A method of automatic focus searching is present that calculates the similarity degree of the two amplitude terms reconstructed with a fixed interval. It is based on the optical field distribution characteristics in a diffraction-limited system, as which a digital holography imaging can be described. The supportive theory is briefed in this paper, and then several typical similarity algorithms are introduced that are used as the focus degree measurement. The autofocus procedure and precisely focus searching of this proposed autofocusing method are given. Each similarity algorithm based autofocusing is validated in simulation. The results demonstrate their applicability and reliability.
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.
An autofocusing method is proposed that utilizes cosine score of inner angle between the vectors resulted from vectoring the axial adjacent reconstructed images in digital holography. It is based on the fact that the images near the focus contain more regular features of object than in the defocused region, therefore, the neighboring reconstructed images are more similar to each other at the focus position than defocused and a cosine score is employed to evaluate such similarity. However, the cosine scores between the axial adjacent amplitude images are so close that it is difficult to distinguish the extremum. Therefore, a modified cosine algorithm is presented to offset such problem on consideration of the correlation of the elements, by subtracting the inner product term from the denominator of the cosine algorithm. The cosine and modified cosine score based autofocusing method procedure is first introduced, and then it is utilized in simulation and real holographic data. In simulation, it precisely judges out the actual recording distance and the focus curve shows good focus function criteria, which verifies the method as an ideal circumstance. In real experiment, it can easily search out the focus distance from the focus curve, and it shows good focus judgement ability than most traditional focus metrics selected. Therefore, the feasibility and validation of the proposed autofocusing method are proved by simulation and experiment results.
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