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Optical profilometry of cylindrical openings for transparent objects

Owing to the capability of noncontact measurement, optical profilometry is required for measurement of cylindrical openings for thin transparent objects, which are easily deformed and damaged by the contact of mechanical probes. In our previous work, we have developed optical profilometry for opaque and translucent objects. During measurement, disk shaped beam illuminates across the objects, and the intersection of beam with the objects is collected by an image sensor. When the surface of objects is rough enough to scatter light, the cross-sectional profile images are detectable by the sensor. The transparent objects have smooth surface so that reflection from the surface is much stronger than scattering. Consequently, the capture of intersection image becomes very difficult. In this work, we introduce the method to measure dimension of cylindrical openings for transparent objects. To generate the cross-sectional profile, the disk shaped beam illuminates the objects obliquely. The dynamic range of the oblique/ tilted angle was investigated for transparent objects measurement. Several transparent objects with cylindrical openings were employed for practical measurement. Measurement results showed the wall thickness strongly affects the measurement accuracy.