It is a Schlieren technique, but in this case a series of images taken over a textured background are then processed with software in a way that amplifies the d

It is a Schlieren technique, but in this case a series of images taken over a textured background are then processed with software in a way that amplifies the differences in air density. This page goes into a little more detail: https://technology.nasa.gov/patent/TOP2-271 https://technology.nasa.gov/patent/TOP2-271