Determination of the diameter of an isolated surface defect based on Fraunhofer diffraction

A determination of the diameter of an isolated surface defect based on Fraunhofer diffraction is presented. A Fraunhofer diffraction pattern from an isolated circular defect on a smooth reflective surface illuminated at normal incidence by coherent parallel light can be observed in the transverse plane in which a lens focuses the light retroflected from a defect-free surface. On the basis of Fraunhofer diffraction, a relation was derived from which the diameter of the defect can be calculated by substituting the measured distance at which the ratio of intensity when the defect is present to the intensity from a defect-free surface is at a maximum.