On-line non-contact surface inspection with high precision is still an open problem. Laser triangulation techniques are the most common solution for this kind of systems, but there exist fundamental limitations to their applicability when high precisions, long standoffs or large apertures are needed, and when there are difficult operating conditions. Other methods are, in general, not applicable in hostile environments or inadequate for on-line measurement. In this paper we review the latest research in Conoscopic Holography, an interferometric technique that has been applied successfully in this kind of applications, ranging from submicrometric roughness measurements, to long standoff sensors for surface defect detection in steel at high temperatures.
Early surface defects inspection in hot steel products is a difficult task, but can help to reduce significantly production costs. This is the case of steel slabs when they are produced in the continuous casting line. Conoscopic holography phase-based long stand-off profilometers have shown to be a great tool for this kind of inspection, and a breakthrough system based on them is being used for more than 2 years in production conditions with high reliability and economical impact. This paper presents the results of this system and the challenges it has overcome: hot material up to 900ºC, dust, scale over the inspected surface, …
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