"Through a glass darkly?"The optical inspection of float glassInnovative technology for the optical inspection of flat glass quality has been developed through an exciting collaboration between business and academia. Initial basic research in a university laboratory has been successfully exploited by a high-technology company which manufactures instruments for optical metrology, and the flat glass industry. The technology, which is currently undergoing further development, should significantly improve the quality glass output of factory lines.
Flat or sheet glass is currently manufactured by the glass industry using the floating method. This is a simple but ingenious technique for producing high-quality flat glass relatively quickly. Based on an idea patented by Pilkington, during the manufacturing process the glass material is drawn off from the furnace in a vertical direction. It is then channelled to a molten tin bath, where the glass forms a flat sheet. Flat glass is often processed further: it may be tempered, toughened and laminated to improve its strength and safety qualities. In Finland, there is only one glass factory, Lahti Glass, belonging to the Pilkington Group. Flat glass has a range of applications which includes window panes for the building industry, windscreens for the automotive industry, and laptop displays for the electronic industry. Depending on the application, the actual thickness of the glass used varies from less than 1mm to the more usual thickness of a few millimetres. In many of the applications - such as vehicle windscreens and computer displays - high quality is of the utmost importance since optical defects affect the appearance of an object seen through the glass. For example, the object may appear distorted, or it may appear to be moving when the observer is moving, even though the object itself is actually stationary. In windscreens, optical defects may generate blind spots or cause drivers to develop painful headaches. Currently glass manufacturers
measure various quality parameters, such as glass thickness and defects,
using on-line instruments, and different metrological methods have been
developed. Optical measurement methods are already widely used because
of their real-time operation and reliability. Nevertheless, there will
always be a need to find increasingly accurate and reliable methods of
measurement to assess the quality of the glass and to optimise the glass
manufacturing process.
The Optix project The Department of Physics at the University of Joensuu (Finland) is running a joint project with Lahti Glass, PanOptix Ltd and Mannet Ltd (the project co-ordinator), aimed at developing accurate optical measurement instruments for the flat glass industry and float glass manufacturers. Glass properties such as wedge shape, edge distortion and thickness are the main areas which have been studied, and the results of the research have been exploited in the design and construction of new instruments. The project, which is termed
Optix, is funded by the EU and is part of the Technology Transfer Nodes
(TTN) programme of High-Performance Computing and Networking (HPCN). The
TTN co-ordinator in Finland is Mr Antti Soini of Finnish Automation Support
Ltd.
The optical inspection of glass quality In the production process the glass sheet is moved forwards on a roller conveyor, the movement being facilitated by the use of rotators at both edges of the solidifying glass. However, the rotators leave traces on strips of a certain width on both edges of the glass sheet, which appear as a marked edge distortion in the direction of the machine. Gravity also affects the glass quality due to possible differences in the local specific gravity of the glass. In the glass industry it is important to find the correct cutting lines to optimise the width of the product. The traditional method of edge distortion detection is based on the zebra test, but unfortunately this relies on human inspection and is therefore fallible. Such an inspection can be replaced by modern machine vision technology. In the Optix project, we used optical interferometry for the quantitative assessment of wedge shape and distortion of glass. Interferometry is a well-known technique in optics that has been employed for the accurate inspection of, for example, the quality of large astronomic mirrors. Such an inspection is usually performed in the laboratory and under steady-state conditions. Thanks to the development of interferometric set-ups and CCD cameras, today it is possible to scan the object and use image data for object data analysis.  Figure 1 shows a schematic diagram of the device described above, which can provide various statistical parameters that describe the quality of the flat glass. One of these is related to the optical power of the glass expressed in millidioptres (mdpt). Opticians use the dioptre as a unit of measurement of the power of a lens. If the power of a lens is one dioptre, two adjacent light rays penetrating the lens will intersect each other 1m from the lens on its optical axis. The intersection point is the focal point of the lens. The dioptre number expresses the inverse value of the focal length of the optical system.  A device similar to the one in Figure 2 is needed in glass factories for off-line quality assessment. The principle behind the instrument is a specific optical phenomenon: when laser light-wave fronts from the upper and bottom surfaces of the flat glass are reflected and interfere, an interference pattern appears. Interference ´fringes´ consist of alternate light and dark bands, seen when similar beams of light interfere, and these patterns carry information about local slope on the inspected area, comparable to 1cm x 1cm. The image information of the fringes is recorded by the CCD camera and analysed using a PC. In this way, it is possible to detect thickness variations as small as 0.2µm.  The current stage in the Optix project is the development of an intelligent on-line optical device, equipped with machine vision and high-speed computation, which will be capable of measuring the thickness of glass accurately and reliably. Initial prototype experiments in industrial environments have already proved successful. As the experience at Pilkington Glass Lahti Ltd demonstrates, there is no doubt that these devices could help glass producers to save on material costs. At Lahti the production unit has used the off-line device since 1995, and it has proved useful in the quality inspection in glass production as the Plant Manager, Reijo Sihto, testifies: "Optical off-line measurement has improved our quality control and process development. The new on-line device will bring these tools to bear in real-time process management. We expect to get 2-4% more high-quality glass from the same line."
Contacts: Professor Kai-Erik Peiponen
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