Mr. Raymond Li Profile

Raymond Li

at Western Univ

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Proceedings Article | 13 March 2024 Presentation + Paper

Optical resolution and MTF of a low-cost Fourier ptychography microscope using a Raspberry Pi computer

Haechan Yang, William Wasswa, Raymond Li, Brandon Pautler, Aidan Fry, Rajan Leung, David Holdsworth, Ian Cunningham

Proceedings Volume 12832, 1283205 (2024) https://doi.org/10.1117/12.3003269

KEYWORDS: Modulation transfer functions, Microscopes, Optical resolution, Light emitting diodes, Image restoration, Optical testing, Spatial resolution

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Introduction: Low-cost optical microscopes are under development by several groups to bring point-of-care malaria diagnosis services to under-served communities in low-resource settings. We describe a Fourier ptychography microscope (FPM) system based on an open-source design and a method of measuring spatial resolution in terms of the optical modulation transfer function (MTF) using images of the 1951 USAF resolution test device.

Methods: The FPM system uses a Raspberry Pi computer and 196-LED matrix light source. FPM images were reconstructed using in-house Python code. The MTF was determined from bar-patterns using a least-squares analysis to fit a square wave, including odd-harmonic terms, to image profiles through the bars. These were normalized to larger uniform regions of the pattern and combined to generate the MTF.

Results: Total component cost of the FPM was less than $200. The theoretical diffraction limit imposed by the pupil function of this system was 280 cycles/mm, slightly less than the measured MTF 10% frequency of 300 cycles/mm. The 10% frequency in FPM images was 550 cycles/mm.

Conclusions: The USAF test pattern provides a practical method for assessing FPM performance in terms of the achieved MTF of the $200 Raspberry Pi based Fourier ptychography microscope. The limiting frequency in FPM images, 550 cycles/mm, was slightly less than the Nyquist sampling cut-off frequency of 670 cycles/mm imposed by pixel spacing.

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