# Identification of Hybrid Okra Seeds Based on Near-Infrared Hyperspectral Imaging Technology
## 3 Results and Discussion
### Selection of Effective Wavelengths  
The whole band of spectral data contains redundant information, and, in order to increase the processing speed of the models and reduce the modeling time, two different algorithms that can extract the characteristic wavelength were applied in this study. Furthermore, a combination of two methods was used to obtain the optimal number of characteristic wavelengths. Figure 2(a) shows the characteristic wavelengths which were extracted by the successive projection algorithm (SPA). SPA is a forward feature variable selection method, which selects the combination of variables with minimal redundancy information and minimal collinearity, and it therefore has a wide range of applications in spectral feature wavelength selection [32; 33; 34; 35]. Fourteen characteristic wavelengths were acquired. The band found near 1065 nm is related to the O-H stretching vibration [36]. The spectral regions which include 1041-1143 nm, 1211-1225 nm, 1360-1390 nm, and 1621-1654 nm are related to the C-H stretching vibration [36]. The band at around 1472 nm is related to the N-H stretching vibration [36]. These groups exist in amino acids and other substances found in okra seeds, such as leucine, lysine, valine, and phenylalanine [37], indicating that the selected characteristic wavelength is representative and it can be used to establish an effective and reliable discriminant analysis model. CARS, a characteristic wavelength selection method, is based on Monte Carlo sampling and PLS regression coefficients. It was used to choose the optimal wavelengths. Initially, 86 characteristic wavelengths were extracted from the 200 full-band wavelengths. Although there is a certain degree of deletion for the whole band, it could be more concise. Figure 2(b) shows the distribution of characteristic wavelengths that were selected by CARS. Therefore, in this study, in order to further reduce the number of characteristic wavelengths, CARS was further screened in conjunction with SPA. Finally, 14 characteristic wavelengths were selected. The changes of the distribution of optimal wavelengths are shown in Figure 2(c). All of the bands that are selected by CAR + SPA are related to the stretching vibration of the functional groups, which include N-H group, C-H group, and C=O group [36]. According to some studies, the unsaturated fatty acids, proteins, and hydrocarbons of okra seeds also contain the corresponding functional groups [38]. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Methods**} & \multicolumn{3}{c}{**Full Wavelength**} \\ \cline{2-4}  & **Parameter** & **Calibration Set** & **Prediction Set** \\ \hline PLS-DA & 9 & 83.36\% & 82.59\% \\ SVM & (256, 5.2780) & 99.31\% & 93.62\% \\ \hline \hline \end{tabular} Note: PLS-DA model’s parameter means the optimal number of LVs; SVM model’s parameter means different penalty parameters (c) and kernel function parameters (g), shown as (c, g).  
\end{table} 
Table 1: Comparison of discrimination results obtained by partial least squares (PLS) and support vector machine (SVM) models with the complete spectral data. PLS-DA: partial least squares discriminant analysis. Figure 3: The distribution spectral image of characteristic wavelengths selected by different methods: **(a)**the spectral image of characteristic wavelengths selected by successive projection algorithm (SPA); **(b)**the spectral image of characteristic wavelengths selected by competitive adaptive reweighted sampling (CARS); **(c)**the spectral image of characteristic wavelengths selected by CARS + SPA. Classification Results and Analysis by the Discrimination Models Based on the Characteristic Band Spectrum  
Discrimination models were built based on the characteristic wavelengths to simplify the complexity and increase the operating speed. When it came to practical breeding applications, a detection device of hybrid okra seeds required faster processing speed and a more reliable model. SPA, CARS, and CAR + SPA were used to select the optimal wavelengths.