Vignetting effect in Fourier ptychographic microscopy

TL;DR

This paper investigates how vignetting effects in Fourier ptychographic microscopy, caused by space-variant optical phenomena, impact image reconstruction quality and proposes models and countermeasures to mitigate these artifacts.

Contribution

The study introduces a space-variant model for FPM considering diffraction effects and validates countermeasures to reduce vignetting artifacts.

Findings

Vignetting significantly affects FPM image quality in practical setups.

A modified space-variant model better explains the vignetting effects.

Countermeasures can partially alleviate vignetting-induced artifacts.

Abstract

Fourier ptychographic microscopy (FPM) is a computational imaging technique that overcomes the physical space-bandwidth product (SBP) limit of a conventional microscope by applying angular diversity illuminations. In the usual model of FPM, the microscopic system is approximated by being taken as space-invariant with transfer function determined by a complex pupil function of the objective. However, in real experimental conditions, several unexpected "semi-bright and semi-dark" images with strong vignetting effect can be easily observed when the sample is illuminated by the LED within the "transition zone" between bright field and dark field. These imperfect images, apparently, are not coincident with the space-invariant model and could deteriorate the reconstruction quality severely. In this Letter, we examine the impact of this space-invariant approximation on FPM image formation based on ray-based and rigorous wave optics-based analysis. Our analysis shows that for a practical FPM microscope with a low power objective and a large field of view, the space invariance is destroyed by diffraction at other stops associated with different lens elements to a large extent. A modified version of the space-variant model is derived and discussed. Two simple countermeasures are also presented and experimentally verified to bypass or partially alleviate the vignetting-induced reconstruction artifacts.