Shading devices have long been used to control solar gain and daylighting through windows. The optimal design and use of such devices requires a detailed knowledge of their optical and thermal properties, including their variation with the angle of incidence of the solar radiation. We describe the experimental methods used to characterise two types of double-glazing units (DGU) incorporating Venetian blinds. The first consists of a DGU with an adjustable blind located in the space between the panes. The second incorporates a fixed blind element bended between two glass sheets. The spectral bi-directional transmittance functions (BRTF) were measured for a range of incidence angles, using the Cardiff goniospectrometer. The thermal properties were measured with an illuminated hot box. These measurements are laborious, and it is desirable to develop simple models, that could be used for product development to estimate the effect of changing the properties of the blind material and its design, without the need to make detailed measurements on each variation. A model is described, that calculates the properties of the DGU using as input the optical proper ties at normal incidence of the component elements (glass and the blind's slats). The model is applied to the variable blind DGU, varying the angle of rotation of the blind. Despite the simple nature of the model, it is shown to satisfactorily predict the properties of the DGU. The model can easily be incorporated in building energy simulation programs.