## 2 Imaging Capture System  
The surface quality is determined by the waviness, roughness, and microdefects of the detection object. Since the minimum defect size is 0.005 mm\({}^{2}\) and the size of the detection object is bigger than 150*60 mm, it is difficult for the plane array camera to achieve such a high resolution with only a single shot. In order to improve the imaging efficiency, the line-scan cameras were applied for the imaging capture system. In the classical detection system of ordinary transparent glass, the dark-field imaging system was always the first choice [4; 6; 24; 28], as the low-angle light irradiates on the smooth glass plane, and the CMOS camera only accepts the scattered light of defects since the scattered light of the background is always very weak. However, the scattering properties are really different when the surface of the glass is covered with ink, as the scattered light of background becomes stronger, thus bringing more noise disturbance to the image and resulting in a poor imaging performance of shallow scratch and dent defects. The imaging characteristics of different defects can be very different. In order to obtain a high-quality image for the screen printing mobile phone back cover glass, a coaxial bright-field (CBF) imaging system and low-angle bright-field (LABF) imaging system are applied in this paper; the bright-field type of imaging system captures the reflected light, as shown in Figure 1. The direction of the reflected light is opposite to that of the incident light with the help of the crucial optical element beam-slitter in the CBF system, as shown in Figure 1. A higher contrast image of the shallow scratch defect and discoloration defect can be obtained in CBF while the imaging quality of the dent defect is higher in the LABF system. The low angle means the angle between the direction of incident light and the direction of X axis is small. Two 8K line-scan CMOS cameras are utilized to capture the images with the unit cell size of the CMOS sensor being 7.04 um \(\times\) 7.04 um. Imaging systems are fixed above the transmission system. To avoid the mutual interference of incident light sources, the focus position of CBF system is at a distance of about 30 mm away from that of the LABF system. Normal scratch defects can be easily obtained with a high variance value. However, sometimes, there are some scratch defects with a shallow depth below 50 nm. The shallow defect is too weak to be detected in the dark-field imaging system. As shown in Figure 2, the average grayscale value of the images of the CBF system is higher than that of LABF. The region of the shallow scratch defect has a higher variance value in the CBF system, as shown in Figure 2c. The dent defect is produced by the surface irregular waviness. The region of the dent defect has a much higher variance value in the LABF system, as shown in Figure 3c. Figure 1: Sketch of the coaxial bright-field (CBF) imaging system and low-angle bright-field (LABF) imaging system, composed of an light emitting diode(LED) light source, collimating lenses, beam-splitter, imaging lens, and CMOS. Figure 2: Grayscale distribution of the shallow scratch: (**a**) The image of the shallow scratch defect in the CBF system; (**b**) The image of the shallow scratch defect in the LABF system; (**c**) Grayscale distributions of the shallow scratch in the CBF system and LABF system. The discoloration defect is produced by non-uniform screen printing. The region of the discoloration defect has a much higher variance value in the CBF system, as shown in Figure 4c. As shown in Figure 5, the typical defect images and the corresponding ground truth annotations are presented. It can be seen that the surface of MPBG contains various defects with an unevenly distributed background because of the various structures. The contrast of the shallow scratch can be very low, and the size of the dent can be very small while the size of the discoloration can be very large.