Schlieren imaging

Schlieren imaging for the visualization of indoor air flows

The schlieren imaging is a technique for visualizing and measuring indoor air currents. The measurement setup essentially consists of a concave and extremely finely ground mirror (Ø = 1002 mm, focal length: 3001.5 mm, micro-roughness: <2.7 nm RMS, surface defects: <λ / 9.75 @ 633 nm), a light source (LED), a knife edge and a camera with very good resolution and a lens with very high sharpness.

The functional principle of schlieren imaging is based on the deflection of light beams as a result of refractive index gradients. The light rays from the light source are directed with the help of the schlieren mirror and passes through the measuring section. Density gradients in test section lead to refractive index gradients that deflect the light rays. On the other side of the measuring section, the light beams are focused on a point at the radius of curvature of the mirror and then fall on the camera lens. In order to make the deflected light rays visible, a knife edge e.g. can be placed at the focal point from bottom, so that it can block about half of the light rays. Hence, the downwardly deflected light rays at the test section can no longer fall on the lens of the camera, thus creating dark areas (shadow), while the upwardly deflected light rays passes the knife edge and thus result in brighter areas on the camera lens. These differences in brightness enables to visualize the first derivative of the refractive index.

With the commissioning of this unique schlieren imaging system for examining the indoor climate, the Professorship of Building Physics now has an innovative, non-invasive measurement technique for determining air movement with high temporal and spatial resolution.

In the future, new materials, technologies and processes in the area of ​​heating / cooling / ventilation of buildings can be developed and a contribution can be made in the specialization fields of »Sustainable Use of Energy and Resources« and »Healthy Living and Health Management«.

The project is funded by the Free State of Thuringia under the number 2015 FGI 0005 and was co-financed by funds from the European Union as part of the European Regional Development Fund (ERDF).

The schlieren imaging system and the background-oriented schlieren system were awarded with the Thuringian Research Award 2021 in the category "Applied Research".

Amayu Wakoya Gena M.Arch.
Phone: +49(0)36 43/58 48 34