Analysis of density variation by shadowgraphyKeywords: Shadowgraphy, flow visualization, edge detectionI. BRIEF HISTORY AND WORKING PRINCIPLESShading is one of the oldest and simplest flow visualization techniques. Robert Hooke was the first person to study the techniques of Schlieren and Shadowgraphy around 1665. He studied the shadow of the plumes of a burning candle cast by the sun on a white paper [1]. Hooke published this work in Micrographia [2]. In 1780, Jean Paul Marat published a volume on the physics of fire which apparently contains the first optical flow visualization image ever printed [1]. Ernst Mach mentioned several outdoor visualizations of shadows and schlieren in his popular science lectures [3]. Cooper and Rathert [4] first studied the use of the sun's shadow to reveal the location of shock waves on the wings of high-speed aircraft [3]. Graph shadowing is widely used in flow visualization and analysis of pressure, density and temperature variations in transparent media. Shadowgraphy is preferred for stream visualization due to its simple and inexpensive setup. The basic setup includes a light source and a screen to cast a shadow. When light passes through a transparent medium that has a variation in density or other properties, the effect can then be seen in its shadow because part of the shadow is bright while another part is dark. The basic working principle of graphic shadow is the change of refractive index. media due to change in pressure, media temperature density. Light rays are refracted according to Snell's law. Due to refraction of light in a particular direction, on some parts of the screen the intensity of light is less, resulting in a dark section while in some ports...... middle of paper ......lieren and Shadowgraph Imaging in the Great Outdoors, PSFVIP-2, May 16-19, 1999,[4] Cooper, GE and Rathert, GA, Visual Observations of the Shock Wave in Flight, RM Report A8C25 , NACA, 1948.[5] HW Liepmann, A. Roshko, Elements of Gas Dynamics (Dover Publication, 2001).[6] http://www.thermopedia.com/content/1117/. (accessed 03/06/2014).[7] Yangang Liu, Peter H. Daum, Relationship between refractive index and mass density and self-consistency of mixing rules for multicomponent mixtures like ambient aerosols, Elsevier, Aerosol Science 39 (2008) 974-986 .[8] Hui Hu, Flow Visualization Techniques: Shadowgraph and Schlieren, AerE 344 Lecture Notes, Iowa State University. URL: http://www.public.iastate.edu/~huhui/teaching/2009Sx/AerE343L-AerE311L/Lecture-notes /Reading -07-Notes.pdf[9] The MathWorks, Inc., Matlab 2014a, Image Processing Toolbox-User's Guide.