This report describes a program module of the expert system EXPOSURE. This module, written in Common Lisp, is for calculating the maximum electromagnetic radiation incident upon a building's wall due to theburning of a neighboring building. It is assumed that the burning building has an arbitrary number of rectangular openings emitting radiation. The exposed wall can be considered as being composed of a number of rectangular regions. These regions may be openings on regions of different materials. This program can determine the maximum radiation for each of these regions and for the wall less these regions.