Solar-control films are increasingly being retrofitted to the windows of buildings as a means of reducing solar gain. At present, there is a dearth of information concerning how these films effect the thermal comfort of occupants within buildings where these films have been applied. An illuminated hot box, utilising a xenon lamp to simulate sunlight, has been designed as a testing facility. The illuminated hot box has been used to obtain information on how much thermal radiation enters the internal space from a window fitted with a solar-control film. The data from the experimental apparatus was verified using a finite-difference model written on a personal computer, with the aim of the computer program being used to compare different films, and thereby avoiding expensive experiments. The experimental rig produced usable data for the tested films only when the lamp was orthogonal to the plane of the glass, with errors occurring, and increasing, as the angle of incidence between the lamp and the glass increased. This conclusion was verified by the computer based model. It was seen that the illuminated hot box was too small to give accurate measurements for angles of incidence other than 0°. It is suggested that a larger illuminated hot box which is able to eliminate the problems encountered when measuring for angles of incidence above 10° would be desirable. Furthermore, a more complex transient finite-difference computer based simulation model is needed, taking into account the conclusions that were made during this study.