Why Phase Imaging？
The above figure shows two normal cases explaining how the intensity distribution reflects the phase of the optical field according to the object which distorts or reflects the optical rays. In Figure (a), suppose the object is transparent, it does not change the amplitude of the light passing through it, but introduces phase delays due to the refractive index. The delays reflect the shape and density of the object, which can be measured. Therefore the material properties of the object, such as pressure, temperature, humidity can be obtained from the measured phase delay. In a reflection mode, as figure (b) shows, the measured intensity carries information about the topology of a reflective object, which can be used for surface profiling.
In the research of optical imaging, both amplitude and phase of light wave are necessary. Since the phase carries important information about a wavefront, we can use it to measure the shape, surface profile and other properties of an object. However, it is difficult to measure the phase by general camera sensors due to its high frequency. Therefore, we need to use special techniques to rebuild both the amplitude and phase of a light wave.
|We propose a fast-converging algorithm for wavefront calculation using single-beam illumination. The captured intensity images are resampled to a series of intensity images, ranging from highest to lowest resampling. Phase calculation at a lower resolution is used as the initial solution phase at a higher resolution. Iterations on the low-frequency components do not need to be performed on the higher-frequency components, thus making the convergence of the phase retrieval faster than with the conventional method.|
– Ni Chen, Jiwoon Yeom, Keehoon Hong, Gang Li, and Byoungho Lee∗, “Fast converging algorithm for wavefront reconstruction based on a sequence of diffracted intensity images , ” Journal of the Optical Society of Korea 18(3):217– 224, 2014.
– Ao Zhou, Wei Wang, Ni Chen*, Edmund Y. Lam, Byoungho Lee, and Guohai Situ. “Fast and robust misalignment correction of Fourier ptychographic microscopy for full field of view reconstruction”Optics Express, 26(18):23661-23674, Aug. 2018.
– Ao Zhou, Ni Chen*, Haichao Wang, and Guohai Situ. “Analysis of Fourier ptychographic microscopy with half of the captured images”Journal of Optics, 20(9):95701, Jul. 2018.