Developments in Optical Measurements Techniques

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 will work to improve
this digital Schlieren technique and apply it to 3 different experiments of
practical, everyday use.

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 the 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, 1998) 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 the 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 the 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.