## 3 Results
### Distribution of \(R_{total}\) Values for p-Waves Incident at \(\theta_{B}\)
The 1300, 1350 and 1650 nm wavelengths correlate more poorly with vitamin C content; this is because the chemical bonds of vitamin C do not absorb significantly at these wavelengths, thus resulting in low \(\mathrm{r_{c}^{2}}\) values. Characteristic wavelengths that have the largest correlation coefficients were selected to form combinations of wavelengths and were then used to determine the vitamin C content of Indian jujube. Once the reflectance signals at these wavelengths were acquired by the NIR photodetector, the PLS algorithm was used to quantify the vitamin C content of each fruit. The correlation of each wavelength combination was then calculated. The \(\mathrm{r_{c}^{2}}\) and RMSEC values of each wavelength combination (which comprises the most strongly correlated wavelengths) are shown in Table 4.  
The results of the PLS calculations indicate that the \(\mathrm{r_{c}^{2}}\) and RMSEC of combination A were 0.86 and 1.44 mg/100 g, respectively. Combinations B and C (N = 4) resulted in \(\mathrm{r_{c}^{2}}\) of 0.87 and 0.86, respectively, as well as RMSEC values of 1.39 mg/100 g and 1.42 mg/100 g, respectively. Although combinations B and C have similar \(\mathrm{r_{c}^{2}}\) values, the RMSEC of combination B was slightly lower. Therefore, another wavelength was added to combination B to form five-wavelength combinations (N = 5). Among  
\begin{table}
\begin{tabular}{c c c c} \hline  & \multicolumn{1}{c}{**Wavelength Combinations (nm)**} & \multicolumn{1}{c}{**Coefficient of**} & \multicolumn{1}{c}{**Root-Mean-Square Error of**} \\  & & **Calibration (\(\mathrm{r_{c}^{2}}\))** & **Calibration (\(\mathrm{r_{c}^{2}}\))** & **Calibration (\(\mathrm{r_{c}^{2}}\))** \\ \hline A & N = 3 & 1200, 1400, 1450 & 0.86 & 1.44 \\ B & N = 4 & 1200, 1400, 14  
combinations D, E and F (N = 5), D has the highest \(r_{\rm C}^{2}\) (\(r_{\rm C}^{2}\) = 0.89) and lowest RMSEC (1.31 mg/100 g). Because the correlation of 1600 nm with vitamin C content is rather low (\(r_{\rm C}^{2}\) = 0.55), the inclusion of this wavelength failed to enhance the accuracy of combination E and F. Another wavelength was added to combination D to form N = 6 combinations, thus forming combinations G, H and I, which all have slightly higher \(r_{\rm C}^{2}\) values than the N = 5 combinations. However, upon further analysis, it was discovered that the \(r_{\rm C}^{2}\) and RMSEC values of the N = 6 combinations were slightly better than those of the N = 5 combinations. We speculate that this is because the \(r_{\rm C}^{2}\) values of 1050 nm and 1150 nm are relatively low (0.70 and 0.68, respectively); therefore, the inclusion of these wavelengths in a wavelength combination did not significantly improve the \(r_{\rm C}^{2}\) of the combination. The wavelengths of combination D (1200, 1400, 1450, 1500 and 1550 nm) correlated strongly with OH and CH\({}_{2}\) absorptions. This result is consistent with the NIR absorptions of OH and CH\({}_{2}\) that were observed by [PERSON] et al. [24], [PERSON] et al. [25], [PERSON] et al. [26], [PERSON] et al. [27] and [PERSON] et al. [28]. Based on these results and the practical requirements of actual detection systems (e.g., size, cost and performance), we decided to use combination D (N = 5) in our detection system, whose \(r_{\rm C}^{2}\) and RMSEC are 0.89 and 1.31 mg/100 g, respectively. The results obtained using combination D are shown in Figure 5. This combination was then used to measure the vitamin C contents of the prediction set. The corresponding \(r_{\rm P}^{2}\) and RMSEP values were then calculated.