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Works on the principle of formation of dark and bright fringes, due to constructive/destructive interference of the test
and the reference beam, when there is a relative change in the path lengths of the test and the reference beam.
Detailed description of this technique
2. Schlieren:
Works on the principle of deflection of light ray when it passes through a medium
in which there is a component of refractive index gradient normal to the ray.
Detailed description of this technique
Shadowgraph system measures the linear displacement of the perturbed light rather
then the angular deflection as in the Schlieren system.
Image indicates second derivative of refractive index field.
Detailed description of this technique
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