1. Schlieren Method: Some History
Robert Hooke
Discovered “Schlieren Method”  1665
Used two candles and a convex lens
Low quality instruments OK!
August Toepler
Used conventional Schlieren System
Trying to find impurities in lenses
More complicated system, better equipment
Although Schlieren looks and sounds like a German name, it is not the name of a scientist. The Schlieren method originated in Schlieren, Switzerland, and it has become an adjective describing the method itself.

2. Schlieren Method: How It Works
Produces image of an object
Deflection of light by refractive index gradient
Converging Lens
Knife Edge  Blocks unwanted light
Lens places image on screen
Simple Schlieren Setup
where f is the focal length of the lens, do is the distance from the object to the lens and di is the distance from the image of the object to the lens

3. Schlieren Optics Point source of light illuminates an object or image.
Image is focused on a knife edge.
When index of refraction of the medium surrounding the object or image changes
Image focal point will be perturbed
Diffracted rays will interfere
Perturbed focal point

4. References http://en.wikipedia.org/wiki/Schlieren
Hect, Eugene and Alfred Zajac. Optics. 3rd ed. New York: Addison Wesley Publishing Company, 1997.
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