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Schlieren and Shadowgraph Photography

light screen knife edge

PHRED PETERSEN
RMIT University

Schlieren and shadowgraph photography are related methods of imaging that reveal localized changes of refractive index found in transparent media, including gases, liquids, and solids. These gradients may be static, such as irregularities seen in glass, or dynamic, such as those induced by pressure, density, composition, or temperature gradients in fluids. Both methods use optical systems that show localized displacements of light rays against a uniform background illumination gradient, which is projected onto a viewing screen or camera focal plane.

Shadowgraphy is the simplest method of visualizing refractive index gradients, requiring only a point source of light and a screen or surface upon which to cast the subject’s shadow. Direct rays from a point source will evenly illuminate a viewing screen. The deflection of some rays by a local change in refractive index results in decreased illuminance at the point on the screen from which those rays have been displaced, and increased illuminance at the location those rays now strike the screen. Examples of natural shadowgrams visible on a sunny day are the shadows cast by imperfect window glass or active heat sources. This effect also produces the “shadow bands” briefly visible during an eclipse of the sun.

While a shadowgram is simply a shadow, a schlieren image is an optical image formed by a lens or mirror system. Schlieren methods offer higher sensitivity than shadowgraphy, but are more difficult to set up. This technique was first demonstrated by Robert Hooke (c. 1672), but found little contemporary interest outside of a few who used it as a method for testing the optical quality of glass lens blanks. August Toepler “reinvented” the technique between 1859 and 1864, and was the first to apply it to generic inhomogeneities in transparent media. He named it schlieren, from the German " schliere ," meaning streak or striation.

Schlieren images are most often made in a parallel beam of light, as shown in Figure 98, which illustrates a compact setup. A light source is focused on to a slit that is placed off-axis and exactly one focal length from a lens or mirror. This produces a parallel beam of light that illuminates the test specimen. The test field is limited by the size of the major optical elements, so mirror arrangements are more common than refracting systems because large mirrors are easier to find. The collimated light containing the specimen image is focused onto a finely adjustable knife edge by another, similar mirror or lens, where it is partially blocked. Any change of refractive index in a transparent medium causes part of the light to be refracted in or out of the part of the collimated beam that passes the knife edge, thus appearing brighter or darker than the background in the focused image. The inclusion of the knife edge is the fundamental distinction between schlieren and shadowgraph methods.

The final image is brought into focus on a viewing screen or in a camera. Color filters may be used in place of the opaque knife edge or elsewhere in the setup to make a multi-colored image that can be interpreted to determine the direction of ray displacements. This moveable knife-edge method is also the basis of the Foucault knife edge used for focusing telescopes and testing optics. Larger schlieren systems can be created with an arrangement of a large light source grid and a corresponding negative cut-off grid placed in camera. Subject areas as large as 2 × 3 m have been studied in this manner.

Schlieren methods have applications in glass technology, aerodynamics, ballistics, heat transfer and convection studies, explosion and shockwave research, gas leak detection, boundary layer studies, and combustion research. It has also found application as a component of large screen television light valve projection systems.

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