The Schlieren technique was a well-known coherent processing method that was usually applied to the visualization of phase objects. It was demonstrated here that, when the Schlieren processing was applied to a light wave modulated in amplitude and possessing some periodicity, the harmonic contents of the resultant image decreases (i.e., the higher harmonics were suppressed). Also, it was shown that, when the amplitude-modulated (periodic) light wave possesses faults, the Schlieren processing produces an enhancement of the faults relative to the periodic carrier. This technique could be applied to defect detection in periodic structures such as photomasks used for LCD panels, integrated-circuit masks, or semiconductor wafers.

Harmonic Suppression and Defect Enhancement using Schlieren Processing. E.Garbusi, J.A.Ferrari, C.D.Perciante: Applied Optics, 2005, 44[15], 2963-9