shadowgraph

1. An image produced by casting a shadow on a screen.
2. See shadow play.
3. See radiograph.

An optical method of rendering fluid flow patterns visible by using index-of-refraction differences in the flow. The method relies on the fact that rays of light bend toward regions of higher refractive index while passing through a transparent material. The fluid is usually illuminated by a parallel beam of light. The illustration depicts the method as it might be applied to a fluid sample undergoing thermal convection between two parallel plates, with the lower plate being kept warmer than the upper one. As illustrated, the rays bend toward the cooler down-flowing regions, where the refractive index is higher, and away from the warmer up-flowing ones. After they have passed through the fluid layer, the rays tend to focus above the cooler regions and defocus above the warmer regions. If an image of the light beam is recorded not too far from the sample, brighter areas of the image will lie above regions of down flow, where the rays have been concentrated, and darker areas will lie above regions of up flow. Because the light passes completely through the sample, the bending effect for each ray is averaged over the sample thickness. See also Convection (heat); Refraction of waves.

Schematic of the shadowgraph method applied to reveal convection patterns in a fluid layer. A cross section of the apparatus perpendicular to the convection rolls is shown.

In convection experiments the refractive index varies because of thermal expansion of the fluid, but the method is not restricted regarding the mechanism responsible for disturbing the refractive index. Thus the same method may be used to visualize denser and less dense regions in a gas flowing in a wind tunnel, including Mach waves and shock waves, where the denser regions have a higher-than-average refractive index. See also Shock wave; Supersonic flow; Wind tunnel.

Images are usually recorded by means of a charge-coupled-device (CCD) camera, digitized, and stored in a computer. Such a digitized image consists of an array of numbers, each number being proportional to the brightness at a particular point in the image. The image points (pixels) form a closely spaced rectangular grid. A reference image may be taken in the absence of any fluid flow, and the reference image may be divided point by point into images taken with the fluid moving. See also Charge-coupled devices.

Shadowgraph, an image produced on photographic material without a camera; the subject matter is interposed between a point source (which can be infrared, X-rays, or ultrasound, as well as visible light) and the material. Examples of shadowgraphs are silhouettes of opaque objects, photograms of transparent and translucent material, and radiographs.

— Graham Saxby

The noun has one meaning:

Meaning #1: a photographic image produced on a radiosensitive surface by radiation other than visible light (especially by X-rays or gamma rays)
  Synonyms: radiogram, radiograph, skiagraph, skiagram

a) prehistoric shadowgraphy, b) sunlight shadowgram of a martini glass, c) "focused" shadowgram of a common firecracker explosion, d) "Edgerton" shadowgram of the firing of an AK-47 assault rifle (images and artwork by Gary S. Settles, Penn State Gas Dynamics Lab)

Shadowgram of the turbulent plume of hot air rising from a home-barbecue gas grill. Photograph by Gary S. Settles, Penn State University.

Shadowgraph is an optical method that reveals non-uniformities in transparent media like air, water, or glass. It is related to, but simpler than, the schlieren and schlieren photography methods that perform a similar function. Shadowgraph is a type of flow visualisation.

In principle, we cannot directly see a difference in temperature, a different gas, or a shock wave in the transparent air. However, all these disturbances refract light rays, so they can cast shadows. The plume of hot air rising from a fire, for example, can be seen by way of its shadow cast upon a nearby surface by the uniform sunlight.

Contents 1 Sunlight shadowgraph 2 Applications 2.1 Shadowgram 2.2 Cartoons 2.3 Postcards 3 See also 4 References 5 External links

Sunlight shadowgraph

This technique is as old as nature itself. For example, some aquatic predators detect their transparent prey by way of their shadows cast upon the ocean floor. Nevertheless it was Robert Hooke^[1] who first scientifically demonstrated the sunlight shadowgraph and Jean-Paul Marat^[2] who first used it to study fire. A modern account of shadowgraphy is given by G.S. Settles.^[3]

Applications

Applications of shadowgraphy in science and technology are very broad. It is used in aeronautical engineering to see the flow about high-speed aircraft and missiles, as well as in combustion research, ballistics, explosions, and in the testing of glass.

Shadowgram

According to F. J. Weinberg,^[4] the result of applying the shadowgraph technique should be known as a shadowgram. A shadowgram is not a focused image, rather it is a mere shadow. In the shadowgram, the differences in light intensity are proportional to the second spatial derivative (Laplacian) of the refractive index field in the transparent medium under study. Once the distance from the transparent disturbance to the cast shadow becomes too large, then the shadow no longer constitutes a useful representation of the disturbance that caused it.

Cartoons

The Shadowgraph, and shadowgram have been used in animation, where they reinforce the cartoon's realism. One first use was made by Disney Studios on the Three Blind Mouseketeers (1936) Silly Symphonies.^[5]

Postcards

Additionally the term Shadowgraph was used by English postcard publishers E.T.W. Dennis & Sons Ltd. of London and Scarborough for a series of 'Hold up to the Light' postcards in the 1950s. In these a saucy image can be seen through what seems an innocent picture when a light is shone through the card.^[6]

See also

• Mach-Zehnder interferometer
• Schlieren
• Schlieren photography

References

1. ^ Hooke, R., "Of a New Property in the Air," Micrographia, Observation LVIII,217-219, London(1665).
2. ^ Marat, J.-P., Recherches physiques sur le feu, Paris, France:Cl. Ant. Jombert, 1780.
3. ^ Settles, G. S., Schlieren and shadowgraph techniques: Visualizing phenomena in transparent media, Berlin:Springer-Verlag, 2001.
4. ^ Weinberg, F.J., Optics of flames: including methods for the study of refractive index fields in combustion and aerodynamics, London:Butterworths, 1963.
5. ^ Russel Merritt & JB Kaufman, Walt Disney's Silly Symphonies : A Companion to the classic cartoons series, Italy:La Cinecita del Friuli, 2006, ISBN 88-86155-27-1 p 46
6. ^ "MetroPostcard Publishers D". Metropolitan Postcard Club of New York City. http://www.metropostcard.com/publishersd.html. Retrieved on 2008-10-02. 

External links

Wikimedia Commons has media related to: Shadowgraph

• The Penn State University Gas Dynamics Lab

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