shadowgraph

Shadowgraph is an optical visualization technique used to image refractive-index variations in transparent media by producing shadow-like light patterns on a screen or detector. It is related to schlieren photography but is simpler and generally less sensitive to small gradients, while still revealing qualitative flow structures and density variations.

The method relies on light rays refracted by gradients in the medium’s refractive index. When a region with a density or index variation causes light to bend, the rays do not converge to the same image as in uniform regions, producing bright and dark regions in the recorded pattern. In shadowgraphy, the image intensity is roughly proportional to the second spatial derivative of the refractive-index field along the light path, so it highlights rapid changes in density.

A basic shadowgraph setup uses a light source, a condenser or lens system, the test section containing the transparent flow, and a focusing lens to project an image onto a screen or a camera. The arrangement emphasizes large-angle deflections and yields high-contrast patterns for substantial density variations. Because it is simple and inexpensive, the technique is widely used for rapid visualization of flow phenomena.

Applications encompass fluid dynamics and aerodynamics, including visualization of convection, jets and plumes, shock waves, turbulence, and combustion flames in transparent media. It is also employed in microfluidics and educational demonstrations to illustrate qualitative flow behavior.

Shadowgraph is qualitative and provides a two-dimensional projection of a three-dimensional field. It is typically easier to set up than schlieren systems but offers lower sensitivity to small gradients, making it complementary rather than a replacement in optical flow visualization.