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Shading devices are used to reflect direct sunlight, reduce glare and prevent overheating. A correctly designed shading system will strongly influence the entire building energy consumption, peak loads and indoor comfort. The use of the venetian blinds is particularly common due to its relatively low cost and high versatility. The present study focuses on the algorithms for calculating both short-wave (solar) and long-wave (infrared) transmittance, absorptance and reflectance of the venetian blinds. The model is based on the net radiation method, allows for different slat discretization schemes and is suitable for integration into the building energy simulation tools. Once the effective optical properties are computed, the system can be regarded as an additional layer in a multilayer glazing system. Computed blind solar transmittance has been compared with other numerical models and in situ experimental results. A general good agreement has been found, except in cases where incident radiation is mostly diffuse. In this study, the effect of different design parameters on effective optical properties of the venetian blinds is also investigated: (a) the sun profile angle, (b) the slat angle and (c) the venetian blind geometry. Moreover, the effect of the discretization of each slat is examined. [ABSTRACT FROM AUTHOR]