Schlieren

Schlieren (German, plural of Schliere, meaning streak or striation) is an optical technique used to visualize density variations in transparent media. These variations, often caused by differences in refractive index, would otherwise be invisible to the naked eye.

The principle behind schlieren imaging relies on the fact that light bends when passing through regions of differing refractive index. This bending, although typically small, is amplified in the schlieren system. A uniform beam of light is projected through the test area. Density gradients within the medium cause the light rays to be refracted (bent). These refracted rays are then focused using a lens or mirror system. At the focal plane of this system, a knife-edge or similar obstruction is placed to partially block the light.

Light rays that have been refracted due to the density gradients will be either blocked or allowed to pass through the knife-edge to a greater or lesser degree than the unrefracted rays. This difference in intensity is then projected onto a screen or captured by a camera, creating a visible image of the density variations. Areas with higher density gradients appear as brighter or darker regions, depending on the orientation of the knife-edge.

Schlieren techniques are particularly useful for studying fluid flows, heat transfer, shock waves, and other phenomena where density gradients are present. Variations in temperature, pressure, or composition within a transparent medium will all contribute to density differences that can be visualized using schlieren methods.

Several variations of schlieren techniques exist, including traditional knife-edge schlieren, color schlieren, and background-oriented schlieren (BOS). These variations offer different sensitivities and advantages for specific applications. The key element in all schlieren techniques remains the manipulation of refracted light to reveal otherwise invisible density variations.