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. 2018 Nov 21;7:92. doi: 10.1038/s41377-018-0094-x

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© The Author(s) 2018

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 2 — a Combined foci (sum projection) at the distal MMF facet demonstrating the power uniformity of focusing at different locations. b Intensity distribution and azimuthally averaged profile of a chosen focal point (indicated by a blue square in panel a) in logarithmic scale. c Validation of fluorescent imaging using 4-µm fluorescent particles. d–h Assessment of spatial resolution using a negative USAF-1951 test target. Image of the test target (d) and contrast measurements of selected target elements with 265 line pairs per millimetre (lp/mm) (e), 512 lp/mm (f), and 645.1 lp/mm (g). h Imaging contrast as a function of the normalised spatial frequency for the target elements in (d), and the fit of the modulation transfer function (MTF) corresponding to an aberration-free imaging system with a circular aperture. The fit allows an estimation of the effective numerical aperture (NA) as 0.225 ± 0.008 and the corresponding resolution limit (Abbe criterion) as 1.18 ± 0.04 µm