Geometric measurement of preform rods and starting tubes

A computerized, noncontacting optical technique for measuring the outside diameters of preform rods and starting tubes and the wall thicknesses of the tubes is presented. The method utilizes the fluorescence of an index-matching liquid in which the nonfluorescing silica rod or tube is immersed to define the boundaries. The liquid is excited by a thin helium-cadmium laser beam (lambda = 325 nm) at right angles to the axis of the object. The fluorescent light intensity is detected by a video camera and digitized for computer processing. The computer makes use of the sharp light-to-dark transitions to calculate the overall diameter and wall thickness. The measurements are precise to better than 1% and agree to within this amount with independent measurements made on the rods and tubes with an electronic digital caliper.