*Article* **Research on Polarized Multi-Spectral System and Fusion Algorithm for Remote Sensing of Vegetation Status at Night**

**Siyuan Li, Jiannan Jiao and Chi Wang \***

> Department of Precision Mechanical Engineering, Shanghai University, Shanghai 200444, China; lsyyj@shu.edu.cn (S.L.); m160018@e.ntu.edu.sg (J.J.)

**\*** Correspondence: wangchi@shu.edu.cn; Tel.: +86-021-6613-0801

**Abstract:** The monitoring of vegetation via remote sensing has been widely applied in various fields, such as crop diseases and pests, forest coverage and vegetation growth status, but such monitoring activities were mainly carried out in the daytime, resulting in limitations in sensing the status of vegetation at night. In this article, with the aim of monitoring the health status of outdoor plants at night by remote sensing, a polarized multispectral low-illumination-level imaging system (PMSIS) was established, and a fusion algorithm was proposed to detect vegetation by sensing the spectrum and polarization characteristics of the diffuse and specular reflection of vegetation. The normalized vegetation index (NDVI), degree of linear polarization (DoLP) and angle of polarization (AOP) are all calculated in the fusion algorithm to better detect the health status of plants in the night environment. Based on NDVI, DoLP and AOP fusion images (NDAI), a new index of night plant state detection (NPSDI) was proposed. A correlation analysis was made for the chlorophyll content (SPAD), nitrogen content (NC), NDVI and NPSDI to understand their capabilities to detect plants under stress. The scatter plot of NPSDI shows a good distinction between vegetation with different health levels, which can be seen from the high specificity and sensitivity values. It can be seen that NPSDI has a good correlation with NDVI (coefficient of determination R<sup>2</sup> = 0.968), PSAD (R<sup>2</sup> = 0.882) and NC (R<sup>2</sup> = 0.916), which highlights the potential of NPSDI in the identification of plant health status. The results clearly show that the proposed fusion algorithm can enhance the contrast effect and the generated fusion image will carry richer vegetation information, thereby monitoring the health status of plants at night more effectively. This algorithm has a grea<sup>t</sup> potential in using remote sensing platform to monitor the health of vegetation and crops.

**Keywords:** vegetation health monitoring; remote sensing; NDVI; polarization; image fusion
