How does a schlieren testing system work?

A Schlieren test is an optical system that detects changes within a test area medium (air) and records the changes in the form of an image on a screen. The image is formed by refraction and scattering from what is introduced into the test area, which are areas of varying refractive index. A source directs light onto a spherical mirror, which collimates the light and redirects it onto a second identical mirror. The light is then focused onto an included screen. The space between the mirrors is the test area, where the small particles introduced are made visible by the light source and seen as shadows on the screen. Brightness variations on the screen will occur according to changes within the test area. Spherical mirrors are used due to the slight off-axis nature of the set-up. Applications include the determination of refractive index, fluid and air current flow, and flame analysis. Film or video cameras can also be added to the system in place of the imaging screen. Two systems are available with different size F10 mirrors in order to select the test area size that best matches the application.