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Partial coherent imaging, which provides high robustness and twice the imaging resolution of the coherent diffraction limit, has become a hot research method in quantitative phase imaging. Asymmetric illumination is one of the most common methods to generate phase contrast for weakly absorbed samples. By establishing a strict intensity-phase model, the quantitative phase distribution of the sample is then obtained by inverse algorithm. In order to linearize the imaging process, weak phase approximation, which imposes restrictions of small value phase on sample, is introduced into the partially coherent imaging model to separate the sample absorption and phase. However, the weak phase approximation introduces an uncertain phase loss in quantitative phase imaging, especially for samples with a large phase. In this paper, we investigate the quantitative definition weak phase approximation for partial coherent quantitative phase imaging under asymmetric illumination by simulations. According to the simulation results, we find that the reconstruction accuracy of the weak phase approximation is not only determined by the absolute phase value of the sample, but also a
effected by the illumination aperture. Furthermore, a quantitative definition of the weak phase approximation is given to provide a basis for the phase reconstruction accuracy for quantitative phase imaging based on asymmetric illumination.
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