2.4.1. The Normalized Vegetation Index

The analysis based on the vegetation index has become a main way for scholars to study and practice the remote sensing detection of pests and diseases. So far, many different types of vegetation indices have been proposed one after another. With certain biological or physical and chemical significance, these vegetation indices are an important application form of the plant spectrum. At present, many studies have tried to establish the relationship between remote sensing information and the occurrence and degree of diseases and pests through various vegetation indices. In this study, the spectral image data of 680 nm and 760 nm in the central band were used to calculate the NDVI, of which the formula is as follows:

$$\text{NDVI} = \frac{\text{R}(760 \text{ nm}) - \text{R}(680 \text{ nm})}{\text{R}(760 \text{ nm}) + \text{R}(680 \text{ nm})} \tag{1}$$
