For specific configurations, the surface areas can be evaluated from the geometric dimensions and the view factors are equal to 1. Therefore the expression above simplifies (see Ref. 21 for details).

The surface areas A1 and A2 satisfy

The surface areas A1 and A2 satisfy

The surface areas A1 and A2 satisfy

These expressions are predefined options in the Radiative Thermal Resistor feature. Further simplifications can be made on the net radiation exchange when the blackbody assumption holds (ε1 = ε2 = 1).