# A High-Dynamic-Range Optical Remote Sensing Imaging Method for Digital TDI CMOS
## 2 The Digital TDI Imaging Model and Analysis of Its Features  
Then we discuss the particularity of digital TDI CMOS by comparative analysis and experimentation with the TDI CCD. The digital TDI CMOS completes the shifting accumulation of images in the form of digital signals within the register, while the TDI CCD completes the accumulation in the form of electric charges within the sensor. This is the essential difference between them. We assume that the TDI stage is set as \(N\). During the process of digital TDI pushbroom imaging, the image of the scene produced by the optical lens rapidly moves on the surface of image sensor. At one point, the CMOS closes the electronic shutter after one-time exposure and obtains an image of \(N\) lines; when the image of the scene moves the displacement of one pixel, the CMOS obtains another image of lines, and then repeats the above process. Hence, each point of the scene is shot \(N\) times and recorded in different rows of the consecutive \(N\) images. Afterwards, we perform row-by-row shifting accumulation of the \(N\) images, which can correct the motion-blurring and obtain the digital TDI image of the scene. In order to simplify experimental conditions while not going against the basic purpose and principle to verify the HDRI method, we choose to image the still scenes rather than the moving scenes. There are two advantages in doing so. First, we can use the same CMOS camera to simulate the digital TDI and TDI CCD imaging process, which excludes the interference of other factors except the imaging method. Specifically, the simulative digital TDI CMOS obtains \(N\) images in the same exposure time and adds them up to obtain the integration image. The simulative TDI CCD uses the \(N\)-fold exposure time to get the equivalent integration image in the charge domain. Second, it greatly simplifies the experiments, and helps to simulate the complex scenes with targets that have low illumination or low contrast. The experimental arrangement is shown in Figure 2. The scene includes a simulated cloud and simulated building with elevated bright roofs and dark shadows. The experiment setup can simulate the typical remote sensing scene with high contrast. By adjusting the luminance of the light source and the environment, we can set different conditions of contrast to check the efficiency of the algorithms. In the process of simulative digital TDI imaging, the exposure time of each image is set as 0.5 ms and the \(N\)-stage (TDI stage = \(N\)) image is the superposition result of \(N\) images. The simulation result of \(N\)-stage TDI CCD is the image obtained with an exposure time of (0.5 \(\times\)\(N\)) ms. When \(N\) = 10, for example, the obtained images of the two methods and their histograms are shown in Figure 3.  
Figure 3: \(Cont.\)  
Figure 2: Arrangement of simulated imaging experiments. It can be seen that it is difficult to discern the shadow-covered area in the image obtained by TDI CCD clearly. In comparison, the digital TDI CMOS has a worse effect, because the targets in the shadow have low illumination and low contrast, so they are easily drowned out by cumulative read noise and dark noise. On the other hand, owing to the fact that the pixels formed by digital value accumulation will only saturate but never overflow for the highlighted target, the digital TDI obtains an extraordinary advantage, as the details of targets on the 'roof' are clearer. This can also be verified in the low gray-scale area of the histograms. Most significantly, the digital TDI CMOS has a unique advantage over the TDI CCD. That is, the imaging process of digital TDI generates different exposure degree images from the 1 to \(N\) TDI stage in the register. Different exposure degrees mean different features of histograms and different expression capabilities of target details. Taking these \(N\) images as material, this paper aims to achieve HDRI ability for complex scenes by using the method of image processing before producing the final acquired image.