Schlieren

Schlieren ( SHLEER-ən; German: [ˈʃliːʁn̩] , German for 'streaks') are optical inhomogeneities in transparent media that are not necessarily visible to the human eye. Schlieren physics developed out of the need to produce high-quality lenses devoid of such inhomogeneities. These inhomogeneities are localized differences in optical path length that cause deviations of light rays, especially by refraction. This light deviation can produce localized brightening, darkening, or even color changes in a...