Machine Vision and Applications (2010) 21:653–666 DOI 10.1007/s00138-010-0260-5 SPECIAL ISSUE PAPER Multi-modal defect detection of residual oxide scale on a cold stainless steel strip D. Martin · D. M. Guinea · M. C. García-Alegre · E. Villanueva · D. Guinea Received: 31 August 2008 / Revised: 8 February 2010 / Accepted: 2 March 2010 / Published online: 13 April 2010 © Springer-Verlag 2010 Abstract This work presents two initial approaches and a novel technique for the industrial inspection of residual oxide scale on a cold stainless steel strip. The research aims to develop real-time systems to detect 50-μm defects. Initially, a spectrophotometric analysis provides the wavelength regions where differences between stainless steel and residual oxide scale reflectance are highlighted. The multi-modal approach is based on laser techniques that comprise three different strategies to gradually achieve a robust stainless steel indus- trial inspection through the evaluation of their performance. First, an inspection system based on a single commercial laser has been designed with a dynamic threshold module. In the second approach, the inspection task is accomplished by volatilizing a reduced area of the stainless steel surface with short pulses of a high-power ultraviolet laser and then ana- lyzing the generated plasma with an intensifier camera. The third technique consists of an innovative smart vision system for surface visual inspection based on laser diode diffuse illumination. This vision system can be configured to work with two laser illumination modes: the diffuse coaxial light- ing and the diffuse bright-field lighting. These techniques aim to gradually improve surface defect detection of a cold D. Martin (B ) · D. M. Guinea · M. C. García-Alegre · E. Villanueva · D. Guinea Industrial Automation Institute (IAI), Spanish Council for Scientific Research (CSIC), 28500 Arganda del Rey, Madrid, Spain e-mail: dmartin@iai.csic.es D. M. Guinea e-mail: dmguinea@iai.csic.es M. C. García-Alegre e-mail: maria@iai.csic.es E. Villanueva e-mail: eugenio@iai.csic.es D. Guinea e-mail: domingo@iai.csic.es stainless steel strip. Furthermore, some of the results of the defect detection level obtained with each approach are dis- played and discussed. Keywords Surface inspection · Residual oxide scale · Cold stainless steel strip · Machine vision · Diffuse coaxial lighting · Diffuse bright-field lighting · Vision industrial applications 1 Introduction In the current global competitive industry, stainless steel companies are interested in quality control processes [6, 20]. The effective surface inspection of stainless steel plays an important role in the quality of the end product [36]. This inspection process in current visual inspection systems is mainly based upon on-line defect detection, identification, and visualization to provide optimum performance in defect detection and improve the quality control level [14]. Cold-rolled stainless steel strips are produced by further rolling of a strip obtained through hot strip mills. Prior to cold rolling, the stainless steel is annealed to obtain maxi- mum softness and ductility and to relieve stresses and insure maximum corrosion resistance. The annealing process takes place in a furnace where the temperatures and the heating and cooling rates are carefully controlled [1, 11]. When stainless steel has been heated to high tempera- tures, such as during annealing, an oxide scale is formed on the surface unless the material is completely surrounded by a protective atmosphere (excluding oxygen from the furnace, which is replaced with an atmosphere containing nitrogen or hydrogen as the dominant gas). Any such oxides should be removed to restore the stainless steel to its optimum corrosion resistant condition [27, 28, 38]. 123