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摘要:
调制传递函数（MTF）是遥感相机的重要评价指标，但是目前对于数字域 TDI CMOS 相机动态MTF特性研究十分有限，为了深入研究其像质下降机理，结合数字域TDI CMOS成像原理，建立了像元、电子快门、曝光时间、振动引起的数字域TDI成像MTF下降数学模型。结合推导模型开展了预估分析和实验验证。结果表明：传感器的有效像元区域分布会影响图像MTF，且开口率越小影响越大；CMOS传感器的卷帘快门会导致数字域TDI成像MTF下降，卷帘速度越慢影响越严重，其中卷帘速度从6 μs变为10 μs时，对应的图像MTF从0.191下降为0.177；曝光时间越短则MTF越高，尤其当存在低频像移失配时更为明显，曝光时间从180 μs减小为100 μs时，图像MTF从0.126提高为0.155，但同时也会影响图像信噪比，因此在实际应用中应合理选择曝光时间。
Abstract:The modulation Transfer Function (MTF) is an important evaluation index for remote sensing cameras, but at present, the research on the dynamic MTF characteristics of digital domain TDI CMOS cameras is very limited. In order to deeply research its image quality degradation mechanism, combined with the principle of digital domain Time Delay Integration Complementary Metal Oxide Semiconductor (TDI CMOS) imaging, a mathematical model of digital domain TDI imaging MTF degradation caused by pixel, electronic shutters, exposure time and vibration is established. Combined with the derived model, the prediction analysis and experimental verification are carried out. The results show that the regional distribution of effective pixels of the sensor will affect the image MTF with a greater intluence from smaller opening rates, and the rolling shutter of the CMOS sensor will lead to the decrease of the digital domain TDI imaging MTF with a more serious impact for slower rolling shutter speeds. When the rolling shutter speed changes from 6 μs to 10 μs, the corresponding image MTF decreases from 0.191 to 0.177. As the exposure time shortens, the MTF grows higher, especially when there is low-frequency image shift mismatch. When the exposure time is reduced from 180 μs to 100 μs, the MTF increases from 0.126 to 0.155 with some influence on the image signal-to-noise ratio. Therefore, the exposure time should be reasonably controlled in practical applications.
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Key words:
- TDI /
- image sensor /
- rolling shutter /
- MTF /
- image motion
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图 4 运动目标点在传感器投影成像为线
Figure 4. The moving target point is imaged as a line in the projection of the sensor
图 7 MTF与振动频率及曝光时间的关系图
Figure 7. Relationship between MTF, vibration frequency and exposure time
图 8 不同开口率下扇形靶标仿真图像：从左到右的开口率分别为0.8、0.5、0.3
Figure 8. Simulation images of the sector target under different aperture ratios: 0.8, 0.5 and 0.3 (from left to right)
图 10 行读出时间分别为（a）6 μs；（b）10 μs时不同卷帘速度的TDI图像
Figure 10. TDI images with different rolling shutter speeds. (a) 6 μs; (b) 10 μs
图 11 不同曝光时间的TDI图像：曝光时间（从上到下）分别为100、140、180 μs
Figure 11. TDI images with different exposure times. 100, 140 and 180 μs (from top to bottom)
表 1 图像MTF和SNR测试结果
Table 1. The MTF and SNR test results of images
测试图像 MTF SNR/dB 上图 0.155 17.496 中图 0.136 23.012 下图 0.126 26.004 
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