Bathel and Weisberger describe an elegant method for greatly simplifying focusing schlieren imaging. Standard schlieren systems use a knife edge to selectively diminish or enhance refracted light, giving a visualization of density changes in an otherwise transparent object. Rich images and a relatively simple optical configuration make this a widely used technique, however in some applications the extended sensitivity of standard schlieren can be problematic. A common example is in wind tunnel testing - schlieren is very well suited to flow visualization but the desired data will be confounded with any imperfections in the tunnel's viewing windows. A focusing schlieren system overcomes this by replacing the standard point-like illumination with a grid or line illumination pattern, and the knife edge with a grid/line detection pattern. The overall effect is similar to superimposing a series of rotationally-offset standard schlieren imagers, so that the focusing system is sensitive primarily in the region of overlap. In our wind tunnel example, this means focusing schlieren emphasizes the flow volume and is comparitively insensitive to window imperfections.

While focusing schlieren is a useful technique, it can be hard to implement. Precisely matched illumination and detection grids must be constructed, and carefully aligned in position, orientation and magnification. Further, distortions in the schlieren lenses can degrade the grid matching, even for optimal positioning. In this paper the authors describe how a retroreflective background allows a single grid to be used for both illumination and detection. The retroreflective background effectively reverses the ray paths in the system, resulting in a double-pass through the object, double duty for the grid (it performs both illumination and detection roles), and an inherent alignment of the grid with itself. Additionally, the authors show how manipulation of the polarization state allows a deliberate and controllable spatial offset to be introduced, and used to adjust system sensitivity. This is analogous to how the position of the knife edge can be tuned on an experiment-by-experiment basis in standard schlieren imaging. This paper describes the improved focusing schlieren system precisely and shows compelling wind tunnel experiments demonstrating its utility. This work will be of significant interest to anyone involved in schlieren imaging, flow visualization and/or wind tunnel testing.

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