Comparison of Fringe Pattern Obtained by Scanning Polished Metal Cylinders and Sharp-edged Objects with Laser Beam

INTRODUCTION In laser-scanning measurement of cylindrical objects extremely complex interfering signals occur. They are due to superimposition of reflected, unobstructed and scattered light. The proportions between these components vary in time and also the total intensity distribution changes. The considerations applying Fraunhofer theory are static and fragmentary, and may be concluded that the existing solutions for diffraction of 3D bodies do not fit to engineering applications. A rigorous diffraction theory is based on Maxwell’s equation and the boundary conditions associated with the obstacle [1]. The boundary conditions are used to calculate a field scattered by the obstacle. The origins of this scattered field are currents introduced in the obstacle by the incident field. The scattered field is allowed to interfere with the unobstructed field to produce resultant diffracted field. Within the Kirchhoff theory of diffraction there are two possible ways of interpretation the diffraction phenomena. The first one, based on so called “edge wave”, which refers to T. Young idea was later extended by A. Rubinowicz [1] and the second – Fresnel diff raction model, is based on the “zones concept”. T. Young interpreted the diff raction phenomena as a result of interference of the geometrical wave propagating in free space with “the edge wave” [2]. A. Rubinowicz theoretically proved the possibility of division the Kirchhoff diffraction field into two components. One of them has the character of incident wave and the second represents the wave created by interaction between primary field and the edge of object, and this interaction is of reflection type. These mathematical considerations are commonly accepted in the scientific world, but were never proved experimentally. In the paper the geometrical wave is the undisturbed part of incident wave. The diffracted (or edge) wave is the wave generated by the edge of an object. The obtained results give evidence to the presence of diff raction wave within the undisturbed (free from the other interferences) region. Having the above in view, the close analysis of detector signal was carried out. The obtained differential intensity characteristics allow determining the important metrological qualities of diffraction field.