A detailed evaluation of several fluence rate distribution models was performed. These models included line source integration (LSI), multiple points source summation (MPSS), multiple segment source summation (MSSS), UVCalc3D, RAD-LSI, view factor and discrete ordinate (DO). As part of the evaluation, a complete MSSS model, which accounts for the quartz sleeve thickness when calculating the refraction angles, was developed. In addition, a simple attenuation factor was introduced to integrate the physics of reflection, refraction and absorption effects into the LSI model. As an alternative simple correction to the LSI, the RAD-LSI incorporates the RADIAL intensity model into the original LSI formulation. All models were compared with experimental measurements using spherical actinometers, which measure the fluence rate at specific points in space. Experimental measurements were performed in air and water. Experiments in water were performed at two different ultraviolet transmittance (UVTs) (77 and 88%). The results showed that models that neglected the effects of refraction deviated significantly from the experimental data. In addition, the MSSS approach or models that incorporated the MSSS concept were found to best match the experimentally measured fluence rate distribution. Moreover, little difference was found between the results of MSSS with quartz sleeve thickness and UVCalc3D, which does not model the quartz sleeve thickness in the refraction angle calculation but uses a factor to account for the effects of the quartz sleeve on the fluence rate. The attenuation factor combined with the LSI model was found to match the MSSS model predictions, while reducing the computational cost.

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