On BOS and its progenitor, the Background-Distortion Schlieren Technique

My twin goals for this half hour are to describe a few unique aspects of the BOS technique that I have observed over 20 years of playing with it, and hopefully to raise your awareness of the close kinship between BOS and the much-older background-distortion schlieren technique from which it came; a relationship that I suspect many are not even aware of. This requires no heavy math but is conveyed, instead, mostly by examining schlieren images.

BOS is simple and easy to use, which is its greatest advantage, but that simplicity also conveys disadvantages, particularly non-parallel illumination and a sharp focus on the background rather than on the schlieren object. A second great advantage of BOS is its ability to image large fields-of-view without expensive optics, and a third is its adaptability to outdoor use.

Background-distortion schlieren, first discovered by Robert Hooke over 350 years ago, is now taken so much for granted that it usually goes nameless, or is simply referred to by terms such as “backlighting”. BOS and background-distortion schlieren share almost the same simple optical diagram, but while BOS necessarily focuses on a background pattern, background distortion schlieren focuses instead on the phenomenon under study with defocused background. Examples are drawn from nature and common experience, and include many experiments on multiphase flows such as drops and bubbles, where there are large differences in refractive index between the phases. A few simple guidelines help one to obtain both useful and attractive background-distortion schlieren images. Finally, a popular modern do-it-yourself macro-photography technique, sometimes called “oil and water photography,” is revealed to be a kitchen-table version of the ancient background-distortion schlieren method.