Determination of the diameter of an isolated surface defect based on Fraunhofer diffraction.
When an isolated circular defect on an otherwise very smooth reflective surface is illuminated at normal incidence by coherent parallel light, a Fraunhofer diffraction pattern from the defect can be observed in the transverse plane in which a lens focuses the light retroreflected from a defect-free surface. The ratio of the intensity when the defect is illuminated to the intensity in the absence of the defect vs distance r from the center of the focused light spot exhibits a maximum at some distance r = r(m) in the interval 0 < r < r(1 min), where r(1) min is the radius of the central bright spot in the defect diffraction pattern. On the basis of Fraunhofer diffraction, a relation was derived from which the diameter of the defect can be calculated by substituting in the measured distance r(m) at which the ratio of intensity when the defect is present to the intensity from a defect-free surface is a maximum. Experimental data were obtained verifying the theory.
Klingsporn
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