Abstract: A method and arrangements for determining an at least approximately circular contacting surface is disclosed. The primary object of finding a simple way of objectively determining the contacting surface of a deformable medium on a plane surface with high accuracy and at a low technical expense is met according to the invention in that a film waveguide is used as a plane surface. An incoherent, parallel, homogeneous bundle of light whose width widens one-dimensionally is coupled into this film waveguide and the decoupled light is fed to a photoreceiver array for recording one-dimensional intensity distributions. An evaluating unit generates a difference curve from a normal curve without contacting medium and from a measurement curve with contacting medium and the exact diameter of the contacting surface is determined from the distance of two associated extremes of the difference curve.