Schlieren

History

Schlieren were first observed by Robert Hooke in 1665 using a large concave lens and two candles. One candle served as a light source. The warm air rising from the second candle provided the schliere. In 1858, Léon Foucault described a knife-edge test to accurately measure the shape of concave curved mirrors by reflecting light into a knife edge at or near the mirror's centre of curvature. German physicist August Toepler adapted the principle in 1864 to detect schlieren in glass used to make lenses. In the conventional schlieren system, a point source is used to illuminate the test section containing the schliere. An image of this light is formed using a converging lens (also called a schlieren lens). This image is located at the conjugate distance to the lens according to the thin lens equation: \frac{1}{f}=\frac{1}{d_o}+\frac{1}{d_i} where f is the focal length of the lens, d_o is the distance from the object to the lens and d_i is the distance from the image of the object to the lens. A knife edge at the point source-image location is positioned as to partially block some light from reaching the viewing screen. The illumination of the image is reduced uniformly. A second lens is used to image the test section to the viewing screen. The viewing screen is located a conjugate distance from the plane of the schliere.

Most-observed species

• European roe deer

  Capreolus capreolus (Linnaeus, 1758) · Mammalia
  23,793
• Annual Fleabane

  Erigeron annuus (L.) Pers. · Magnoliopsida
  23,643
• Meadow Brown

  Maniola jurtina (Linnaeus, 1758) · Insecta
  16,629
• Common Blue

  Polyommatus icarus (von Rottemburg, 1775) · Insecta
  11,909
• red fox

  Vulpes vulpes (Linnaeus, 1758) · Mammalia
  11,418
• Narrow-leaved Ragwort

  Senecio inaequidens DC. · Magnoliopsida
  9,950
• Carrion Crow

  Corvus corone Linnaeus, 1758 · Aves
  8,830
• Meadow Grasshopper

  Pseudochorthippus parallelus (Zetterstedt, 1821) · Insecta
  8,694

Most recent

• M 3.7 — 2026-04-26
  4 km N of Quarten, Switzerland
• M 3.5 — 2024-06-27
  1 km SSW of Todtmoos, Germany
• M 4.2 — 2024-06-04
  2 km NNW of Unteriberg, Switzerland
• M 3.9 — 2022-09-10
  1 km WSW of Zimmersheim, France
• M 3.8 — 2022-09-01
  2 km WSW of Sevelen, Switzerland
• M 4 — 2020-10-25
  5 km ESE of Ennenda, Switzerland