[aerohead]

I've only seen it used in ballistics research where shockwaves were inspected.I have to admit that I never considered doing it.I thought that perhaps,the target passed through a gate,with projecting screen behind,and the optics of the camera were pre-focussed on the screen,to capture the" shadow" of the "air",at varying density behind the target vehicle, cast by the sunlight shining across the void in between.Hmmmmmmmmmmmm!