USE OF THE MOIRE EFFECT TO IMPROVE DIAMETER MEASUREMENTS WITH CHARGE COUPLED IMAGERS

An optoelectronic setup has been developed for in-situ measurements of thin glass cylinder diameters. The principle of the method is based on the in-line far field diffraction pattern analysis. The photometric signal is recorded with a 2-D CCD array sensor. The running time is reduced (0.25 s) using the Newton method to solve the inverse problem of the diffraction. It is worth noting that the 2-D analysis of an 1-D diffraction pattern provides several spatial sampling lines at a given time. Thus, the signal to noise ratio (SNR) is improved with an average along the cylinder axis. Other filtering techniques and interpolation algorithms can be applied. After this signal processing, a moire pattern is observed. This pattern is strongly dependent on the object angular position and the distance between the object and the detector. Then, it provides additional information which can be used to validate the recording cycle. The new version of our optoelectronic setup has been tested on a manufacturing line. The measured diameter range is 8-40-mu-m for a distance Z (between the object and the CCD matrix) lying in the interval 20-40 mm.