Spot defects and linear faults, not accepted inarchitecture glass,are easily revealed using a camera

The optical quality of vehicle windscreens has been officially regu-lated for decades and machine vision systems have been developed to control the visual distortion, as well as cosmetics, in the produc-tion of automotive glass. One system, SAFEGLASS, was featured in this publication in the issue of 2005 with the title ’’Camera control of windscreen production pays back in one year’’. The same theme was also discussed by the author in the Glass Processing Days Con-ference, held in Tampere, Finland, in 2005. See the presentation in the Proceedings; ’’Every good automotive glass machine deserves a reli-able camera control gate’’ (www.gpd.fi/programme/saturday.htm).

What do the standards say?

The optical quality of glass in buildings has been recently defined. The standard EN 1096-1 gives numerical values for accepted pin holes in coated glass, while the standard EN 572-2,-8,-9 (October 2004) gives measures for transparent distortion, visible spots, and linear faults. The value of distortion is determined by looking a black & white zebra target through a tilted glass. The size of spot de-fects is simply measured using an accurate caliper and the extension of linear faults, i.e. scratches, is observed by looking at them from the distance of two meters. The spots are preferably seen against a white background and scratches are evaluated in diffuse illumination conditions against a dark background. The standards do not say much about the ability of the inspector’s eye; ’’good enough, not blind’’ is probably OK.

The spot defects are classified in categories according to their dimensions: in Category A the diameter of the spot nucleus is be-tween 0.2 mm…0.5 mm, in B 0.5 mm…1 mm, and in C between 1 mm…3 mm. Spot defects having a diameter of more than 3 mm are classified in the Category D. The accepted number of Category A defects is not limited in Jumbo-sized glasses, but only five class B spots are permitted in one pane, and no more than three of such spots per glass pane are allowed in an average for a delivery of 20 tons of glass. The presence of C and D category defects is more critical. The scratches are judged in a similar manner.

Camera lends an eye to glass operators

The quality of raw glass is generally controlled already in the pro-duction line of float glass. Inspection devices are based on laser light scattering from bubbles or on the deviation of collimated light beams caused by optical distortions. Some defects, like tin marks, often remain unnoticed and some faults, like scratches, may appear in glass after packing and shipping.

The SAFEGLASS camera system has been developed by CCD Photonics Ltd (Espoo, Finland) for inspecting the quality of automotive and architecture glass in the cutting line after washing and drying. The development has been partly funded by the Euro-pean Cluster Integrated Machine Vision (EUTIST-IMV) and by the National Technology Agency, TEKES, Finland. A typical installation site of the above mentioned camera sys-tem is shown in Picture 1. Two separate parallel camera lines are used to reveal non-transparent spot defects (TR for Transmission mode) and linear scratch faults (DF for Dark Field). The illumina-tion and detection optics are designed to simulate the sensation of a human eye, as required by the glass standards. The system can be in-stalled in existing automotive or architecture glass production lines after minor mechanical modifications. A free space of 70 mm is re-quired between the rolls of a horizontal automotive glass conveyor. The cameras are looking through the glass inside this space. A cor-responding free opening should be machined in the support plate of a vertical architecture glass conveyor. The production of architec-ture glass elements differs significantly from automotive glass mak-ing, where sidelites or windscreen elements come into inspection in a constant way: same darkness, same size, and same shape. Building glass elements are assembled from clear and coated glass panes and the inspection camera should adapt to their varying light transmis-sion and varying sizes.

The activities of glass production operators are illustrated in Picture 2. The view is shown in the direction of glass movement. Ar-chitecture glass panes are coming out from the washing machine and the operators are searching for occasional glass defects. The con-veyor on the left side is capable of handling Jumbo-sized glass panes and the conveyor on the right is for smaller glasses. The system of four cameras has the total amount of 4 x 2048 (= 8096 ) light sens-ing elements for revealing spots and scratches. The smaller convey-or can be handled using a single 8000-element camera. The opera-tors and cameras need not to be rivals in finding faults. The final de-cision of the fault seriousness is always made by the operator. Cam-eras only help to point the location where a defect might hide.