Schlieren

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.

Schlieren were first observed by Robert Hooke[1] in 1665 using a large concave lens and two candles.

Toepler's original system[2] was designed to detect schlieren in glass used to make lenses.

In the conventional schlieren system,[3] a point source is used to illuminate the test section containing the schliere.

A knife edge at the point source-image location is positioned as to partially block some light from reaching the viewing screen.

The word schlieren originates from the German schliere, meaning "streak".

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.

Projection display systems such as the now-obsolete Eidophor and Talaria have used variations of this approach as far back as 1940.