Gonio-photometry: a measurement technique for an accurate assessment of the directional radiative properties of building and urban materials

Construction materials for building and urban applications are generally characterised by a diffusing behaviour in response to the incident radiation. With this assumption, reflectance and absorptance values are constant and independent on the incidence angle. This assumption is not valid for regular materials, like glass or polished surfaces, where an angular dependence of radiative properties is observed. However, many opaque construction materials often show a mixed behaviour, which includes reflectance regular and diffusing (or scattering) components. Moreover, the apparent roughness of the materials surface changes according to the angle of incidence of the solar radiation: in particular the higher the latter, the smaller the former. This issue is relevant for cool materials application, where polished and smooth solutions are adopted to increase global reflectance. Directional reflectance properties are to be measured in order to accurately assess the response of the materials surfaces. Gonio-photometry ensures reliable directional measurements on opaque and construction materials and the results can be used for a more accurate building and urban integration of the construction materials: Glare and visual discomfort risk for materials with significant regular components; angular dependent reflectance and the impact on the solar gains and energy balance of the building envelope. Eight typical construction material for roofing and façade systems are measured with a gonio-photometer and a spectrophotometer. The comparison between reflectance values in the visible band obtained with both devices shows a good agreement in four cases with differences ranging between 0.02 and higher discrepancies for the other four samples, with a maximum difference of 0.09. The results provide information about the directional response of such materials and set the frame for new experimental and calculation analyses aimed at a more accurate urban and building integration.