Schlieren photography

Any technique for the photographic recording of schlieren, which are regions or stria in a medium that is surrounded by a medium of different refractive index. Refractive index gradients in transparent media cause light rays to bend (refract) in the direction of increasing refractive index. This is a result of the reduced light velocity in a higher-refractive-index material. This phenomenon is exploited in viewing the schlieren, with schlieren photographs as the result. Electronic video recorders, scanning diode array cameras, and holography are widely used as supplements. See also Holography; Optical detectors; Photography; Refraction of waves.

There are many techniques for optically enhancing the appearance of the schlieren in an image of the field of interest. In the oldest of these, called the knife-edge method (see illustration), a point or slit source of light is collimated by a mirror and passed through a field of interest, after which a second mirror focuses the light, reimaging the point or slit where it is intercepted by an adjustable knife edge (commonly a razor blade). The illustration shows the “z” configuration which minimizes the coma aberration in the focus. Mirrors are most often used because of the absence of chromatic aberration. See also Aberration (optics).

Knife-edge method of viewing schlieren, employing the “z” configuration.

Rays of light that are bent by the schlieren in the direction of the knife edge are intercepted and removed from the final image of the region of interest, causing those regions to appear dark. Consequently, the system is most sensitive to the density gradients that are perpendicular to the knife edge. The knife edge is commonly mounted on a rotatable mount so that it can be adjusted during a measurement to optimally observe different gradients in the same field of interest. The intensity in the processed image is proportional to the refractive index gradient. A gradient in the same direction as the knife edge appears dark. Gradients in the opposite direction appear bright. This method, employed with arc light sources, is still one of the simplest ways to view refractive index changes in transparent solids, liquids, and gases. A well-designed schlieren system can easily detect the presence of a refractive index gradient that causes 1 arc-second deviation of a light ray.

Except for locating and identifying schlieren-causing events such as turbulent eddies, shock waves, and density gradients, schlieren systems are usually considered to be qualitative instruments. Quantitative techniques for determining density are possible but are much more difficult to employ. The most common of these is color schlieren. The knife edge is replaced with a multicolored filter. Rays of light refracted through different angles appear in different colors in the final image.

The availability of lasers and new optical components has expanded the method considerably. When a coherent light source such as a laser is used, the knife edge can be replaced by a variety of phase-, amplitude-, or polarization-modulating filters to produce useful transformations in the image intensity. See also Interferometry; Laser; Polarized light.

Schlieren is German for ‘striations’. The term was coined by Albert Töpler, who developed the technique in 1906 from a related technique used to identify figuring errors in telescope mirrors. Schlieren photography is a way of visualizing density variations in a gas, and is useful in wind tunnel studies and investigations into heat flow. It employs a shadowgraph principle. A collimated (i.e. parallel) beam of light passes through the test space and is brought to a focus at a knife edge; it then diverges on to a screen or a camera system. Any gas density gradient with a component perpendicular to the knife edge will deviate the light from the region, so that it either clears the edge, giving a bright area on the screen, or is intercepted by it, giving a dark area. The resolution can be improved by a further knife edge at the first focus of the system. Where large spaces are to be imaged, off-axis parabolic mirrors are used rather than lenses to collimate and focus the beam (Fig. 1). An alternative to the knife edge is a band of three colour filters, red above and blue below, with a narrow strip of green in between.

Schlieren photography is sensitive enough to record the pattern of warm air rising from a human hand, but a more sensitive test uses interferometry, in a kind of hybrid of Schlieren photography and holography. A laser beam replaces the white light beam, and a beamsplitter and beam combiner form a Mach-Zehnder interferometer set-up (Fig. 2). This shows density differences directly, rather than density gradients.

Fig. 1

Fig. 2

— Graham Saxby

Bibliography

• Settles, G. S., Schlieren and Shadowgraph Techniques (2001)

A schlieren photograph showing the compression in front of an unswept wing at Mach 1.2

Shock waves produced by a T-38 Talon during flight

Color schlieren image of the thermal plume from a burning candle, disturbed by a breeze from the right. Photographed by Gary S. Settles, Penn State University

Schlieren photography is a visual process that is used to photograph the flow of fluids of varying density. Invented by the German physicist August Toepler in 1864 to study supersonic motion, it is widely used in aeronautical engineering to photograph the flow of air around objects. Its role is changing due to the increasing use of computational fluid dynamics, where the same principle is used to display the computed results as flow images.

Contents 1 Optical system 2 Variations 3 Synthetic schlieren 4 See also 5 References 6 External links

Optical system

The basic optical schlieren system uses light from a single collimated source shining on, or from behind, a target object. Variations in refractive index caused by density gradients in the fluid distort the collimated light beam. This distortion creates a spatial variation in the intensity of the light, which can be visualised directly with a shadowgraph system.

In schlieren photography, the collimated light is focused with a lens, and a knife-edge is placed at the focal point, positioned to block about half the light. In flow of uniform density this will simply make the photograph half as bright. However in flow with density variations the distorted beam focuses imperfectly, and parts which have focussed in an area covered by the knife-edge are blocked. The result is a set of lighter and darker patches corresponding to positive and negative fluid density gradients in the direction normal to the knife-edge. When a knife-edge is used, the system is generally referred to as a schlieren system, which measures the first derivative of density in the direction of the knife-edge. If a knife-edge is not used, the system is generally referred to as a shadowgraph system, which measures the second derivative of density

If the fluid flow is uniform the image will be steady, but any turbulence will cause scintillation, the shimmering effect that can be seen on hot surfaces on a sunny day. To visualise instantaneous density profiles, a short duration flash (rather than continuous illumination) may be used.

Variations

Variations on the optical schlieren method include the replacement of the knife-edge by a colored "bullseye" target, resulting in Rainbow Schlieren which can assist in visualising the flow. The adaptive optics pyramid wavefront sensor is a modified form of schlieren (having two perpendicular knife edges formed by the vertices of a refracting square pyramid).

Few complete schlieren optical systems are commercially available today, but details of theory and construction are given in Settles' 2001 book.^[1] The USSR produced a number of sophisticated schlieren systems based on the Maksutov telescope principle, many of which still survive in the former Soviet Union and China.

Synthetic schlieren

The synthetic schlieren method is a technique similar to schlieren photography which makes use of digital photography and image processing rather than optics to visualize the density variations of a fluid.

See also

• Mach-Zehnder interferometer
• Schlieren
• Shadowgraph

References

1. ^ Settles, G. S., Schlieren and shadowgraph techniques: Visualizing phenomena in transparent media, Berlin:Springer-Verlag, 2001.

External links

Wikimedia Commons has media related to: Schlieren photography

• The Penn State University Gas Dynamics Lab
• Grady, Denise (2008-10-28). "The Mysterious Cough, Caught on Film". The New York Times: p. D3. http://www.nytimes.com/2008/10/28/science/28cough.html. Retrieved 2008-10-28. 
• Schlieren Photography - How Does It Work?

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