Abstract: Total reflection method has been commonly used to detect real contact area, however the boundary of the contact spots in the contact image obtained was vague.The principle of total reflection technique was analysed thoroughly in this paper. It is found that under the condition of total reflection, there is still the energy loss of the light wave due to the absorption by the asperity surface. The functional relation between light intensity of reflection, and contact surfaces distance was derived, and accordingly, the photomicrographs of the contact spots were analysed using an image processing system to identify the contact area. The contact pair was coupled with the white synthetic sapphire and metal specimen. The Hertz' contact test was also performed so as to verify the accuracy and the reliability of this technique, in the meanwhile, the limitations of Hertz' contact theory was analysed. Finally, the contact curves of typical surfaces in engineering practice were given. The results shown that the contact of rough surfaces is discrete and the relationship between the real contact area and the load is nonlinear.