Vision systems for the optical inspection of super thin glass

Date: 4 December 2004
Source: Isra Vision
Super thin glass backs the trend towards increasingly flat and light-weight products. As a result, this material is of increasing importance for modern flat-panel or cellular phone displays.

Longevity, brilliance and color stability are among its valued properties. Modern coating technologies optimize the optical characteristics of the glass, such as transmission and reflection. High planity and flawlessness allow op-timal finishing of the material in the production process and are of significant importance to the overall appearance of the end product. As a result, the expectations on the quality of the material have risen notably, and with them also the requirements on the employed in-spection technology: A 100% quality inspection is thus the definition of our goal. Optical inspection systems guarantee the required high quality in the production of thin glass.

In the past, glassblowers used special pipes to prepare glass sheets. A hot, viscous clump of glass was painstakingly blown, the glass bubble cut open and then swung and drawn. When the right size was achieved, the glass was laid out flat to cool. Of course, this process made it impossible to manufacture uniformly thin sheets at a high level of quality. Not until the 17th century did it become possible, thanks to a casting process for large glass panes, to prepare flat glass in a horizontal position. The Hall of Mirrors in the Palace of Versailles is a spectacular display of the result of this development.

Fault recognized, fault averted
Today, we are in a position to manufacture super thin glass. Three manufacturing processes have established themselves to this end. They include the Down Draw Process and the Overflow Fusion Process. Initially, only these two manufacturing processes could guarantee adequate surface quality of the thin glass. Meanwhile, su-per thin glass may also be manufactured using the Float Process – the manufacturing process for flat glass developed in Great Britain in 1959.
Super thin glass sheets with thickness as low as 0.1mm are used for monitors, CRT and TFT displays and cellular phone displays, for ex-ample. Often, it is covered with "hard" or "soft" coatings to improve the optical characteristics of the glass. For LCD, a transparent layer of indium tin oxide (ITO) is used.
Even during production, a 100% inline inspection of the manufac-tured glass must be ensured. Only the early recognition of inclusions, stress points or unevenness in the glass ribbon allows for a timely reaction to the problem. The inspection systems are typically based on CCD line scan cameras aligned through the glass against an illu-mination. Examples of typical material faults detected by such sys-tems include bubbles, drops, spalls, inclusions, scratches, contami-nations and coating defects.
The inspection systems are expandable: So, even large web widths of up to 6m (PDP float lines) may be inspected. The conveyor speeds vary, depending on the application, between 1m/min to 30m/min. The resolution of the systems usually lies in the range of 0.01mm to 0.3mm. Consequently, even micro-defects can be recog-nized.

Modularly constructed Vision Systems
The ISRA GLASS VISION division of the Darmstadt ISRA VISION Group offers a wide range of systems for optical glass inspection. The 2f1 Vision System XT, which may be directly integrated in the various steps of the production process, ensures the zero-defect production of super thin flat glass. The modular construction of the system allows it to be adjusted flexibly to suit the extremely diverse glass production facilities.
The underlying metrological process allows a contact-free and com-plete inspection; even the smallest optical faults can be detected. Thus, both deformations and distortions can be made detectable in a single measurement system, simultaneously and in real time.
In addition to the possible faults in the glass carrier material, further surface defects such as scratches, inclusions and holes in coated glass cannot be avoided, for reasons of manufacturing technology. Therefore, the optical inspection system must reliably recognize and reject faults both in the glass and in the coating. It is applicable for all coatings, such as Low-E, ITO or anti-reflective. The faults are reliably classified and assigned.
The inspection system comprises - depending on the web width - one or more CCD line scan cameras and a special LED line illumina-tion unit. The illumination is maintenance free and has a life expec-tancy of up to ten years. All components, including the air-conditioning system and the control module, are housed in a stable and protective steel construction, which guarantees a high level of interference resistance and, above all, prevents the effects of vibra-tion interference. The air-conditioning system regulates the tempera-ture of the camera casing and the lighting unit.
Faults of a size of 0.01mm and above are recognized as optical de-flections in the inspected material. Thus, the optical quality of the material may also be determined.

Faults lead to optical deflection
The measurement principle works as follows: The cameras are ar-ranged above and focused on the glass that is to be inspected. The LEDs are located under the material. These lighting elements are systematically switched on and off by the control system. When all elements are simultaneously operated, the light intensity signal reaches the 100% mark of the camera pixel amplitude. However, only 50% of the lighting elements are simultaneously on inside of a time t1. Thus, the intensity signal U1 reaches an amplitude of 50%. After the light elements have been reversed, the other half of the lighting system produces the intensity signal U2 inside the time t2, which also reaches 50% of the amplitude in an uninterrupted state. The difference between the signals U1 and U2 always amounts to zero, for fault-free material.
An error in the inspected glass leads to an optical deflection. Then, the difference between U1 and U2 is no longer zero. Dirt particles on the inspected material have no effect on the signal difference, be-cause they reduce both intensity signals by the same amount. In an uninterrupted state, the digital signals reach an amplitude of 128 dig-its. The sum of both signals results in 256 digits. The reduction of the light intensity due to dirt particles reduces the sum, but does not lead to a difference. So, the fault defect recognition is very robust. As described, the lighting elements switch on and off alternately. There are two modes for this, which detect defects both in the X and the Y direction.
The cameras are connected over a simple “datalink” with the image processing board, the “heart” of the system. This is located in the computer rack, and is responsible for the full signal processing as well as relaying the signals to the image-processing computer. The signal processing is controlled by a program loaded to the FPGA chip in the image-processing module at system start. This method allows for the parallel signal processing of different signals, such as e.g. bright area, deflection and reflection.
The system’s PC operation station furnishes the user with detailed information on the material faults detected. This includes printed pro-tocols, images, a map of the error position and statistical analyses. All measurement results are stored in a database and thus remain permanently available for further evaluation.

Your partner for high quality
With the system, manufacturers of high quality glass can ensure fault-free production in a growing market, cost-intensive reclamations and avoid rejects in production. The production process will be opti-mized.
ISRA VISION is a partner who precisely understands the require-ments of the branch, and whose turnkey systems and solutions are specifically designed for glass production. The inspection systems find every defect quickly, automatically and with 100% reliability. Quality becomes transparent. The premium quality control thereby delivers total satisfaction for the glass manufacturer’s customers and lastingly secures market position.

600450 Vision systems for the optical inspection of super thin glass

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