schlieren

thumb|This video demonstrates a single-pass high-speed schlieren system capturing the transitional ballistic sequence of a handgun. thumb|Colored schlieren image of the thermal plume from a burning candle, disturbed by a breeze from the right

Schlieren ( ; , ) 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 an image, depending on the directions the rays deviate.