A new approach for rapid inspection of fast moving, difficult to analyse surfaces, such as glazed or textured ceramic tiles, is being developed by the University of the West of England, in Bristol, UK. The technique is said to allow manufacturers to produce images of complex 2D and 3D features or flaws, and reduce waste caused by defective tiles.
It builds on existing photometric stereo (PS) methods that identify concomitant printing and moulding defects on diffuse tile surfaces. This is achieved by capturing images under different lighting configurations and isolating surface topography detail at high-resolution to distinguish between anomalies in colour, texture, scratches, dents or geometric deformation.
Professor Melvyn Smith, Director of the Machine Vision Lab at the University, states that PS has yet to achieve any form of significant industrial application because differing separate directional illumination configurations must be present in each view.
‘Photometric stereo uses two or more image captures under differing controlled lighting conditions to recover surface reflectance and topography,’ he explains. ‘This is achievable in a static scenario, either by moving the light sources or by switching [them] between image acquisitions, but will not work in industrial applications’.
The team has overcome this by using Dynamic PS techniques, where spectral multiplexing produces separate lighting configurations by temporal spacing. A time controller captures a moving surface at differing times in close synchronisation. Narrow infrared PS enhances this by isolating light channels within an infrared region to separate colour from gradient data.
Existing methods to inspect fast moving surfaces are based on 2D imaging using line scan camera technology or forms of structured lighting. Smith explains that these techniques are unable to reliably differentiate between cracks and scratches and colouring defects, or painted and printed finishes.
‘Imagine you have a conventional 2D image of a tile showing a dark patch. A printed spot or a depression in the surface will appear as a very similar dark patch,’ he says.
‘The rich data provided by the PS technology offers potential for more sophisticated forms of automated and adaptive process control to reduce defects.’
Nigel Leak, industrial expert at materials analysis and quality testing consultancy Ceram, based in Staffordshire, UK, believes it is too early to assess the effectiveness of the technique. ‘The narrow infrared method could help bring PS inspection to the table as a potentially more precise process’ he says. ‘However, in order to make the technology available at an industrial scale an array of intricate tile ranges, colour variations and surface details must be tested. A pilot scale production line would also help identify how scaleable this method could be.’
The research team is developing its system with Fima Surface Inspection Ltd, a supplier of ceramic inspection systems, based in Bristol, UK.