Developments in Shell Surface Characterizations Using Holography

The Digital Holographic Microscopy (DHM) technology allows fast three-dimensional (3-D) surface image acquisition in order to characterize and quantify defects on microshell outer surface. This device captures holograms and reconstructs a double image, one for the intensity and another for the phase. Depending on the used objective lens (20× or 50×) and the microshell diameter, several hundred or thousand pictures must be taken to cover both hemispheres (1170 pictures for a 2200-µm-diameter capsule). A specific handling system using two rotation axes has been developed to acquire automatically all 3-D hemispherical data without any manipulation.

A new version of the 3-D surface rebuilding (using images stitching) and analyzing software is used to characterize (diameter and height measurement) and to localize individual defects, such as bumps or pits. Using a new version of the dedicated software, the pictures are filtered and stitched together to perform a 3-D surface mapping of the capsule. Postprocessing routines are able to detect defects and to sort them out. Specific tools are also dedicated to the characterization of polishing defects, such as concave or clustered defects.

In addition to this, a study of the DHM metrology capabilities, including a comparison with a calibrated atomic force microscope, has been led.