Quantitative phase imaging (QPI) utilizes refractive index and thickness variations that lead to optical phase shifts. This gives contrast to images of transparent objects. In quantitative biology, phase images are used to accurately segment cells and calculate properties such as dry mass, volume and proliferation rate. The fidelity of the measured phase shifts is of critical importance in this field. However to date, there has been no standardized method for characterizing the performance of phase imaging systems. Consequently, there is an increasing need for protocols to test the performance of phase imaging systems using well-defined phase calibration and resolution targets. In this work, we present a candidate for a standardized phase resolution target, and measurement protocol for the determination of the transfer of spatial frequencies, and sensitivity of a phase imaging system. The target has been carefully designed to contain well-defined depth variations over a broadband range of spatial frequencies. In order to demonstrate the utility of the target, we measure quantitative phase images on a ptychographic microscope, and compare the measured optical phase shifts with Atomic Force Microscopy (AFM) topography maps and surface profile measurements from coherence scanning interferometry. The results show that ptychography has fully quantitative nanometer sensitivity in optical path differences over a broadband range of spatial frequencies for feature sizes ranging from micrometers to hundreds of micrometers.

Phase calibration target for quantitative phase imaging with ptychography.

A wavelength scanning Ptychographic Iterative Engine (ws-PIE) is proposed to reconstruct high-quality complex images of specimens. Compared with common ptychography, which required the user to transversely scan the sample during data acquisition, the ws-PIE fundamentally reduces the data acquisition time and can avoid the heavy dependence on the accuracy of the scanning mechanism. This method can be easily implemented in the field of material and biological science as the wavelength-swept laser source is currently commercially available. The feasibility of the ws-PIE is demonstrated numerically and experimentally.

Ptychographic microscopy via wavelength scanning

pdf/ptychography-by-changing-the-area-of-probe-light-and-scaled-33yhbybsd5.pdf

Ptychography by changing the area of probe light and scaled ptychography

Because of the limited dynamic range of common detectors, high-frequency components weaker than the threshold of the detector used are lost in digital holography (DH) even with the aid of coherent diffraction imaging (CDI)-based iterative computation, and this makes DH less preferable for many interesting applications. To overcome this problem, the partially saturated diffraction pattern was adopted to enhance the resolution of DH iteratively. Since more high-frequency components were retrieved in comparison with our former CDI-based digital holographic method, the resolution of DH was improved further. The feasibility of the proposed method is demonstrated both numerically and experimentally.

Partial saturation-aided resolution enhancement for digital holography

Resolution improvement for enhancing interference fringes of digital holograms with near infrared illuminance

The image reconstructed in ordinary digital holography was unable to bring out desired resolution in comparison to photographic materials; thus making it less preferable for many interesting applications. A method is proposed to enhance the resolution of digital holography in all directions by placing a random phase plate between the specimen and the electronic camera and then using an iterative approach to do the reconstruction. With this method, the resolution is improved remarkably in comparison to ordinary digital holography. Theoretical analysis is supported by numerical simulation. The feasibility of the method is also studied experimentally.

High-resolution digital holography with the aid of coherent diffraction imaging.

A modified extended-ptychographical-iterative-engine (ePIE) algorithm is proposed to overcome the
disadvantages of ePIE technique and reduce the influence of stage hysteresis or backlash error. The exit wave
of a rotatable "screen" illuminated by plane wave is used as the illumination on the specimen, and the complex
transmission functions of the rotatable object and specimen can be simultaneously reconstructed. Compared with
the standard x-y scanning PIE algorithm, the proposed algorithm can completely avoid the influence of stage
hysteresis (or backlash error). The proposed algorithm also has higher convergence speed and better accuracy
than the standard PIE algorithm.

Phase imaging with rotating illumination

An algorithm is proposed to enhance the resolution of digital holography by retrieving the frequency components lost in common holograms. A pinhole is placed directly behind the specimen to record the hologram, and an iterative scheme commonly used in coherent diffraction imaging is adopted for the reconstruction. Since some of the frequency components lost in common digital holography can be properly retrieved, the resolution of the reconstructed image is remarkably improved. Theoretical analysis and computer simulations are presented to demonstrate the feasibility of this proposed technique.

https://iopscience.iop.org/article/10.1088/0256-307X/30/5/054209/pdf

Super-Resolved Digital Holography Based on the Coherent Diffraction Imaging Scheme

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High-resolution digital holography with the aid of coherent diffraction imaging.

Phase imaging with rotating illumination

Super-Resolved Digital Holography Based on the Coherent Diffraction Imaging Scheme