GLASS INSPECTION
Heart of glass
Tim Hayes provides a window into how to find defects in glass
G
lass is everywhere, from glazed buildings to smartphone displays. More than 65 million tonnes of
float glass – the form most widely used in construction and consumer products – was manufactured in 2021, to support a market currently estimated at $265bn and expected to hit $352bn by 2028. Technically speaking, only good quality
glass is everywhere. Any sub-standard material is weeded out before it reaches the consumer, with the glass industry going to great lengths to inspect its product for encapsulated defects, surface flaws and manufacturing errors. Tis process originally involved
mechanical measuring gauges and the gaze of skilled human experts, but now exploits a range of sophisticated optical processes and high-performance machine vision systems. ‘Defects can take a multitude of
different forms, including bubbles, chips, scratches and defects in thin film coatings or in ceramic print,’ said Angus Duncan, global R&D portfolio manager for process control and digitalisation at Pilkington, the long-established UK glass manufacturing company that forms part of Japan’s NSG Group. ‘We typically need to detect discrete
Glass inspection normally involves different illumination methods
defects in glass at sizes below 0.5mm. But in some cases, the defects can be considerably smaller. Tin defects down to a few microns on coated glass can cause functional issues. And the resolution of the measurement systems involved usually needs to be a factor of 10 better than these tolerances.’
Making light work Spotting these tiny flaws means bringing a range of optical techniques to bear. Today’s basic armoury of inspection tools includes focused laser scanners, moiré line scanners, bright and dark field platforms, and line scan cameras. And for specific cases, a manufacturer such as Pilkington can also deploy confocal chromatics, polarisation, fringe analysis and interferometry. Spectrometry is also important for
measuring spectral properties of thin film coatings, according to Duncan, while deflectometry is becoming widely used to assess reflective distortion, curvature and flatness, all of which influence the aesthetic effects of glass. Since any one of these imaging
techniques is rarely sufficient to visualise 12 IMAGING AND MACHINE VISION EUROPE APRIL/MAY 2022
different defect types with the high contrast necessary, optical inspection usually involves a number of different modalities working in harness. In principle, even a basic optical
inspection approach for glass is likely to need three illumination approaches. A dark field arrangement will highlight scattering defects, like scratches or bubbles, with a low angle illumination close to the inspected object. Tis means that only light scattered from a defect is detected by a watching camera positioned appropriately. In addition, direct reflected light may
be essential for registering the presence of defects in surface layers. And light transmitted through the material from back lighting sources on the opposite side of the glass will reveal material inclusions and contaminants. Te basic rules of physics still apply,
however, with image resolution limited by diffraction even as new and improved lenses come along. And adding more pixels to a camera does not necessarily equate to the detection of more defects.
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