相衬成像中的空间滤波研究

【作者】 袁圣平；

【导师】 李重光；

【作者基本信息】 昆明理工大学 ， 光学， 2011， 硕士

【摘要】 相衬成像基于空间滤波方法将物体的相位信息转化为相应的振幅信息,实现透明相位物体的衬度显示。相衬成像的关键在于改变频谱的相位,它的本质是空间频域滤波。本文从空间频域滤波和光学系统空间频率响应特性的角度去探究相衬成像。相衬成像系统中空间频域滤波所使用的滤波器是相位滤波器,传统的做法是对零级衍射光产生一定的相位补偿,但不改变光波振幅。在此基础上我们提出了两种不同于传统做法的新思路。一是给非零级衍射光一定的相位补偿,等效代替对零级衍射光的相位改变,这种方法能够获得与传统做法相同的衬度图像,并且有利于相位滤波器的工艺制作；二是给零级衍射光一定的相位补偿的同时,给零级和非零级衍射光一定的振幅衰减,经模拟验证,改进后的滤波器可使成像衬度大大提高。衍射受限系统是一个低通滤波器,系统的频率响应特性决定了光学系统的成像能力,系统传递函数能够全面地评定系统的频率响应特性,与星点法和分辨率法相比具有优越性。我们将相衬成像系统视作衍射受限系统,求出了相干照明下相衬成像系统的相干传递函数,它有利于相衬成像系统频率特性分析。照明光源相干性是影响相衬成像质量的另一个重要因素,现有的相衬成像研究中通常将相干光源作为照明光源。然而实际光源均有一定大小和谱线宽度,当这些影响因素大到一定程度时,相干理论不再适用于相衬系统成像的分析,而应使用部分相干理论来解释。我们定性地描述了部分相干光照明下的相衬成像,通过数值模拟初步比较了相干和部分相干光照明下相衬成像的差别。本文最后对文中所述方法进行了实验验证,实验实现了观测样品的相衬成像。我们还结合实验的具体情况进行分析,分析讨论了一种相衬成像方法——遮挡零频法,它与干涉相衬成像法思想相同,但该方法对实验环境要求更低,便于实现。更多还原

【Abstract】 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.更多还原