Schlieren (from German; singular "schliere") are optical inhomogeneities in transparent material not visible to the human eye. Schlieren physics developed out of the need to produce high-quality lenses void of these inhomogeneities. These inhomogeneities are localized differences in optical path length that cause light deviation. This light deviation is converted to shadow in a schlieren system.
Schlieren flow visualization is based on the deflection of light by a refractive index gradient. The index gradient is directly related to flow density gradient. The deflected light is compared to undeflected light at a viewing screen. The undisturbed light is partially blocked by a knife edge. The light that is deflected toward or away from the knife edge produces a shadow pattern depending upon whether it was previously blocked or unblocked. This shadow pattern is a light-intensity representation of the expansions (low density regions) and compressions (high density regions) which characterize flow.
US Patent Issued to Chang Gung University on June 11 for "Schlieren Type Ultrasonic Wave Observer System" (Taiwanese Inventors)
Jun 11, 2013; ALEXANDRIA, Va., June 11 -- United States Patent no. 8,462,326, issued on June 11, was assigned to Chang Gung University...