Supersonic wind tunnels are commonly used by engineers to test and improve the aerodynamic performance of aircraft designs and engine components. During a test, the aircraft model is placed in the test section of a wind tunnel and air is made to flow past the model. Schlieren imaging is a trusted method for visualising density variations in these ‘invisible’ air flows. In one of the most common types of Schlieren imaging system - a source directs light onto a spherical mirror, which collimates the light and redirects it onto a second identical mirror. The resultant image may be captured by an imaging camera or on a screen.