This work investigates experimentally and mathematically the effect of sand dust layer on beam light transmittance at a photovoltaic module glazing surface. A transmittance coefficient for beam light has been found with respect to the number of sand dust particles per unit area of glazing surface, size of the particles, beam light incidence angle and wavelength. It has been shown that the incidence wavelength has a negligible effect on the light transmittance in the visible region due to the fact that the size of the sand dust particles was much greater than the incidence wavelength. A new correlation has been found to help photovoltaic system designers to predict the amount of beam solar radiation that would reach the surface of the modules as a function of sand dust particles size and amount accumulated on the surface when they are used in dusty environments. Furthermore, the results from this work could improve the predictive capability of existing solar energy simulation models by incorporating the factors which account for sand dust accumulation on photovoltaic surfaces.