Schlierenfotografie

Schlierenfotografie, or schlieren photography, is an optical method for visualizing small changes in the refractive index of transparent media. It is commonly used to reveal density gradients in gases such as air, produced by temperature differences, pressure fields, or compositional variations. The resulting images show regions of differing light deflection as high-contrast patterns, enabling researchers to study flows, heat transfer, combustion, and other phenomena that are otherwise invisible.

The technique relies on the deflection of light rays by refractive-index gradients. A nearly parallel light beam passes through the test region; rays are bent slightly by spatial variations in density, and this angular deviation is converted into brightness differences in an image by placing a knife edge or another cutoff at the focal plane of the imaging lens. The brightness pattern therefore encodes the gradient of refractive index along the light path.

Typical schlieren installations include a bright light source, a collimator to create an almost parallel beam, the test section containing the flow, a focusing optic to form an image, and a knife-edge or diaphragm that blocks part of the light in the focal plane. The camera records the resulting schlieren image. Variants may use mirrors instead of lenses, or implement digital or color enhancements.

Schlieren photography originated in the 19th century with August Toepler, and was refined in the following decades. It became a standard tool in aerodynamics, enabling visualization of shock waves, boundary layers, and jets. Modern approaches include background-oriented schlieren (BOS), which uses a patterned background to produce density-contrast via image distortions, and digital schlieren techniques for quantitative analysis.