Schlieren imaging is a technique that lets you see small changes in refractive index .
How it Works
Schlieren imaging essentially uses a spatial filter to enhance the path differences on light passing through air currents. The setup relies on a point source of light (in this case a white LED inside a casing) and a pair of concave mirrors. The mirrors are set up so that the first collimates the light from the source and passes it through the ‘test volume’. The second mirror focusses the collimated light into a point. An edge (which is where the name comes from) is placed at this focus so that some of the light hits it and some passes. You can then view air currents moving in the test volume either with your eyes or a camera.
The kit that I used is originally from Edmund Optics and has two 1.5m focal length concave mirrors in it. The kit was originally sold in the USA, so has a 110V bulb and US-style mains plug; I used a white LED that I had lying around and used my bench power supply to control the voltage and current so that the LED didn’t burn out.
At a recent social night at the Oxford Hackspace, used this technique to look at the air currents around some flames from a candle and a camping stove. Local artist Hugh Pryor brought his camera and took a few shots. He added some coloured filters to the LED so that the images appear in colour rather than monochrome.