*2.1. Plant Materials and Experimental Design*

The spectral and polarization data analyzed in the study are from the leaves of plants on the campus of Shanghai University: spotted laurel (*Aucuba japonica*), money plant (*Epipremnum aureum*) and Malabar chestnut (*Pachira aquatica*). Figure 1a is a potted money plant in the laboratory, and Figure 1a1,a2 are the regions of interest for collecting vegetation index and polarization data, respectively. Figure 1b shows the leaves of spotted laurel, money plant and Malabar chestnut, and Figure 1b1–b5 shows the regions of interest collected by time series experiment at 3-day intervals. Figure 1c is a spotted laurel on the campus of Shanghai University, and Figure 1c1–c4 is defined as the healthy leaf region, level-2 stress leaf region, level-1 stress leaf region and withered leaf region, respectively.

**Figure 1.** Plant materials. (**a**) Money plant: (**a1**,**a2**) are the regions of interest for the NDVI and polarization data collection, respectively. (**b**) The leaves of a spotted laurel, money plant and Malabar chestnut: (**b1**–**b5**) are the images of the changes of a money plant leaf every 3 days. (**c**) A spotted laurel at Shanghai University: (**c1**–**c4**) are the healthy leaf region, the level-2 stress leaf region, the level-1 stress leaf region and the withered leaf region, respectively.

Three experiments were designed in this paper, including: (1) an experiment was carried out under different illuminances of 0.01, 0.1, 0.5, 1 and 5 lux on a pot of money plant in a dark laboratory room, and the LED light group and LED controller were used to control the illumination of the dark room. The illumination was measured with a high-precision illuminometer (model: tes1330A, Taishi). (2) In the dark laboratory room, a time series experiment was carried out to collect data on the leaves of spotted laurel, money plant and Malabar chestnut every three days. Vegetation leaves were pasted on a display board printed at Shanghai University and placed in a dark room. The LED light group and LED controller were used to control the illumination of the dark room, and a high-precision illumination meter (model: tes1330A, Taishi) was used to measure the illumination. (3) An outdoor experiment was conducted on a spotted laurel on campus at night.

### *2.2. Polarized Multispectral for Low-Illumination-Level Imaging System*

The polarized multispectral for low-illumination-level imaging system (PMSIS) developed for outdoor plant imaging at night consists of a scientific CMOS (SCMOS) monochrome camera with 2048 pixels × 2048 pixels (model: PCO. edge 4.2, PCO, GER), coupled to a Nikon MF 50-mm (1:1.8) fixed focus camera lens, narrow band interference filters fitted on a rotating filter mount and a linear polarizer fitted on a rotating polarizer mount (Figure 2). Blue (FF02-482/18-25) and green (FF02-520/28-25) band filters of Semrock and red (FB680-10) and near-infrared band (FB760-10) filters of Thorlabs were used. The camera was connected to a desktop computer, and the control and data acquisition were carried out through a Cameralink port.

**Figure 2.** (**a**) Indoor polarized multispectral for a low-illumination-level imaging system. (**b**) Outdoor polarized multispectral for a low-illumination-level imaging system.

As shown in Figure 2a, an indoor polarized multispectral for low-illumination-level imaging system was established. In order to simulate a night environment, the system was placed in a dark room made of optical shading material, and the shading rate of the dark room was greater than 99%. During the experiment, the illuminance of the dark room was controlled at 0.01, 0.1, 0.5, 1 and 5 lux, respectively (Note: dark night: 0.001–0.02 lux, moonlit night: 0.02–0.3 lux and cloudy indoor: 5–50 lux). The illuminance of the dark room was controlled by the LED lamp set and LED controller to carry out the experiment of monitoring the vegetation health status with different illuminances at night.

As shown in Figure 2b, an outdoor polarized multispectral for a low-illuminationlevel imaging system was built to conduct an outdoor night monitoring experiment on a

spotted laurel. The time selected was from 8:00 at night. The ambient light included an atmospheric glow, starlight, moonlight and city light. The outdoor experiment location was on the campus of Shanghai University, Baoshan District, Shanghai (Season: Winter (Longitude: 121◦241.95", Latitude: 31◦1910.57")), and the illuminance of 0.22 lux was actually measured with an illuminance meter.

### *2.3. Measurement of Chlorophyll and Nitrogen Content*

The chlorophyll and nitrogen contents of plant leaves were measured with a handheld chlorophyll analyzer (Medium Kelvin, Model: TYS-4N). The chlorophyll analyzer can measure the relative chlorophyll (SPAD) and nitrogen contents (mg/g), leaf moisture (RH%) and temperature of plants instantly. In the test, two LED light sources emit light in two wavebands, one with a red light (center wavelength: 650 nm), the other with an infrared light (center wavelength: 940 nm). After penetrating the blade, the light is received and processed by the receiver, and finally, the SPAD value is calculated and displayed on the screen.
