## 3 Results and Discussion  
### Spectral Characteristic Analysis and Pretreatment  
The typical visible and near infrared spectra of the five kinds of leaves, including the slight, the moderate, the serious, nutrient deficiency and normal, are shown in Figure 2. The main bands of visible spectrum are 540-640 nm and 750-850 nm. The former corresponds to the part of green light, yellow light and red light. The latter is mainly related to C-H, O-H, N-H chemical bonds, caused by its 3-4 order stretching vibration. Near infrared spectral band is located at 1400-1600 nm and 1900-2100 nm, where the former is mainly caused by O-H, C-H, N-H level 1 stretching vibration, and the latter is mainly related to the secondary deformation vibration of O-H, C-O and N-H bonds [13; 29]. Two distinct reflection peaks are located at the spectral wavelength of 560 and 760 nm in Figure 2a. The main reason is that the chlorophyll in leaves with citrus greening is less than that of the normal, leading to the phenomenon where two reflection peaks of the leaves with citrus greening are higher than that of the normal leaves, and gradually rises with the increase of the disease grade [37; 45]. The fact that the reflection peak of nutrient deficiency leaves is lower than that of the normal may be related to the lack of nutrients in leaves. Two comparatively distinct absorption peaks in Figure 2b are located at 1930 and 1450 nm. For the leaves that are rich in the hydrogen group, the former is mainly caused by the expansion and deformation of the O-H group, while the latter is caused by the first order frequency double-stretching vibration of the O-H bond of water and sugar [46; 47; 48]. The reason why the leaf absorption peak of greening disease was lower than two kinds of nutrient deficiency and normal leaves may be that the pathogen of the greening disease blocks the absorption of water, resulting in low absorbance, and gradually dropping off with the decrease of the disease grade. The absorbance of nutrient deficiency leaves was higher than that of greening disease, which may be caused by the low dependence of the lack nutrients on light. It is shown in Figure 2 that the ranges of 325-500 and 1000-1135 nm in the visible spectra have little effect on the whole spectrum, thus the ranges of the above bands were removed. The near infrared spectra in the range of 835-1000 nm is close to a straight line, because the leaves absorbance in this band is mainly caused by the 3-4 order frequency multiplier of functional groups, which has limited influence on the whole spectrum, thus the band was also removed. Since the data types of the visible and near infrared spectral domains are inconsistent, the absorbance is converted into reflectance by Equation (6) and because of the mutual interference between the spectral variables. \[q=\lg(1/n) \tag{6}\]  
Figure 2: Typical spectra of 5 kinds of leaves, respectively. where \(n\) represents absorbance and \(q\) represents reflectivity. In order to optimize the model, the second derivative was used to preprocess the spectrum. The method not only amplified the effective spectral information, but also eliminated the spectral baseline drift, and increased the number of spectral curves and reduced the peak overlap, thus improving the resolution and facilitating the model's construction [49]. The spectrum of the second derivative pretreatment is shown in Figure 3. It is shown in Figure 3(a) that the visible reflection peaks of the five-type citrus leaves are obviously different at 690 nm, and the reflection peak of citrus leaf with serious citrus greening is the highest. In Figure 3(b), significant differences can be found in the near infrared reflection (NIR) peaks of the five-type leaves at 1840 nm, which may be caused by the amplifying the sensitive information of disease leaves during pretreatment [50; 51].