Keywords: Shadowgraphy, flow-visualization, edge detection
I. BRIEF HISTORY AND WORKING PRINCIPLE
Shadowgraphy is one of the oldest and simplest of the flow visualization techniques. Robert Hooke was the first person to study the Schlieren and Shadowgraphy techniques around 1665. He studied the shadow of plumes of a burning candle casted by sun on a white paper . Hooke published these work in Micrographia . In 1780, Jean Paul Marat published a volume on the physics of fire that contains apparently the first optical flow visualization image ever printed . Ernst Mach mentioned several outdoor shadowgraph and schlieren visualizations in his Popular Scientific Lectures . Cooper and Rathert  first studied the use of sunlight shadowgraphy to reveal shock wave locations on high-speed aircraft wings .
Shadowgraphy is extensively used in flow visualization and analysis of variation of pressure, density and temperature in transparent media. Shadowgraphy is preferred for flow visualization because of its inexpensive and simple setup. The basic setup include a light source and a screen to cast a shadow. When light is passed through a transparent media having variation in density or other properties, then the effect can be seen in its shadow as some portion of the shadow is bright while some portion is dark.
The basic working principle of shadowgraphy is change in refractive index of media due to change in pressure, density of temperature of the media. The light rays are refracted in accordance with the Snell’s Law. Due to refraction of light in a particular direction, on some portion of screen, intensity of light is less resulting in dark section while in some port...
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...lieren and Shadowgraph Imaging in the Great Outdoors, PSFVIP-2, May 16-19, 1999,
 Cooper, G.E. and Rathert, G.A., Visual observations of the shock wave in flight, Report RM A8C25, NACA, 1948.
 H.W. Liepmann, A. Roshko, Elements of Gas Dynamics (Dover Publication, 2001).
 http://www.thermopedia.com/content/1117/. (accessed on 06/03/2014).
 Yangang Liu, Peter H. Daum, Relationship of refractive index to mass density and self-consistency of mixing rules for multicomponent mixtures like ambient aerosols, Elsevier, Aerosol Science 39 (2008) 974–986.
 Hui Hu, Flow Visualization techniques: Shadowgraph and Schlieren, AerE 344 Lecture Notes, Iowa State University.
 The MathWorks, Inc., Matlab 2014a ,Image Processing Toolbox-User’s Guide.
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