In recent years, vision-based polarization systems have proven to be reliable technologies for specific applications: Executing transmission analyses based on photoelastic effects, it is possible to detect and evaluate stress states in transparent materials. Polarization measurements, for example, allow for the inspection of non-metallic transparent or reflective materials such as water, plastic, or glass, as well as the monitoring of challenging surfaces or the reading of black text on black backgrounds — a particularly demanding task in tire manufacturing. In the pharmaceutical industry, machine vision systems with polarization cameras are especially well-suited for blister inspection, ensuring that defective tablet packaging or loading is reliably detected and sorted out.
The Process Engineering research group at the Institute of Production Engineering and Photonic Technologies at TU Wien, however, is focusing on a relatively new imaging method in the field of polarization. Ferdinand Bammer, who is leading the project, explains the basics as follows: “Unlike transmission technology, we use ellipsometry, which means we examine the polarization states of light reflected from test objects and measure the intensity of this reflected light at linear polarization angles of 0°, 45°, 90°, and 135°. By analyzing the ratios of these intensities with a suitable mathematical model, we can determine the quality and thickness of layers.”
To enable complete detection of the polarization state, the researchers draw from the optical toolbox: since the rotation direction of the measured polarization ellipse cannot initially be determined using this method, they employ an additional quarter-wave plate. This allows the measurement of light intensities at 45° and 135° linear polarizations as well as left- and right-handed circular polarizations. “From these intensities, we derive the 45° ratio R45 and the circular ratio Rz,” explains Bammer. “This generally enables full characterization of the polarization state. The resulting parameters R45 and Rz — or the classic ellipsometric angles ψ and Δ, which can be directly calculated from them — depend on the properties of the sample. The thickness of any coating on the sample has the greatest influence on these parameters. Thanks to the use of ellipsometry, we can therefore draw conclusions about the quality of layer thicknesses. Our goal is to use this method that has proven to be robust in the lab and transfer it into a system powerful and reliable enough for use in industrial inline applications.”
The demand for such inline ellipsometry systems is real and continuously growing, Bammer explains: “In electronics manufacturing, it is essential in countless processes to accurately determine the quality of thin layers made of semiconductors, metals, polymers, or dielectric materials. This is also true for the production of OLED displays, fuel cell membranes, battery electrodes, photovoltaic components, as well as in the pharmaceutical, food, and beverage industries. Ellipsometric measurements, for example, can help ensure that the thickness of inner coatings in plastic and glass containers meets all necessary requirements, both to protect the contents and to prevent them from diffusing.”
The level of precision required when measuring and assessing layer thicknesses depends heavily on the specific application. For example, in the case of barrier coatings on bottles and films, sliding layers in pharmaceutical containers, or primers on metal sheets used as adhesion promoters or surface pretreatments for improved paint bonding, an accuracy of about 10 percent of the layer thickness is generally sufficient, according to Bammer. In such cases, the typical layer thickness ranges from 10 to 200 nanometers. However, more complex applications — such as those found in organic photovoltaics (OPV) or display technologies — may demand significantly higher levels of accuracy.
“To meet these requirements, we searched the market for suitable polarization cameras — and found them thanks to a recommendation from a research partner,” says Bammer. “According to that recommendation, SVS-Vistek provides the most robust and, above all, the lowest-noise cameras, which is why we chose this manufacturer. We've had positive experiences with both product quality and service. Currently, we’re using the GigE Vision polarization camera model exo250ZGE, which offers a 5-megapixel resolution and data rates of up to 24.5 frames per second, meeting our needs in simple applications.” For higher measurement speeds, Bammer installs the USB3 polarization camera exo250ZU3, which supports frame rates of up to 75 fps. For applications requiring higher resolution, the USB3 polarization camera exo253ZU3 — with a resolution of 12.3 megapixels — is used.
In addition to the technical features of SVS-Vistek’s polarization cameras, other factors also convinced Bammer: “Even in our initial discussions with the experts from our partner, their strong competence in camera technology and clear understanding of our requirements quickly became evident. The collaboration with SVS-Vistek was fast, friendly, and efficient — which greatly supported us in realizing our ellipsometry systems.”
Bammer identifies the greatest challenge of the ellipsometry project as understanding the relationship between polarization and the measured quantity. “Grasping this relationship and translating it into a simple and robust measurement method is already a complex task in itself. Equally challenging, however, is communicating the technical background to decision-makers in the industry and achieving their acceptance. Only then, the method’s proven performance in the lab can be successfully transferred into practical applications. SVS-Vistek’s polarization cameras provide an excellent foundation for this — one we will continue to build on as we further develop our systems.”
Several systems based on this technology are currently being transitioned into practical use. Initial adopters are now looking to implement the Vienna-developed ellipsometry in various laboratory systems, as well as in industrial applications, including the measurement of layer thickness and quality in PET bottles, among others. While the extremely high precision of laboratory ellipsometry generally cannot be replicated on the production line, the measurement quality already achieved as of today is already sufficient for certain applications.
Thanks to foreseeable advances in the technology, its potential will continue to grow in the future, Bammer is convinced: “In the production of components essential to many emerging technologies including displays, lighting systems, photovoltaic components, batteries and accumulators, fuel cells, or catalysts, there is a steadily increasing demand for reliable, full-surface quality control of coatings. That’s why I am certain the importance of polarization technology will increase significantly in the coming years. At present, there is still too little understanding and knowledge of polarization measurement and ellipsometry within the industry. However, powerful polarization cameras such as those offered by SVS-Vistek could open many doors in this field.”
"The importance of polarization technology will increase in the coming years."Ferdinand Bammer
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