Schlieren imaging was developed in the 1800's to detect flaws or 'schliere' in glass. The technique is frequently used today as a means of visualising shock waves in wind-tunnels and temperature gradients around objects. Schlieren imaging relies on the refractive index (the ability to 'bend' light) of a transparent medium (often air) changing with density to produce an image. In a simple schlieren system a parallel beam of light is passed through the subject and is focussed on to a knife edge using lenses or spherical mirrors. A change in density in part of the subject causes that part of the light beam to be refracted and to fall above or below the knife edge creating lighter or darker areas in the image. Colour filters are sometimes used in place of a knife edge to produce an image using different colours to denote different areas of density.
Optical Set up: Illumination for schlieren imaging must be produced from a point light source. For high speed photography the intensity of the light source and sensitivity of the imager should allow recording at the desired frame rate. A parallel beam of light is created using a pair of lenses or mirrors. The diameter of the lenses or mirrors determines the size of the working area or 'measurement volume' in which the image is produced. A knife edge or specially designed colour filter is mounted on a vernier adjustment close to the image plane. The schematic diagram above shows the light path and position of the camera sensor.
High-speed schlieren photography of combustion process reveals the flame front progression. FASTCAM ultima APX-RS Images courtey of Marintek, Norway
Airflow visualisation in a high velocity wind tunnel. FASTCAM ultima APX
Further Information on Schlieren Imaging
Schlieren and Shadowgraph Techniques: Visualizing Phenomena in Transparent Media, Prof. Gary Settles, Gas Dynamics Laboratory, Pennsylvania State University,
Springer-Verlag, Nov. 2001, ISBN 3-540-661557
Schlieren Photography Priniciples, Prof. Andrew Davidhazy, Imaging and Photographic Technology, Rochester Institute of Technology (view detail)
An Air Flash Lamp Advances Color Schlieren Photography, C. L. Strong, Scientific American, Vol. 231, No. 2 August 1974