DEFECT ENHANCEMENT USING FRESNEL IMAGING AND A TN-LCD

In this paper, we discuss defect enhancement in periodic images by Fre snel imaging. A periodic image is written in a liquid crystal display (matrix LCD) and read out by a coherent beam, a plane wave. The LCD is of twisted-nematic type (TN). The polarization of the read-out wave i s rotated as a function of the gray value of the image. Because the in put image is periodic, self-imaging (also called Fresnel imaging or Ta lbot imaging) occurs at certain distances (Talbot planes) behind the L CD. A mirror is put in one of these planes and the wave is redirected back to the LCD. The wave reads out the LCD again, and the rotation of polarization is performed again for all pixels, but in opposite direc tion; consequently, the input polarization state is reached again. If the input image has a defect, then this pixel is not present in the re directed wave, because self-imaging only occurs for periodic parts of an image. This pixel leaves the LCD with a different polarization stat e and can be detected. (C) 1998 Elsevier Science B.V. All rights reser ved.