Abstract
Fourier ptychographic (FP) microscopy is a coherent imaging method that can synthesize an image with a higher bandwidth using multiple low-bandwidth images captured at different spatial frequency regions. The method's demand for multiple images drives the need for a brighter illumination scheme and a high-frame-rate camera for a faster acquisition. We report the use of a guided laser beam as an illumination source for an FP microscope. It uses a mirror array and a 2-dimensional scanning Galvo mirror system to provide a sample with plane-wave illuminations at diverse incidence angles. The use of a laser presents speckles in the image capturing process due to reflections between glass surfaces in the system. They appear as slowly varying background fluctuations in the final reconstructed image. We are able to mitigate these artifacts by including a phase image obtained by differential phase contrast (DPC) deconvolution in the FP algorithm. We use a 1-Watt laser configured to provide a collimated beam with 150 mW of power and beam diameter of 1 cm to allow for the total capturing time of 0.96 seconds for 96 raw FPM input images in our system, with the camera sensor's frame rate being the bottleneck for speed. We demonstrate a factor of 4 resolution improvement using a 0.1 NA objective lens over the full camera field-of-view of 2.7 mm by 1.5 mm.
Highlights
Fourier ptychographic microscopy (FPM) is a recently developed computational imaging system capable of acquiring the complex and quantitative field distribution of a sample [1, 2]
FPM achieves this by a simple modification in sample illumination without the need for a separate reference beam or mechanical movement within the system as in other phase imaging systems
An iterative algorithm that reconstructs the aberration of the microscope system simultaneously with the sample spectrum allowed for removal of spatially varying aberrations throughout the microscope’s field of view [8] and made FPM suitable for imaging samples with uneven surfaces [9]
Summary
Fourier ptychographic microscopy (FPM) is a recently developed computational imaging system capable of acquiring the complex and quantitative field distribution of a sample [1, 2]. It uses a coherent light source to image different components of the sample’s Fourier spectrum, and uses a phase retrieval algorithm to synthesize these images into a high-resolution complex field distribution It can linearly improve the numerical aperture of the imaging lens by the illumination NA. [26] demonstrated for the first time using a high-power laser beam coupled with a DMD which allowed for shot-noise-limited image capturing process, overcoming the power limitation of LEDs. Ref. The FOV was limited to around 50 μm by 50 μm for a 0.04 NA 1.25x objective, which is much smaller than the FOV typically offered by such an objective lens Another feature overlooked by many FPM illumination schemes implemented so far is the efficient usage of the illumination beam to improve capturing speed: an LED’s radiation profile typically follows a Lambertian distribution [27] and only a small portion of it ends up illuminating the sample; and a DMD only utilizes a tiny fraction of the input laser beam for each sample illumination angle. Our laser FPM demonstrates wide FOV and high quality image reconstruction at a high capturing speed
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