Computing the flux densities provided by solar concentrators or focusing heliostats can be done in two different ways: A grid ray-tracing (GRT) procedure that uses of a large number of ray bundles, starting from the solar disk and finally impinging the focal plane of the concentrator. The method is reliable and accurate, but requires extensive computing times. Alternatively, the flux densities can be estimated by using convolution algorithms. This latter method requires much less computing time, but is known to be less accurate when the incidence angle of the sunrays on the reflector increases. The objective of this contribution is to define an algorithm based on convolution products and fast Fourier transforms having high accuracy. The results show that RMS error differences between both models are approximately.