Schlieren

Schlieren (from German; singular "Schliere", meaning "streak") are optical inhomogeneities in transparent material not necessarily visible to the human eye. Schlieren physics developed out of the need to produce high-quality lenses devoid of these inhomogeneities. These inhomogeneities are localized differences in optical path length that cause light deviation. This light deviation can produce localized brightening, darkening, or even color changes in an image, depending on which way the ray deviates.