Laser Diffuse Lighting in a Visual Inspection System for Defect Detection in Wood Laminates

Abstract

Nowadays, wood companies are ever more interested in automatic vision systems (Li & Wu, 2009), (Astrand & Astrom, 1994), for an effective surface inspection that greatly increases the quality of the end product (Smith, 2001), (Armingol et al., 2006). The inspection process, in most visual inspection systems, pursues online defects identification, to reach optimum performance (Malamas et al., 2003), (Spinola et al., 2008). The usual wood inspection systems are visual ones, based on standard cameras and lighting (Batchelor & Whelan, 1997), (Cognex, 2011), (Parsytec, 2011), (Pham & Alcock, 2003) to operate in highly structured environments (Silven et al., 2003). The quality control in visual surface inspection systems must be robust to cope with wood variable reflectance and high speed requirements. The surface inspection methods proposed in the literature for visual inspection aim at adhoc surface inspection systems to solve each specific problem (Pham & Alcock, 1999). Usual inspection systems are based on visible lighting and few of them use diffuse components to illuminate the rough and bright surfaces. A visual wood defect detection system proposed by (Estevez et al., 2003) is composed by a colour video camera, where the standard lighting components are a mixture of two frontal halogen and ceiling fluorescent lamps. The commercial light diffusers use a light source and different components to illuminate the surface in an irregular way to eliminate shadows, but present some problems such as, short useful life, extreme sensitivity and high cost. On the other hand, one of the major drawbacks in automated inspection systems for wood defect classification is the erroneous segmentation of defects on light wood regions, (Ruz et al., 2009). Moreover, the speed of wooden boards at the manufacturing industry is at about 1 m/s, which implies high computational costs (Hall & Astrom, 1995). Current work presents a surface inspection system that uses laser diffuse lighting to cope with different type of defects and wood laminated surfaces to improve defect detection without any previous defect information. The work will not only highlight the specific requirements for a laser diffuse lighting in a visual inspection system but also those of unsupervised defect detection techniques to cope with the variability of wood laminated surfaces and defect types, leading to a heterogeneous and robust visual surface inspection system. The manuscript is organized as follows: section 2 displays images of different wood laminated surfaces, captured by a visual surface inspection system with standard lighting. In Section 3, an innovative surface inspection