Citation: | Xie Jing, Hu Yongfu, Tang Qi, et al. Automatic exposure technology for space camera[J]. Opto-Electronic Engineering, 2018, 45(11): 180033. doi: 10.12086/oee.2018.180033 |
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Overview: The automatic exposure technology controls the imaging brightness by adjusting the exposure time of thecamera, so that the image is suitable for visual observation. In space field, space camera based on CMOS image sensorhas been widely used for its high integration, small volume and light weight. It plays a very important role in spacecraftkey action records, earth remote sensing and near earth observation. Because of the particularity of space environment, the irreversibility of space tasks and the lag of manual intervention, the camera can be intelligent and autonomous, which is very important. The camera can quickly and automatically adjust the imaging parameters and get the best imaging results for the first time. In view of the particularity of space environment and the irreversibility of space tasks, afast adaptive exposure algorithm is proposed, which can separate the target from the background and adjust the exposure time quickly and steadily. It is very suitable for deep space exploration and remote sensing. First of all, taking theremote sensing image as an example, the imaging link simulation is carried out. Special software is used to analyze theimaging link and get the target radiance. On the basis of the simulation analysis of the imaging link, according to theimaging characteristics of the lens and the sensitivity of the image sensor, the energy of the target to reach the imagesensor and the number of electrons converted from the lens are calculated to estimate the exposure time range, thussetting the initial exposure value. The target and background are separated by simple histogram distribution statistics.The different weights are applied to the target and the background. The luminance characteristics of the image are calculated by the weighted statistics. The exposure time is obtained by the look-up table method according to the result ofthe luminance characteristic. In the process of adjusting the exposure time, a dual target adjustment range is set up tomake the auto exposure algorithm converge well. The algorithm can be conveniently implemented on FPGA with lessresource occupancy and no complex operation. The algorithm is fast in computing speed and large in throughput, andcan be easily transplanted on various platforms. The experimental results show that the algorithm can quickly and stablyobtain the best exposure time, fast convergence speed, high stability, and less resource occupancy, which is very suitablefor space scene detection. The correlation algorithm has been successfully applied to multiple on orbit models, and alarge number of effective images have been obtained.
High contrast scene image
Imaging link of space camera
Simulation process of imaging link of space camera
Weighting coefficient setting
The background recognition area of the image. (a) The image taken by a camera; (b) DN < 16 dark background area; (c) 16 < DN < 32 background transition region
Coefficient of image lookup table
Automatic exposure control flow chart
Block diagram of automatic exposure based on FPGA
Automatic exposure image of camera
Image effect of under exposure
Image effect of overexposure
A special scene image taken by a camera (outside scene)
The histogram of a special scene. (a) Full image histogram; (b) Remove background histogram
An on-track image taken by an automatic exposure. (a) Imaging effect of Chang'e three landing camera; (b) Imaging effect of technical test camera in orbit; (c) Automatic exposure imaging effect of video camera on orbit