This study aims to develop a mathematical algorithm to simulate the heliostat field behavior in solar tower power plants in order to estimate the optical loss mechanisms of these systems and consequently to assist in new projects design. The loss mechanisms calculated in the code are reflectivity, cosine effect, atmospheric attenuation, spillage, blocking and shadowing; according to the geographical location of the plant and the mirrors arrangement in the field. In result, it is performed a case study investigating different configurations in fields located at low latitudes: a circular layout a southern layout, which have achieved an annual efficiency of 67.0% and of 60.3%, respectively.