Schlieren optics

Schlieren optics

Schlieren optics is a visual process used to photograph the flow of fluids of varying density. Invented by the German physicist August Toepler in 1864 to study supersonic motion, it is widely used in aeronautical engineering to photograph the flow of air around objects.

The technique is based on the deflection of light rays by a medium with varying density, which causes changes in the light's speed and direction. When light passes through a medium of uniform density, it travels in a straight line. However, if there are density variations within the medium, the light rays are bent, creating areas of light and dark in the resulting image. Schlieren optics makes these variations visible.

The basic setup for a schlieren system includes a light source, a collimating lens to produce parallel light beams, an object of interest through which the light passes, a schlieren stop to block part of the light, and a focusing lens to capture the image. The schlieren stop is crucial; it blocks the light rays that have not been deflected, allowing only the deflected rays to reach the recording medium (camera or eye), thus visualizing the flow patterns.

Schlieren optics can be used to study a wide range of phenomena, including the flow of air, the shape of flames, the sound waves, and even the thermal plumes rising from warm objects. It is a powerful tool in fluid dynamics, aerodynamics, and thermodynamics, providing insight into complex flow phenomena that are otherwise invisible to the naked eye.