A new, more accurate and considerably simpler version of the Perez[1] diffuse irradiance model is presented. This model is one of those used currently to estimate short time step (hourly or less) irradiance on tilted planes based on global and direct (or diffuse) irradiance. It has been shown to perform more accurately than other models for a large number of locations worldwide. The key assumptions defining the model remain basically unchanged. These include (1) a description of the sky dome featuring a circumsolar zone and horizon zone superimposed over an isotropic background, and (2) a parameterization of insolation conditions (based on available inputs to the model), determining the value of the radiant power originating from these two zones. Operational modifications performed on the model are presented in a step by step approach. Each change is justified on the basis of increased ease of use and/or overall accuracy. Two years of hourly data on tilted planes from two climatically distinct sites in France are used to verify performance accuracy. The isotropic, Hay and Klucher models are used as reference. Major changes include (1) the simplification of the governing equation by use of reduced brightness coefficients; (2) the allowance for negative coefficients; (3) reduction of the horizon band to an arc-of-great-circle; (4) optimization of the circumsolar region width; and (5) optimization of insolation conditions parameterization.