The archaeologically accepted plan of the House of the Faun at Pompeii serves as the basis for an initial 3D computer model that employs a simulated lighting script to render the lighting conditions of interior spaces at different times of the year. The model reveals that the Alexander Mosaic, lifted in 1843 and moved to the Museo Nazionale in Naples, was apparently never seen under optimum lighting conditions in its original setting. The problem is solved by providing a new interpretation of modifications made to the colonnades associated with the mosaic and then reconfiguring the model accordingly. The result is a new reconstruction of the architectural setting of the Alexander Mosaic and a new understanding of its original lighting conditions.