Schlieren Method

a technique that detects optical inhomogeneities in transparent refractive media and defects of reflecting surfaces.

The schlieren method is used to detect striae in optically transparent materials and to determine the quality of mirrors and other optical parts. It is used to study the density gradients of air flows around wind tunnel models and to project images onto screens (the images are obtained in the form of optical inhomogeneities); it is also used in bubble chambers, large-screen television systems, systems used to reproduce images from thermoplastic and vesicular photographic materials, and other devices. The schlieren method was proposed by the German scientist A. Töpler in 1867.

Figure 1

In the schlieren method, a beam of rays from a point or slit source of light (Figure 1) is directed through the object being studied (3) by a lens or a system of lenses and mirrors (2–2’) and is focused on an opaque screen (5) with a sharp edge (a Foucault knife-edge) such that the image of the source is projected on the very edge of the screen. If the object in question has no optical inhomogeneities, all the rays are stopped by the screen. If an optical inhomogeneity (4) is present, the rays will be scattered by the inhomogeneity, and some of them, having been deflected, will pass above the edge of the screen. By placing a projection lens (6) behind the screen, it is possible to project these rays onto the screen (7) and to obtain an image (8) of the inhomogeneities that have scattered the rays. Optical inhomogeneities can be viewed directly by the observer through an eyepiece mounted on the device instead of a projection lens.

Optically conjugate gratings (rasters) that overlap the path of the rays when inhomogeneities are present in their path are sometimes used instead of a point source of light and a Foucault knife-edge. Gratings with slits in the form of light filters are also used. These make it possible to determine more graphically the character of the optical inhomogeneities. A cruder (shadow) pattern of the zones of abrupt change in the optical density of an object can be obtained if the rays are overlapped by a Foucault knife-edge or gratings. Illumination of the object by two optical systems mounted at an angle to one another makes it possible to obtain a stereoscopic pattern of the distribution of the object’s inhomogeneities.