Lens-free auto-focusing imaging algorithm for the ultra-broadband light source

Abstract

Auto-focusing is an essential task for lens-free holographic microscopy, which has developed many methods for high precision or fast refocusing. In this work, we derive the relationship among intensity derivation, the derivative of spectral distribution, as well as the distribution of the object, and propose a new auto-focusing criterion, the Robert critical function with axial difference (RCAD), to enhance the accuracy of distance estimation for lens-free imaging with the ultra-broadband light source. This method consists of three steps: image acquisition and preprocessing, axial-difference calculation, and distance estimation with sharpness analysis. The simulations and experiments demonstrate that the accuracy of this metric on auto-focusing with the ultra-broadband spectrum can effectively assist in determining the off-focus distance. The experiments are conducted in an ultra-broad-spectrum on-chip system, where the samples including the resolution target and the cross-section of the Tilia stem are employed to maximize the applicability of this method. We believe that the RCAD criterion is expected to be a useful auxiliary tool for lens-free on-chip microscopes with ultra-broadband spectrum illumination.