You can also do it without any mirror, using Background Oriented Schlieren. Basically, you print a bunch of random dots on paper, and use a camera to see the su

You can also do it without any mirror, using Background Oriented Schlieren. Basically, you print a bunch of random dots on paper, and use a camera to see the subpixel shifts in the dots. ( see http://en.wikipedia.org/wiki/Synthetic_schlieren http://en.wikipedia.org/wiki/Synthetic_schlieren ). By cross-correlation, you get the change in refractive index/density of air. One of the main limitations on a Schlieren system is that you can only image objects that are smaller than your mirror. That makes the BOS systems are pretty neat, because at least in theory, you can just print out large backgrounds to image large areas.