Contact method :

In the contact method (Fig.11.4.1), the test surface is placed in contact with the master surface with a tissue paper in between them. The tissue paper is then pulled out slowly. The two surfaces are now in contact with each other with a small wedge angle between them. If test surface does not depart significantly from flat surface, a fringe system is observed with slight distortion at the points where surface is not smooth. (Fig.11.4.2). The flatness is given by where s is the amount by which the fringe is shifted, x is the fringe width and is the wavelength of light used. If surface departs considerably from flatness but is of regular shape (such as concave or convex lens), a circular fringe pattern is observed. By changing the direction of viewing one can find if the surface is concave or convex. If the fringes move inward when the angle of viewing is increased, the surface is concave. If there are N circular fringes, the surface departs from flatness by and has a radius of curvature where ‘2d ' is diameter over which N fringes are observed.