Shadowgraph

Shadowgraph is an optical method that reveals non-uniformities in transparent media like air, water, or glass. It is related to, but simpler than, the schlieren and schlieren photography methods that perform a similar function. Shadowgraph is a type of flow visualisation.

In principle, we cannot directly see a difference in temperature, a different gas, or a shock wave in the transparent air. However, all these disturbances refract light rays, so they can cast shadows. The plume of hot air rising from a fire, for example, can be seen by way of its shadow cast upon a nearby surface by the uniform sunlight.

Some aquatic predators detect their transparent prey by way of their shadows cast upon the ocean floor. Nevertheless it was Robert Hooke who first scientifically demonstrated the sunlight shadowgraph and Jean-Paul Marat who first used it to study fire. A modern account of shadowgraphy is given by Gary S. Settles.

Applications of shadowgraphy in science and technology are very broad. It is used in aeronautical engineering to see the flow about high-speed aircraft and missiles, as well as in combustion research, ballistics, explosions, and in the testing of glass. Ideal for identification of flow patterns.

According to F. J. Weinberg, the result of applying the shadowgraph technique should be known as a shadowgram. A shadowgram is not a focused image, rather it is a mere shadow. In the shadowgram, the differences in light intensity are proportional to the second spatial derivative (Laplacian) of the refractive index field in the transparent medium under study. Once the distance from the transparent disturbance to the cast shadow becomes too large, then the shadow no longer constitutes a useful representation of the disturbance that caused it.

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