A new method of image comparison based on phase -only image processing has been applied to printed circuit board inspection. The technique has advantages over both standard template matching and rulebased approaches. The method makes use of a phase -only image comparison technique to compare a test board to a "golden board" image. Phase -only imaging is insensitive to translation errors (misregistration) between the golden board and the board under inspection. It is also very insensitive to lighting variations and can even handle cases of contrast reversal. The theoretical basis for the use of phase -only techniques will be presented. The results of applying this algorithm to real PCB inner layer images with simulated errors and image to image variations will also be shown.
We report on a new method for image recovery from speckle patterns, based on a complex phase retrieval method. Earlier work discussed both the theory1 and experimental demonstration of image recovery2 from speckle patterns. The previously reported work processed each speckle pattern to estimate autocorrelation, averaged autocorrelation estimates to reduce noise, and then did phase retrieval from the power spectral density to recover the image. The current work uses phase retrieval to directly recover a complex image field from each speckle pattern. The magnitudes of each recovered image are then averaged. The resulting imagery shows improved signal-to-noise ratio (SNR) compared with previous work. The use of sufficiently tight support constraints allows reconstruction of imagery with an SNR equal to the square root of the number of the speckle patterns processed. Improved SNR means that the required laser illumination energy may be reduced in long-range high resolution imaging systems based on speckle pattern sampling.
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