A description of the stray-light problem based on a radiative transfer approach is presented. The 2D convolution that describes the effect of the stray-light is recovered in this formalism. The controversy raised by the old normalization condition is satisfactorily eliminated within the present description. The present formalism is applied to aureole observations and derived the spread function parameters. These parameters are similar to the ones used in the standard approach but now a new quantity appears. It is derived consistently within the present formalism and it allows a description of different atmospheric quality conditions without changing the actual shape of the spread function. It is shown how, for large spots, the contamination of stray-light can be considered as an added constant level of light throughout the umbra.