Dark-field Scheimpflug imaging for surface inspection

Automated Visual Surface Inspection is an important task in industrial quality control and there are a plethora of optical systems with imaging or scanning configurations at work in industry. Surface Inspection on ceramic tiles however, is a task that is still performed manually by most manufacturing companies. The main reason that the Ceramic Tile industry has not yet fully automated its inspection procedures is that the particular surfaces are produced in an extremely wide range of surface textures, colours, dimensions and patterns. Visual surface inspection systems can normally handle a limited range of surface textures. The system described here detects defects such as dents and scratches, using Dark Field Illumination'. The defects are detected by imaging deflected and scattered light respectively. Dark Field Illumination is a technique that in principle can only be used with specular surfaces. In order to cope with the tile surfaces that are not always specular (surface roughnesses ranging from O.O8tm to 1.5am), grazing incidence illumination is used to increase the ratio of specular versus diffuse reflection2. The surfaces are imaged on a Line Scan Camera, which is placed at a large angle relative to the surface normal. This can be up to 23° away from the specular direction; for separation angles wider than that the signal from scratches is too weak to be useful for an industrial application. In the resultant images, the surfaces show dark, while defects show bright. Illumination and imaging take place in a plane that is perpendicular to the direction of motion of the conveyor belt. The imaging is oblique, and the image plane forms an angle with the lens plane. This angle is specified by the Scheimpflug imaging condition. Being able to tilt the camera relative to the lens, enables the use of the small depth of field from a wide aperture imaging lens (low F number). This increases the total light throughput of the system. The image is formed on a Line Scan Camera, thus limiting the perspective distortion due to oblique imaging to one direction. The surface flaws that can be detected with the presented configuration, are topographic (bumps, dents, inclusions) as well as digs and scratches. The two different types of defects are detected by imaging deflected and scattered light respectively, on two separate camera heads.