3.4.1. Various Vegetation Indices

The vegetation index can reduce the influence of sensors and environment on the target through normalization and derivative processing as well as by improving data utilization efficiency through the use of several selected bands [41]. Numerous vegetation indices have been constructed to estimate the biophysical and biochemical properties of plants. In the present study, 12 vegetation indices with good correlation to Anth were selected, as shown in Table 2. Seven two-band vegetation indices and five three-band vegetation indices were included. The bands used in the vegetation indices are marked in Figure 4a. Specifically, the selected bands were mainly concentrated around 500–575, 670–725, 750, and 800 nm.


**Table 2.** Two-band and three-band vegetation indices.

The correlations between the vegetation indices and Anth are shown in Figure 6. Among the two-band VIs, CHLI (700,710), VRI (740,720), and RNDVI (750,705) showed a strong correlation with the Anth of healthy and red leaves. Among the three-band vegetation indices, PSRI was strongly correlated with the Anth of red leaves, and MCARI was strongly correlated with the Anth of healthy leaves. There were significant correlations between the vegetation indices and Anth in healthy leaves (P< 0.01), with GNDVI achieving the highest correlation coefficient of −0.73. In red leaves, CHLI achieves the highest correlation coefficient of −0.75.

**Figure 6.** (**a**) Correlation analysis of healthy leaf VIs; (**b**) correlation analysis of red leaf VIs.

### 3.4.2. VIc Based on Two Arbitrary Bands

The coefficients of determination (R2) between plant-specific variables, and VIc can reflect the predictive power of two independent band combinations. Figure 7 shows the contour maps of R<sup>2</sup> between Anth and NDVI, RVI, DVI, and SAVI using all combinations of two wavebands at *i* and *j* nm, which are very useful for selecting effective bandwidths and various combinations of wavelengths. For healthy leaves, the most significant areas noted around NDVIh, RVIh, DVIh, and SAVIh were (R573, R507), (R572, R507), (R547, R522), and (R547, R518), respectively, and DVIh was the most significant VIc with an R<sup>2</sup> of 0.65. The contour map of NDVIh was very similar to that of RVIh, and their significant bands and

maximum R<sup>2</sup> values were close. For red leaves, the most significant areas noted around NDVIr, RVIr, DVIr, and SAVIr were (R689, R461), (R460, R692), (R690, R656), and (R685, R667), respectively, and DVIr was the most significant VIc with an R<sup>2</sup> of 0.68. With the exception of RVIh and RVIr, the other vegetation indices showed symmetric spatial distribution patterns of R2.

**Figure 7.** Contour maps of R<sup>2</sup> between Anth and VIc of healthy and red leaves.
