## 3 Experimental Setup and Results
The linear scaling of the retrieved CNR strongly depended on the FWHM of the speckles' pixel intensity distribution for the same number of independent iterations. The contrast and resolution can be seen in the horizontal section of the normalized Y-axis data of the vertical bar of the letter \"u\" plotted in Figure 2c, where the experimental results of the CNR agree well with the evolution of the CNR in Figure 2d. As the FWHM of the speckles' pixel intensity distribution decreased, the CNR degraded gradually, as displayed in Figure 2b, as did the quality of the images. However, because of the well-known trade-off between resolution and CNR, the resolution did not show the same change in this case. Figure 1: Schematic of the microscale pseudothermal ghost imaging experimental setup and the used object. Because of the laser-cutting fabrication process, the object size is certified by conventional microscopy. To have a better understanding of the CNR enhancement of ghost imaging by the speckles' pixel intensity distribution from the nonuniform pseudothermal light in our experiment, we recorded and retrieved images for different peak positions of the Gaussian-like speckles' pixel intensity distribution; the results are summarized in Figure 3. As can be seen in Figure 3a, when moving the position of the pixel intensity distribution peak from the pixel intensity of 80 first to 127, and then to 170, whilst ensuring nearly the same FWHM of the speckles' pixel intensity distribution, the image quality showed a sudden change. Figure 3b shows the image quality inspected by the naked eye. The quality increased from the left position of the speckles' pixel intensity distribution until it reached a maximum, at which point the speckles' pixel intensity distribution was at the center position, and then the quality dropped. As shown in Figure 3c, it was very easy to distinguish the horizontal section of the normalized Y-axis data of the vertical bar of the letter \"\(\mu\)\" if the speckles' pixel intensity distribution was in the center position compared to others. Finally, we estimated the CNR as shown in Figure 3d, providing a quantitative confirmation of the qualitative inspection results. The corresponding CNR of the ghost image started from 0.43, reached a peak at 1.1, and then dropped to 0.7. Thus, the CNR was mainly determined by the FWHM of the speckles' pixel intensity distribution. Therefore, we came to the following conclusion: the best CNR of a ghost image can be achieved if the mean speckle pixel intensity is in the  
Figure 2: The evolution of the contrast-to-noise ratio (CNR) and reconstruction of the object image. (**a**) The measured speckles’ pixel intensity distribution with different full widths at half maximum (FWHMs). Only one symbol is displayed for every 10 data. (**b**) The retrieved images of the letter “\(\mu\)” in different FWHMs of the speckles’ pixel intensity distributions. (**c**) Solid blue lines are the horizontal line-outs of the normalized data of the retrieved ghost images and red dashed curves display the object. (**d**) The CNR as a function of the FWHM of the speckles’ pixel intensity distribution from the nonuniform pseudothermal light. center of the dynamic range of the CCD camera and the distribution has the maximum FWHM. The slight discrepancy may be because of some uncertainties in the FWHM of the speckles' pixel intensity distribution. It is hard to keep the FWHM the same in different peak positions of the speckles' pixel intensity distribution without adjusting the parameters of the CCD camera. At least our data prove that the widest FWHM at the center position of the speckles' pixel intensity distribution can achieve the best CNR for the given speckle size and object in microscale ghost imaging. The observed scaling depicted in Figures 2d and 3d, corresponding to Figures 2a and 3a, has never been reported in the literature. These experimental results reveal some interesting properties of the speckles' pixel intensity distribution on ghost imaging CNRs.