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Detection of a random edge and its application to interior surface measurement

A random edge is the center of an in-focus-strip which is formed by viewing a vertical surface through a conventional microscope. The in-focus-strip is the image of the surface which locates inside the depth of the field of the microscope. The image of the surface which locates outside of the depth of the field is blurred and contains only low frequency illumination profile. The location of this random edge has been used to make internal measurements within holes as small as 200 microns in diameter. A specialized edge detector has been designed to 'find' the center within the in-focused region. The detector first locates the in- focus-strip. It then extracts the surface background detail from the out-of-focus part of the image. This out-of-focus information is used to effectively normalize the intensity distribution over the image. The image is then smoothed to remove random noise. The location of the center of the in- focus-strip is then found from the application of a weighting function constructed from the position of individual details located within the edge region. This method has been used in measuring inside a 0.25 mm diameter hole. The hole is used to carry the coolant through a turbine blade. The accuracy of the measurement is better then +/- micrometers .