A research team from Mannheim develops measurement technology for assessing structural and material differences on surfaces
Whether a surface is roughly soiled is recognizable at a glance. However, this is much more difficult in the case of small and invisible soiling. What’s the best way to ascertain if a surface really is as clean as it’s supposed to be? Such soiling is caused by oil and fat layers, detergents or plastic abrasion. Measuring surfaces is a major interest in forensics as well. Currently, the Center of Mass Spectrometry and Optical Spectroscopy (CeMOS) is investigating this issue at Mannheim University of applied science. This is in collaboration with Steinbeis Transfer Center for Smart Manufacturing Solutions and SME partners in industry.
The project is focused on developing a measurement system for structure measurement. Based on this, the design of a device for the qualitative control of surface structures has been realized. The result is a laser-based scanner that enables a rapid spectral measurement of surfaces.
CeMOS, the largest institute for university of applied sciences in Baden-Württemberg, has been developing solutions for medical technology, image processing, metrology-based material development. Above all, optical device technology is a key component of the institute. The team at the Steinbeis Transfer Center supported the fundamental development of CeMOS for pre-series prototyping. This is commercially available at the Steinbeis Transfer Center “Intelligent Industrial Solutions”.
But how does the surface scanner work? “The developed measurement method is characterized by a mid-infrared laser beam deflected by a complex optical system above the sample. The sample partially absorbs laser light in a different way, depending on its absorbance properties. An infrared (IR) detector acquires the confocal reflected measurement signal.” explains Professor Dr. Matthias Rädle, director of CeMOS and a Steinbeis Entrepreneur at the Steinbeis Transfer Center. The signal is strongly depending on the sample and its absorption properties. Infrared light is particularly well suited for material discrimination: it is not visible to the human eye and does not damage the sample during the entire scanning process. By using different specific laser wavelengths, molecular differentiation is enabled.
Non-contact measurement of human fingerprints
The scanner’s properties enable to detect traces of fat. In criminology, this is of major importance and significantly supports dactyloscopy. This refers to the evaluation of human fingerprints, which are individual, unique, unchangeable and accordingly classifiable for each human being. Dactyloscopy is a scientifically and judicially recognized method of identifying persons. Fingerprints are created by secreted body fat or fat taken up from the environment. Individual lines in the fingerprint, known as papillary lines, leave a unique lipid structure on touched surfaces. “The technology we have developed has the great advantage that fingerprints on different surfaces can be measured and recorded in a contactless way without preparation. Conventional methods of recording, such as powdering using the adhesion method, on the other hand, lead to direct irreversible damage to the fingerprint,” explains Tim Kümmel, Msc. one of the inventors of the system at CeMOS.
Contactless imaging has the profit that further examinations of the unchanged samples or renewed scans are possible afterwards. In addition, the fingerprint must be digitized and compared to a database with already taken fingerprints after using the adhesion method. The developed scanner, on the other hand, reduces fingerprint acquisition times in police laboratories significantly and allows direct comparison of the digital image with the database. Furthermore, the scanner is capable of providing a three-dimensional assessment of finger fat in sample measurement. This would allow the use of new features to uniquely identify a fingerprint.
Contactless measurement of surfaces for quality control
However, the developed scanning technology is not only used in dactyloscopy, but also in surface quality control. Quality control is an important component in all areas of manufacturing, as it serves to verify if a product meets certain requirements. The control depends strongly on the product that needs to be validated. For verification, non-contact methods are preferred, which do not make any changes to the product. A change would mean that the product is no longer usable for control procedures or further processing.
The scanner developed by the project team is not only capable of detecting structural differences. It also distinguishes molecular differences. This enables, for example, the detection of thin oil films on reflective substrates. The ability to achieve very high resolutions in a short scan time was the focus of the development. A high scan resolution of a few microns makes the detection and digitalization of smallest structures on a surface possible.
The scanner has a penetration depth of up to 150 µm when measuring and also records the height profile of the specimen. The information obtained from a scan can therefore not only be converted into a two-dimensional image, but also enable spatial assessment of the scanned surface. This reaches the point where the scanner can be used to look through thin plastics such as PVC tape (polyvinyl chloride). To visualize this, the team masked a two-euro coin with PVC tape in such a way that the euro lettering was hidden, and then measured the coin. In the scan results the covert part is already faintly visible. A subsequent image processing provides a better impression. It processes the scanned image data for the eye, that the euro lettering, as well as the drawn structures on the coin are clearly visible. This examination is also possible for circuit boards. For this purpose, parts of the circuit board are masked off with PVC tape. This experiment illustrates that the developed scanning technique can capture and visualize the board structure and even conductor tracks inside the board.
Further areas of application and possibilities for optimally using the scanner are being added daily and will continue to be researched in the future. In addition, the optimization of the ready-for-sale series model is planned.