Background oriented Schlieren

Background oriented Schlieren (also called synthetic Schlieren) is a technique which visualizes differences in air density (it also works for fluids), and can therefore show heat waves, shock waves and similar phenomena.

The principle behind this technique can be seen in the first example, were a candle was used as a source of heat waves. You take two photos, one of the undisturbed background, and one with the candle between the background and the camera. The warm air from the candle will cause differences in air density, and these differences will bend the light travelling from the background to the sensor, causing the background pattern to look different on the two photos. By obtaining the difference between this photo and the clean background photo, you can visualize the heat waves.

It is important that the background has to have some sort of fine structure (in this case I used a black T-shirt). Courser structure doesn't give as nice results, and no structure gives no difference in the photos and so nothing will be visible.

The difference between the two photos can simply be obtained by loading them as two layers in Photoshop, and setting the blend mode of the upper layer to "difference". However, I have found that the result is slightly clearer if you perform a high pass filter (I used a radius of 200 px) on both layers prior to determining the difference.

[ background || candle added || difference || high pass + difference ] ⇛ Heat waves caused by a candle. (2019-01-12)

Heat waves caused by a candle. (2019-01-12)

Heat waves caused by a candle. (2019-01-12)

Heat waves caused by a candle. (2019-01-12)

Heat waves caused by a candle. (2019-01-12)

Heat waves caused by a candle. (2019-01-12)

Heat waves caused by a candle. (2019-01-12)