The pre-sliding regime is typically neglected in the dynamic modelling of mechanical systems.However,the change in contact state caused by static friction may decrease positional accuracy and control precision.To investigate the relationship between contact status and contact force in pre-sliding friction,an optical experimental method is presented in this paper.With this method,the real contact state at the interface of a transparent material can be observed based on the total reflection principle of light by using an image processing technique.A novel setup,which includes a pair of rectangular trapezoidal blocks,is proposed to solve the challenging issue of accurately applying different tangential and normal forces to the contact interface.The improved Otsu’s method is used for measurement.Through an experimental study performed on polymethyl methacrylate (PMMA),the quantity of contact asperities is proven to be the dominant factor that affects the real contact area.The relationship between the real contact area and the contact force in the pre-sliding regime is studied,and the distribution of static friction at the contact interface is qualitatively discussed.New phenomena in which the real contact area expands along with increasing static friction are identified.The aforementioned relationship is approximately linear at the contact interface under a constant normal pressure,and the distribution of friction stress decreases from the leading edge to the trailing edge.