The Accuracy of RADIANCE Software in Modelling Overcast Sky Condition

A validation study of the sky models of RADIANCE simulation software against the overcast sky condition has been carried out in order to test the accuracy of sky model of RADIANCE for modeling the overcast sky condition in Hong Kong. Two sets of data have been analysed. Firstly, data collected from a set of experiments using a physical scale model. In this experiment, the illuminance of four points inside the model was measured under real sky conditions. Secondly, the RADIANCE simulation has been used to simulate this physical model under overcast sky conditions using gensky sub-program with and without measured sky data, which is defined as Measured Sky and Default Sky, respectively. The analysis was based on the Mean Bias Errors (MBE???s), Root Mean Square Errors (RMSE???s) and Relative Errors (RE???s) between measured and simulated illuminance. This study found that CIE overcast sky model with measured sky data (Measured Sky) shows a very good accuracy in \nsimulating the overcast sky condition. This was corresponded by low values of MBE???s and RMSE???s. While the CIE overcast sky model without measured sky data (Default Sky) shows a very bad accuracy, which has average MBE of more than 200%. The study recommended of using a design sky of 10,000 lux (diffuse illuminance) for the computer simulation under overcast sky condition for Hong Kong.