Edge enhancement methods based on derivative operations

Abstract

Two methods are described for edge enhancement using optical derivative operations. The proposed methods are based on the polarization properties of the twisted-nematic liquid-crystal displays (LCDs). LCDs traduce the image information in changes of the state of polarization of the light, which allows us to generate simultaneously a "positive" and a "negative" (i.e., a contrast-reversed) replica of the digital image displayed on the LCD. In both methods negative and positive replicas are at the same time imagined across a plane. In first case when the negative replica has a lateral differential displacement relative to the original one, an image with enhanced first derivatives along a specific direction is obtained. In the case when the negative replica is low-pass filtered, one obtains the Laplacian of the original image. Unlike the usual Fourier (coherent) image processing, the technique proposed here works with incoherent illumination. Validation experiments are presented.