This paper describes the basic technique of schlieren photography in simple terms and then goes on to discuss the latest trends and developments in schlieren systems. There is some discussion of the use of schlieren photography for non-scientific applications.
T. P. Davies,
"Aspects Of Schlieren Photography", Proc. SPIE 0674, 17th Intl Congress on High Speed Photography and Photonics, (1 September 1987); doi: 10.1117/12.975558; https://doi.org/10.1117/12.975558