## 6 Conclusions  
Polarization measurement at the L1 point of the Earth and the Moon can directly obtain the polarization information of the Earth and the Moon, which can avoid the fitting error caused by the fitting of data from different time series observations. In order to solve the problem that the current polarimeter mainly uses time-sharing to obtain the same target or uses multiple detectors to measure the target, this paper innovatively proposes the concept of a Multi-Spectral Polarization Imager, as shown in Figure 4, which uses a unique method of combining detector surface pixel coating with aperture division polarization information. A complete imaging function for nine spectral bands of 400-420 nm, 433-453 nm, 460-480 nm, 480-500 nm, 522-542 nm, 545-565 nm, 590-610 nm, 660-680 nm and 845-885 nm on a single detector is achieved, which is designed to serve the Earth scene recognition application. The system integrates a four-channel aperture dividing system at the back end of the common aperture telescope system with a focal length of \(f=1016\) mm. In order to achieve a \(2.5^{\circ}\) field of view for the purpose of global observation of the Earth, the focal length of the four-channel lens is set to \(f=200\) mm, corresponding to a spatial resolution of 10 km. While meeting the requirements of lightweight and miniaturization, the MEPI can obtain the Earth's shape, polarization information and spectral information. Among them, geosynchronous polarization measurement can effectively improve the real-time performance of Earth polarization measurement. The optical design results showed that the MEPI has enough image quality to support scene recognition. The spatial resolution of each channel is 10 km. The RMS radius of the system is less than 9.6 \(\upmu\)m in the full field of view. At 52 cycles/mm, the minimum MTF of each channel of the system is higher than 0.5. The images of the four channels can be collected synchronously on a single 53 mm \(\times\) 29.4 mm sensor. It is of great significance to regard the Earth as an exoplanet for global synchronous polarization observation. The atmospheric composition is affected by human activities, and the dynamic fluctuation of concentration profoundly changes the radiation balance. Aerosols are derived from volcanoes, dust, and anthropogenic emissions, and the uneven spatial and temporal distribution interferes with light propagation. The structure of ice crystals and water droplets in the cloud layer is variable, and the difference in optical properties significantly dominates the radiation transmission. Each link of the water cycle is linked to regulate the surface, dry and wet. Through polarization observation technology, we can capture the differences and changes of these elements on the second to day scale, and provide key data for revealing the internal mechanism of the Earth system, predicting climate change and monitoring natural disasters. In view of the highly dynamic nature of the Earth system and the close coupling among the various components, the simultaneous observation of the Earth can not only capture instantaneous phenomena, but also construct long-term time series data sets, open up new horizons for research in the field of earth science, and promote the in-depth exploration of the unique life maintenance mechanism and environmental change law of this 'exoplanet'. Conceptualization, W.Z., W.F. and X.Y. ; methodology, W.Z., Y.L. and X.Y. ; software, W.Z., Y.L. and Q.Z. ; validation, W.Z. and X.Y. ; formal analysis, W.Z. and C.Z. ; investigation, W.Z. ; resources, X.Y. ; data curation, W.Z. ; writing--original draft preparation, W.Z. ; writing--review and editing, W.Z. ; visualization, W.Z. ; supervision, W.F. and X.Y. ; project administration, W.Z. ; funding acquisition, X.Y. All authors have read and agreed to the published version of the manuscript. This research was supported by the National Key R&D Program of China, under grant Nos. 2022 YFB3903200 and 2022 YFB3903201. Not applicable.