Image Reconstruction Using Autofocus in Single-Lens System

Abstract

To reconstruct the wavefront in a single-lens coherent diffraction imaging (CDI) system, we propose a closed-loop cascaded iterative engine (CIE) algorithm based on the known information of the imaging planes. The precision of diffraction distance is an important prerequisite for a perfect reconstruction of samples. For coherent diffraction imaging with a lens, autofocus is investigated to accurately determine the object distance and image distance. For the case of only the object distance being unknown, a diffuser is used to scatter the coherent beam for speckle illumination to improve the performance of autofocus. The optimal object distance is obtained stably and robustly by combing speckle imaging with clarity evaluation functions. SSIM and MSE, using the average pixel value of the reconstructed data set as a reference, are applied on two-unknown-distance autofocus. Simulation and experiment results are presented to prove the feasibility of the CIE and proposed auto-focusing method.

Selection of autofocus algorithms for printed circuit board automated optical inspection system

“…Similar to 2.2.1., this algorithm sums the squared difference between neighboring pixels in the x direction with a distance of 1 [11], [12]. The power of two in (7) results in a steeper gradient than in (1). Consequently, this algorithm provides a bigger difference in sharpness value between images.…”