Automatic focus determination through cosine and modified cosine score in digital holography

Abstract

An automatic focus determination method in digital holography that utilizes the cosine score (CS) of the inner angle between the vectors that result from adjacent axial reconstructed amplitude images is proposed. This method is based on the fact that the optical field near the focus plane contains more regular features of an object than the defocused region. Further, a modified CS (MCS) autofocusing method is proposed to extend the application range and improve the performance of the proposed method. First, a low-pass filter is designed for the object term holograms in the frequency domain. Second, the standardized Z-scores method is applied along the axis to the amplitude images reconstructed from the filtered hologram. Finally, the MCS of the standardized amplitude images is calculated by an improved cosine-like formula. Next, the focus plane is found at the minimum MCS for different types of objects. This method enables accurate focus determination for all types of objects, as well as for the image region with a slow change or small size, which offers time savings and the precise autofocusing of objects with large longitudinal volumes. The simulations and experimental results on a United States Air Force resolution chart and living cells are presented, illustrating the efficiency and potential of the methods.