In recent years, with the advent of large spacecraft, thermal balance testing techniques using infrared lamp sources have become increasingly attractive. In addition to cost savings, the IR technique is more versatile in terms of spacecraft orientation and supplied flux conditions. Because of the differences between solar illumination and IR simulation the IR lamp flux characteristics must be accurately determined. An equation predicting the incident flux distribution of a lamp reflector combination is developed for a Research Inc. lamp model 5236-5. The radiation absorbed by spacecraft surfaces under test conditions can be obtained from a computer code incorporating lamp arrays, specular baffles and surface property variations with lamp power level and radiation angle of incidence.