# Nonuniform Background Correction for Wide-Field Surveillance Camera
## 2 Analysis of the Space Surveillance Image  
The wide-field surveillance images used in this paper are taken by a surveillance system. The sensor is a CMOS image sensor of ChangGuangChenXin company. Its Spectral response is shown in Figure 1. The sensor specifications are shown in Table 1. The dark current with different temperatures is shown in Figure 2.  
Firstly, we obtained surveillance images through ground-based observations. Figure 2(a) is the original two-dimensional image, Figure 2(b) is the one-dimensional image, Figure 2(c) is the 3D image with a size of 30 \(\times\) 30 pixels, and Figure 2(d) is the 3D image with a size of 100 \(\times\) 100 pixels, which can show more details in multiple dimensions. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Photosensitive area & 61.44 mm \(\times\) 61.44 mm \\ Pixel size & 10 \(\upmu\)m \(\times\) 10 \(\upmu\)m \\ Number of active pixels & 6144 (H) \(\times\) 6144 (V) \\ Shutter type & Rolling shutter with global reset \\ Dark current & As shown in Figure 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The sensor specifications. Figure 1: Spectral response of the CMOS sensor. From Figure 3a, we can see that the surveillance images are seriously affected by nonuniform backgrounds. This is because our wide-field imaging system uses fiber image transmission components, which will bring serious nonuniformity to the imaging system. At the same time, due to the loss of the fiber image transmission components in the  
Figure 3: Influence of nonuniform background on wide-field surveillance image in complicated situations. (**a**) Original surveillance image, (**b**) one-dimensional analysis of the nonuniform background, (**c**) 3D display of the target submerged in the nonuniform background, (**d**) three-dimensional analysis of the nonuniform background. Figure 2: Dark current vs. die temperature. image transmission process and the discrete sampling characteristics of the fiber image transmission components themselves, its application will inevitably affect the signal-to-noise ratio, modulation transfer function, and optical transmittance. Therefore, we can see from Figure 3d that the image quality of the star point is degraded seriously, which is almost submerged in the nonuniform background. Secondly, we can also see a lot of noise in the space surveillance image. This noise mainly comes from the sensor and sensor circuit, including reset noise, quantization noise, photon shot noise, and dark current. Moreover, it will also be affected by space radiation noise. As shown in Figure 3b, space targets or stars are basically submerged in dark currents with strong energy, which is known as hot pixels. Hot pixels constitute a part of the nonuniform background of the space surveillance image. Finally, from Figure 3b, we can also see that the background of the space surveillance image also contains the sensor defect, which is due to the fact that our sensor adopts the test piece and has a certain degree of sensor defect itself. This can simulate the possible local damage of the sensor caused by some potential factors, such as direct sunlight or space radiation, when the surveillance camera works in orbit for a long time. The star in Figure 3c,d is the display effect of the same star in different dimensions. The star is strongly affected by the nonuniform background and is basically submerged in the nonuniform background. In general, on the one hand, wide-field and long exposure bring better target detection ability to space surveillance. On the other hand, it will also bring serious nonuniformity to the image. These factors mainly include fiber image transmission components, dark current, possible detector defects, and inevitable stray light. These factors bring serious challenges to space target detection and tracking. In order to better detect space targets, we need to correct the nonuniform background of the space surveillance image, which is also an essential part of improving the imaging performance of the surveillance camera.