We assess daylight scattering by Roman window glass and its effect on the illumination of building interiors. The scattering properties of exemplary findings are characterized by gonio-photometric measurements and analyzed with regard to surface structure, inclusions and aging effects. The measurement data will be compiled into data-driven models of the optical properties of each sample, predicting the distribution of transmitted light for arbitrary incident directions. These models will be employed in conjunction with a simplified room model to assess the effects of the findings' particular properties on the illumination of opaque room surfaces, the perceived brightness of the windows, and the resulting effects on the perception of the illuminated space. In an interdisciplinary discussion the influence of window glasses on building practices in late antiquity and the adequacy of the method for future investigations in archaeology and historic building research shall be discussed. The cooperation between Technische Universität Darmstadt, Lucerne University of Applied Sciences and Arts, LVR-LandesMuseum Bonn and Römisch-Germanisches Zentralmuseum Mainz is supported by the Thyssen foundation. As a pilot study, it shall prepare further research on the quality, meaning and technological foundations of illumination in late antique buildings.