Inclusion of the specular component in the assessment of bidirectional distribution functions based on digital imaging

To describe complex fenestration systems such as novel solar blinds, new glazing or coating materials, daylight and sunlight-redirecting devices, a detailed description of their optical properties is needed, given by their Bidirectional Transmission (or Reflection) Distribution Functions (commonly named BTDFs and BRDFs). These functions are angle-dependent at both the incidence and the emission levels, and are defined as the ratio of the luminance of a surface element in a given direction (after diffuse transmission or reflection) to the illuminance on the sample. However, these functions are capable of describing the specular as well as the diffuse components of emerging light, and their mutual knowledge is necessary to properly assess a glazing or shading system’s daylighting performances and benefit from their potential as energy-efficient and users’ comfort strategies. Although the analytical expression of a BT(R)DF differs whether it is related to specular or diffuse light, a simultaneous assessment of the two components can be achieved under certain conditions. These conditions are analyzed for the particular data acquisition procedure developed for a novel type of bidirectional goniophotometer, based on digital imaging.