Schlieren optics is an optical technique that enables the user to image small changes and non-uniformities in the refractive index of air, or other transparent media; and are most commonly used to image air flows. Schlieren systems are a specific type of optical system that has been around in its most basic form since 1864, but as with all systems, they continue to advance and have become a widely-used imaging approach across the engineering and aerospace sectors. In this article, we look at how these optical systems work, and the applications where Schlieren optics are used.
How Schlieren Optics Work
The first step to understanding how a schlieren system works is to look at how the light propagates through the optics. The most basic type of schlieren system uses a straight line of lenses and parallel beams of light, where the light passes through a slit and is focused by the first lens. The parallel light beam then becomes refracted by an object in the test area and passes through a second lens. The light is then cut off by a knife edge and the image is projected by a third lens onto a screen or photographic sensor.
In more advanced systems, the light is focused by shining it through a slit onto a long focal-length mirror (or a series of mirrors), which focuses the light onto a thin piece of metal. This metal acts as a light blocker and can take the form of a thin wire, a knife edge or a razor blade edge. Behind this light blocker is a camera, and this camera is positioned directly in front of the mirror towards the test area.
Because of the specific focusing of the light and position of the camera, if there are any changes in the refractive index of the air in the test area, then the image of the point light source will change slightly. If this change in light source passes around the light blocker, then it will be imaged as streaks of light in the area where the change in the index of refraction occurred. This is known as the schlieren effect.
The brightness of the schlieren effect, and therefore the resulting image, is highly dependent upon the magnitude of the refraction– i.e. a bigger refraction will produce a much brighter image, and vice versa. This is because a bigger refraction will correspond to more light passing over the light blocker, thus making the image brighter. Bigger effects can also be seen based on how far away the refraction occurs from the light blocker (and therefore how close to mirror the refraction occurs). If the refraction occurs far away from light blocker, then it will have a large path to diffract away from the point of diffraction (and will travel higher from the point of refraction), which ultimately results in more light passing over the light blocker.
There are also some slightly modified techniques which have been developed based around schlieren optic principles and are used to provide different optical information. These are shadowography and schlieren interferometry. Shadowography is a technique that projects the shadow of an object in the test area of a schlieren system onto a flat screen, rather than taking an image through a camera. Schlieren interferometry bears more resemblance to conventional interferometry than it does to the information obtained from a standard schlieren setup. However, it does apply some of the visualization aspects of schlieren systems but uses a prism to induce interference or diffraction within the schlieren image. It is often used to provide quantitative measurements from a schlieren image.
Why Schlieren Optics are Used
Any inhomogeneous refractions within an air/transparent flow can be imaged (and visualized) using a Schlieren setup. Whilst the imaging is a qualitative approach (i.e. there is no determined value backed out), there are many factors that can influence the refractive index of these flows, such as the density, temperature, or pressure, and tailoring these environmental factors during a study can be used to understand the properties of the air flow. However, many areas of research that use Schlieren optics are concerned with how the air behaves around another object, and thus, can be used to determine how well the objects behave in certain flow environments. Aside from air flows, Schlieren optics can also be used to show the diffraction of light around an object.
So which applications benefit from Schlieren optics? Well, the applications range across the engineering and aerospace industries, but the main focus is in applications where the aerodynamics of a flow are important. Some examples of these applications include various types of fluid dynamics research, wind tunnel research to see how air flows around a new aircraft design, to determine the internal characteristics of glass, flame analysis and for determining the mass and sound velocity of particles.
Columbia University: https://mice.cs.columbia.edu/getTechreport.php?techreportID=1542&format=pdf&
Edmund Optics: https://www.edmundoptics.com/f/schlieren-systems/11889/
Harvard University: https://sciencedemonstrations.fas.harvard.edu/presentations/schlieren-optics