This contribution presents a simple, fast, and low-cost method to track in situ and in real time the evolution of the surface topography. The method combines an optical method (collimated light) with image analysis. The method was validated using a complex tribo-system; a reciprocal sliding of a rough cast iron counter-piece under mixed lubrication. We demonstrated that the optical method is well suited to observe the evolution of the contact areas during sliding. We also proved that the contact area occurs on the highest peaks of the surface roughness. Finally, we estimated the wear rate by combining the information of the contact area and the Abbott-Firestone curve obtained by profilometry. The wear rate was found to be 40 times higher in the early stage of sliding as compared to the steady-state. The running-in of this particular system was found to be approximately 10 h. In addition to be simple, fast and low-cost, the proposed method has other advantages. It is adaptable for real industrial tests conditions. The fast data analysis allows the wear rate of a tribo-system being determined in real-time. This method characterise the real contact areas of the observed piece, which is difficult, if not impossible, to obtain by profilometric observation alone.