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Fourier Ptychography Microscopy (FPM) is considered as one emerging technology for the development of high efficiency and low-cost microscopic scanners. One of the major advantages of FPM is its large depth of field (DOF), which significantly reduces the mechanical accuracy requirement of the scanning stages. In this study, we experimentally measured the DOF for our FPM prototype under different illumination conditions. The measurements were based on the theory that the DOF is considered as the range along optical axis for which the contrast is above 80% of the maximum when adjusting the focus location. Accordingly, the contrast is estimated using the bar pattern on the standard resolution target USAF1951 where the modulation transfer function (MTF) curve value drops to 0.5. During the experiment, the FPM prototype is equipped with a 4×/0.13 NA objective lens, and the DOF measurement was conducted with conventional single LED illumination and symmetric illumination. The results demonstrate that the DOF of the single LED illumination FPM is 15.3 µm, which is close to the DOF of the objective lens (14.5 µm). The DOF increases to 22.7 µm when symmetric illumination is adopted, which agrees with the theoretical conclusion. This investigation provides meaningful information for the future optimization of the FPM-based microscopic digitizers.
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