These results provided beneficial information for improvement of the theories of the contact and sliding mechanisms for elastic materials. For example, Schallamach (1971) first discovered a surface instability, such as the propagation of the detachment waves across the contact zone during sliding motion. In recent years, as presented in the references by Chareauminois and Fretigny (2008) and Prevost et al. (2013) , the digital image correlation (DIC) method has been focused on visualizing the

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... rface displacement and stress fields within the contact interface. These research studies will be helpful for the understanding Abstract This study demonstrates optical measurements of a real contact area and tangential contact stiffness in a rough contact interface between a soft adhesive elastomer and a glass plate. A friction tester developed in this study employs a rough contact interface between a rough rubber plate that is made of cross-linked poly-dimethyl siloxane (PDMS) and a smooth optical glass (BK7) hemisphere. This friction tester also equips a transmission optical system that is composed of a white light-emitting diodes (LED) light source, an objective lens system, and a charge coupled device (CCD) camera, for the in situ observation of the space distribution of the real contact area in the apparent contact region. Based on a simplified analysis, in accordance to the measurements of the transmitted light intensity, the amount of the real contact area was determined without any complicated calibrations. As a result, it was found that the amount of the real contact area linearly increased with normal load. Furthermore, using the digital image correlation (DIC) method, the time changes in the surface displacement field within the apparent contact region were visualized from the onset of the sliding motion to the onset of the steady sliding. These results provided a direct estimation of the tangential contact stiffness that resulted from the tangential deformation at the contacting asperity layer.