Interesting link. Schlieren techniques have long been used to image flows in wind tunnel tests, but this seems to be the first serious attempt with a rotorcraft in hover flight. One limitation with this technique is that it only provides an image of pressure gradient, and the results can be affected by variations in the ambient conditions. In a test cell it is fairly easy to control ambient conditions, but it is very difficult to control ambient conditions outdoors. That's why this test was conducted in a quarry pit.