# Multi-Lens Array Full-Field X-ray Microscopy
## 4 Discussion  
For the first time, an X-ray full-field microscope at a laboratory source was built using a multi-focus CRL objective. Making use of the 9.25 keV Ga K\({}_{\alpha}\)-line, several non-overlapping images of a 2.4 \(\upmu\)m period absorption grating were generated with a contrast of 12%. The gain in contrast by a factor of 2.5 when imaging the same FoV with a \(4\times 3\) CRL array from slightly different directions was shown. The theoretical factor of 3.5 was not achieved  
Figure 6: (**a**) Ray-tracing simulation of the setup in Figure 3. (**b**) For increasing the image quality, the 25 individual images are relocated to a center position and summed up; both images are normalized to maximum intensity, and the axes units are in pixels.  
Figure 7: Sketch of a 3 \(\times\) 3 MFCRL array with a sample **1**; the rays demonstrate a ray **2** regularly passing the array; a ray **3** passing the horizontally focusing lens elements but missing the vertically focusing elements; and a ray **4** passing the vertically focusing lens elements but missing the horizontally focusing elements.  
due to not completely homogeneous illumination as well as rays passing through the material blocks between the CRLs. Using ray-tracing simulations, it could be shown that the structured image background between the individual images is created by those rays that have not completely passed through a single CRL but have exited laterally before passing through the entire lens.  
Given this positive proof-of-principle at X-ray energies below 10 keV, a setup for multi full-field microscopy at 24.2 keV is planned. This can be done by making use of the 24.2 keV In K\({}_{\alpha}\)-line, which is possible with, for example, an Indium alloy Excillum MetalJet source [27]. The setup presented here (Section 2.3) contains an alloy of 95% Ga and 5% In, resulting in an approximately 130 times less intense 24.2 keV In K\({}_{\alpha}\)-line, compared to the 9.25 keV Ga K\({}_{\alpha}\)-line [4]. For this reason, the 24.2 keV In K\({}_{\alpha}\)-line was not investigated for this proof-of-principle. The multi-focus CRLs could be improved by adding front and back apertures blocking all rays that did not pass the lenses correctly. Further improving the multi full-field microscopy method includes automated merging of the single images as well as making use of partly overlapping images.  
The following are available online at [[https://www.mdpi.com/article/10.3390/app11167234/s1](https://www.mdpi.com/article/10.3390/app11167234/s1)]([https://www.mdpi.com/article/10.3390/app11167234/s1](https://www.mdpi.com/article/10.3390/app11167234/s1)), Table S1: Summarized parameters of the used MFCRL, Table S2: All aperture sizes of the used MFCRL. The aperture \(A_{\rm i}\) is the same for lens element \(N_{\rm v,i}\) and \(N_{\rm h,j}\), i.e., for the ith vertically and ith horizontally focusing lens element.  
Conceptualization, A.O., D.M. and A.L.; Formal analysis, A.O. and A.L.; Funding acquisition, C.F., J.M. and A.L.; Investigation, A.O., D.M., C.F., A.B., J.M. and A.L.; Methodology, A.O., C.F. and A.L.; Project administration, J.M. and A.L.; Resources, A.O., D.M., C.F., A.B., J.M. and A.L.; Supervision, J.M. and A.L.; Validation, A.O., D.M., C.F., A.B., J.M. and A.L.; Writing--original draft, A.O., D.M. and A.L.; Writing--review and editing, C.F., A.B. and J.M. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
We thank the Karlsruhe Nano Micro Facility (KNMF) for the fabrication of the polymer X-ray optics and microworks GmbH for providing the sample grating. We further acknowledge the support of the Karlsruhe School of Optics and Photonics (KSOP, financed by the Ministry of Science, Research and the Arts of Baden-Wurttemberg as part of the sustainability financing of the projects of the Excellence Initiative II) as well as the support of the KIT-Publication Fund of the Karlsruhe Institute of Technology.  
The authors declare no conflict of interest.