The optical properties of Venetian blinds and other types of slat shading can be determined by first measuring the reflectivity of the slats and then applying an optical model that accounts for the geometry of the system of slats. A radiosity method following the guidelines of ISO 15099 has been implemented in Window6. The radiosity approach fails when the blind slats are highly specular. In that case ray tracing has proven to be successful.
M. Rubin, J. Jonsson, C. Kohler, J. Klems, D. Curcija and N. Stojanovic, Bidirectional optical properties of Slat Shading: Comparison Between Raytracing and Radiosity Methods, accepted for publication in Solar Energy. download pdf.
M. Andersen, M. Rubin, R. Powles, J.-L. Scartezzini, Bi-directional transmission properties of Venetian blinds: experimental assessment compared to ray-tracing calculations Solar Energy 78 (2005) pp 187-198: download pdf
M. Andersen, M. Rubin, and J.L. Scartezzini, LBNL-51169, Comparison between ray-tracing simulations and bi-directional transmission measurements on prismatic glazing, Solar Energy 74 (2003) 157-173. LBNL-51169. download pdf