**3. Results**

### *3.1. Experiments on Different Illumination of Vegetation*

Five groups of illuminance experiments were carried out: 0.01, 0.1, 0.5, 1 and 5 lux, and spectral data were collected to calculate the NDVI. Figure 5 shows the NDVI under different illuminances. The corresponding illuminances are 0.01 (Figure 5a), 0.1 (Figure 5b), 0.5 (Figure 5c), 1.0 (Figure 5d) and 5.0 lux (Figure 5e).

**Figure 5.** The NDVI under different illuminations. (**a,a1**) Color image of a money plant and illumination of the NDVI: (**b**) 0.01 lux, (**c**) 0.1 lux, (**d**) 0.5 lux, (**e**) 1.0 lux and (**f**) 5.0 lux. (**f1**) The region of interest for the NDVI.

As shown in Figure 5f1, some regions of interest in Figure 5b–f were selected. Fifty-one pixels were selected from the black and white regions, respectively, averaged and then divided. Then, as shown in Figure 6, the NDVI changes with the illumination. It can be clearly seen from Figure 6 that the NDVI changes significantly with the increase of the illumination. Therefore, the NDVI is greatly affected by the illuminance of the experimental environment, so it is not reliable to use the NDVI to monitor the vegetation state at night, and it should be supplemented and corrected with other information.

**Figure 6.** Histogram of the NDVI with different illuminations.

Figure 7a–e is the calculated linear polarization (DoLP) images under illuminations of 0.01, 0.1, 0.5, 1.0 and 5.0 lux, respectively. Some regions of interest in Figure 7a–e are selected, as shown in Figure 7e1, and 51 pixels are selected for the black and white regions in Figure 7e1, averaged and then divided. Then, as shown in Figure 8, the image of the DoLP changes with the illumination. It can be clearly seen from Figure 8 that the DoLP of vegetation does not change significantly with the increase of illumination. Thus, the DoLP was almost insensitive to ambient illumination. It can be seen from Figure 7 that the polarization imaging shows detailed information of the foliage, so the use of the DoLP for monitoring the nighttime vegetation state can be used to supplement the information for NDVI monitoring.

**Figure 7.** The DoLP with different illuminations. The illumination of the DoLP: (**a**) 0.01 lux, (**b**) 0.1 lux, (**c**) 0.5 lux, (**d**) 1.0 lux and (**e**) 5.0 lux. (**e1**) The region of interest for the DoLP.

**Figure 8.** Histogram of the DoLP with different illuminations.

Figure 9a–e is the fusion images (NDAI) calculated by the NDVI, DoLP and AOP, and the illuminance is 0.01, 0.1, 0.5, 1.0 and 5.0 lux, respectively. The results show that healthy and nonhealthy vegetation can be well-distinguished from each other. The fusion images (NDAI) highlight more detailed information of the leaf surface than the NDVI image, and the contrast by target monitoring is enhanced.

**Figure 9.** The fusion image (NDAI) with different illuminations. Illumination of the NDAI: (**a**) 0.01 lux, (**b**) 0.1 lux, (**c**) 0.5 lux, (**d**) 1.0 lux and (**e**) 5.0 lux.
