This site allows interested people to access, free-of-charge, average solid angle software developed by R Whitcher without having to contact him directly. Please note the disclaimer on the software.
These papers have been published by R Whitcher on this subject:
Calculation of the Average Solid Angle Subtended by a Detector to Source in a Parallel Plane by a Monte Carlo Method. Radiat Prot Dosimetry, 2002; 102(4):365-9
The use of a Monte Carlo method to calculate the average solid angle subtended by a detector to source in a non-parallel plane. Radiat Prot Dosimetry. 2003;104(2):167-71
A Monte Carlo method to calculate the average solid angle subtended by a right cylinder to a source that is circular or rectangular, plane or thick, at any position and orientation. Radiat Prot Dosimetry. 2006;118(4):459-74
A Monte Carlo method to calculate the average solid angle subtended by a detector to an ellipsoidal surface or solid source at any position and orientation. Radiat Prot Dosimetry. 2012 May;149(4):444-53
Developments in the calculation of average solid angle subtended by detectors to sources at any position and orientation, by a Monte Carlo method. Nuclear Inst. and Methods in Physics Research, A. 2020. Volume 981, article id. 164532.