# Parallel Ghost Imaging with Extra Large Field of View and High Pixel Resolution
## 2 Experimental Methods and Results
The modulator is a 7-layer, 200-mesh sandpaper, and the speckle patterns have a significant impact on the quality of ghost imaging, which will be discussed and analyzed later. The photon energy used in the experiment is 15 keV. The sample chosen for this experiment is a fish brain with high complexity and low absorption contrast, aiming to challenge the ultimate capabilities of parallel ghost imaging. The imaging system consists of a 100 \(\upmu\)m thick scintillation crystal (LuAG:Ce), an optical conversion microscope (Model: Optique Peter MICRX016), and an sCMOS detector (Hamamatsu ORCA-Flash 4.0 C11440). The detector has an effective area of 6.6 \(\times\) 6.6 mm\({}^{2}\), pixel size of 6.5 \(\upmu\)m, and a resolution of 2048 \(\times\) 2048 pixels. The experimental setup for recording the object arm is shown in Figure 1a. Different energy beams are emitted from a bending magnet and filtered by a double-crystal monochromator to select a single energy. Slits are used before and after the monochromator to adjust the beam shape, and filters are employed to block stray light. The movement path of the sandpaper for each measurement is identical, ensuring that the speckle patterns recorded in the object arm correspond perfectly to the multiple sets of speckle patterns recorded in the reference arm. The monochromatic light, after passing through the scintillator, is converted to visible light. For recording the object arm, a 2\(\times\) magnification optical lens is used to capture the image of the sample coupled with the speckle pattern, as shown in Figure 1b. By switching to a 20\(\times\) optical magnification lens, the effective pixel size is reduced to 0.325 \(\upmu\)m, allowing the collection of speckle patterns with sub-micron-level high resolution. The experimental setup for recording the high-resolution reference arm is shown in Figure 1c, and the high-definition speckle image collected is shown in Figure 1d. By aligning and reconstructing the 10 \(\times\) 10 pixels from the reference arm with the corresponding pixel positions in the object arm, high-resolution parallel ghost imaging can be achieved. The reconstruction results in an image size of 2000 \(\times\) 2000 pixels, with a pixel resolution of 0.325 \(\upmu\)m/px, as shown in Figure 2. One of the main challenges in this process is aligning the two sets of data. We leave a margin of redundant pixels around the image edges and use the quality of the reconstructed image as a criterion. The alignment is adjusted to minimize the error to less than one reference arm pixel. The comparison of characteristics exhibited by various ghost imaging algorithms under a bucket detector array was completed in our previous work [30]. Among them, TGI (traditional ghost imaging) shows the worst performance. The reconstruction results of DGI (differential ghost imaging) are slightly better than those of TGI. Even at a higher sampling rate, the image quality of both TGI and DGI shows little improvement, and the discontinuity between subsystems is very obvious. TVAL3 (Total Variation Assisted Linearized Bregman Iteration) performs exceptionally well, as it can still reconstruct the object at a low sampling rate; however, block artifacts remain. We specifically propose global ghost imaging (GGI) for the ghost imaging framework based on a bucket detector array. In GGI, all ghost imaging subsystems are uploaded to a central processor where they are iteratively computed while sharing a global prior. This allows the iterative results to escape local optima and jointly find the global optimal solution. This method significantly improves imaging quality and drastically reduces the number of measurements required for high-resolution ghost imaging, even down to single-digit measurements. More importantly, it eliminates the discontinuities between the various ghost imaging subsystems. This method has been thoroughly validated through simulations and experiments with manually fitted bucket detector arrays.