Spatial Filtering In Phase Contrast Imaging Studies

Phase contrast imaging system transforms phase distributions of objects into corresponding intensity distributions based on spatial filtering, so as to realize the visualization of phase objects. Phase-retardation of the spectrum is the key to phase contrast imaging. The essence of the phase contrast imaging is spatial spectrum filtering.In this paper, the phase contrast imaging is studied from the spatial spectrum filtering point of view and spatial frequency response of optical systems. Spatial spectrum filtering of phase contrast imaging system utilizes the phase filter. The traditional approach provide a certain phase compensation for zero-order diffracted light but does not change the amplitude. On this basis, we propose two new ideas which different from the traditional approach. First of all, using a certain phase-retardation for non-zero-order diffracted light substitutes for phase-retarding of zero-order diffraction equivalently. By this method a same contrast images can be obtained as traditional approach and one phase filter can be fabricated easily. Second, during a certain phase-retardation acts on zero-order diffracted light, the amplitudes of the zero-order and non-zero-order diffraction light are attenuated. After simulation verification, the modified filter can greatly improve imaging contrast.Diffraction-limited system is a low-pass filter. The imaging ability of the optical system is determined by the frequency response characteristics. The system coherent transfer function which has advantages compared to the star-point method and the resolution method can fully assess the system's frequency response. We regard phase contrast imaging system as a diffraction-limited system and have deduced coherent transfer function of phase contrast imaging system. It is conducive to analyze the frequency characteristics of phase contrast imaging system.The coherence of illuminating light is another important factor which affects the quality of phase contrast imaging. Existing phase contrast imaging studies usually take coherent light as illuminating light. However, the actual illuminating source has a certain size and spectral line width. When these factors reach a significant degree, coherence theory no longer applies to the analysis of phase contrast imaging. Instead, we should use partially coherent theory to explain it. We qualitatively describe phase contrast imaging under the partially coherent illuminating light. By numerical simulation, the differences between coherent illumination and partially coherent illumination are compared preliminary.Finally, the method presented in this article is proved by experiments. The experiment demonstrated the phase-amplitude transformation of sample can be achieved and the method is valid. We also analyze the specific situation of experiment, a novel method is proposed for phase contrast imaging by zero-order-block method, which is equivalent to the idea of interference contrast imaging. In addition, this method requires only simple conditions for experiment, so the experimental validation is very easy.