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FRINATEK-Fri prosj.st. mat.,naturv.,tek

Developments in Optical Measurement technology

Alternative title: Utvikling av optiske måleteknologier

Awarded: NOK 6.9 mill.

Hot air rising above a surface such as tarmac may often be seen to cause mirages which appear like a lake or body of water at a distance. The apparent water is in fact a reflection of the sky caused by the refractive index of the hot air. This effect was first noted by scientist Robert Hooke in 1665. In 1864 German scientist August Toepler was studying the quality of optical lenses and invented what we today know as Schlieren imaging (schliere is German for "streak"). Changes in refractive index (caused by density gradients) in a material will give rise to variations in how light is refracted through the material. Toepler introduced a way to limit light which was shifted in one direction and keep the light which was shifted in the other direction. In this way he was able to not only locate where a flaw in his lens was located, but also learn something about the shape of the flaw. The Schlieren technique as invented by Toepler was relatively expensive and required substantial expertise. In the late 1990's two groups of researchers, at University of Cambridge and at the German Aerospace Center in Göttingen, independently discovered a much simpler and cheaper way to measure the same information. They discovered essentially what Hooke had discovered 300 years earlier - variations in the refractive index of an object will give rise to an apparent motion of a background pattern located behind the experiment. By using a digital camera to image a random pattern located behind the measurement object, they could measure the apparent displacement using pattern recognition software and thereby calculate the change in refractive index gradient. The DOMT project is working to improve this digital Schlieren technique and applying it to different experiments of practical, everyday use. The project working hypothesis is that it should be possible to exploit that light refraction through a medium is also a function of wavelength. It is therefore possible to develop a new technique based on two (or more) different wavelenghts of light. The project has termed the technique Bi-Chromatic Synthetic Schlieren (BiCSS) and are in the process of publishing results.

Synthetic Schlieren (SS) is a measurement technique that measures changes in a density gradient field using the refraction of light. The technique is a digital version of the original Schlieren technique developed by Toepler and Focault independently in t he late 1800's. This technique was based on an elaborate optical setup featuring a knife edge placed at the focal point of the experiment. The knife edge would serve to mask out light rays that were refracted in one direction, whereas light rays refracted in the other direction would remain. Hence, the resulting image would contain bright and dark patches depending on the refractive index variations. The digital version of Schlieren was first published by a group at Cambridge University (Dalziel et al, 1 998) and subsequently by a group in Göttingen (Meier, 1999), independently. Their version of the Schlieren technique is based on imaging a known background pattern located behind an optically transparent experiment section. Refractive index changes in the experiment will give rise to apparent displacements of the background pattern, and hence, by applying pattern matching principles one may calculate the depth integrated density gradient field. Up until now Synthetic Schlieren has relied on acquiring two subsequent images of the background pattern with a known time spacing and there has been no way of acquiring instantaneous measurements. This may, for example, be highly important when the density gradients in the experiment changes with both motion of t he fluid and temperature variations. Furthermore, for very long time-series, the original Synthetic Schlieren concept will see a significant impact of temporal fluctuations in the surroundings, such as the air in a laboratory. The primary objective of t he present proposal is to develop and publish a technique that enables instantaneous density gradient measurements. Secondarily, the project will generalize the pattern matching principles of Synthetic Schlieren.

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FRINATEK-Fri prosj.st. mat.,naturv.,tek