Fast Quality Inspection of Micro Cold Formed Parts using Telecentric Digital Holographic Microscopy

¹ BIAS – Bremen Institute of Applied Beam Technology, Klagenfurter Straße 5, 28359 Bremen, Germany
² Aswan University, Faculty of Science, Department of physics, 81528 Aswan, Egypt
³ BIBA - Bremer Institut für Produktion und Logistik GmbH at the University of Bremen, Hochschulring 20, 28359 Bremen, Germany
⁴ University of Bremen, Bremen Institute for Metrology, Automation and Quality Science (BIMAQ), Linzer Str. 13, 28359, Bremen, Germany
⁵ University of Bremen, Faculty 01: Physics and Electrical Engineering and MAPEX Center for Materials and Processes, 28359 Bremen, Germany

^* Corresponding author : agour@bias.de

Abstract

Quality inspection is an integral part of the production process and often part of the quality management agreements between manufacturer and customer. Especially when it comes to safety-relevant parts, i.e. in the automobile or medical industry, often a 100% quality inspection is mandatory. Here, we present a solution comprised of a digital holographic measurement system, as well as fast algorithms for geometric evaluation and surface defect detection that paves the way for the inspection of metallic micro cups in less than a second. By use of a telecentric lens instead of standard microscope objective, we compensate scaling effects and wave field curvature, which distort reconstruction in digital holographic microscopy. Due to limited depth of focus of the microscope objective, depth information from different object layers are then stitched together to yield 3D data of its geometry. The resulting point cloud data is automatically decomposed into simple geometric shapes in order to analyse geometric deviations. Amplitude as well as phase distribution images are then analysed for surface defects. Our approach is demonstrated by inspecting cold formed micro cups. Defects larger than 2 μm lateral resolution and 5 μm depth can be detected.