# Application of Hyperspectral Imaging for Assessment of Tomato Leaf Water Status in Plant Factories
## 3 Results and Discussion
### Relationship between Individual Wavelength and Water Status  
Many different types of random noise exist in an HSI system, including a dark current from the camera, light from the environment, noise from digitization owing to analog to digital conversion, and so on. These noise values will obviously impact the results obtained from subsequent image analysis [26]. To improve the assessment accuracy of the leaf water status, the spectral derivative technique and the logarithmic transformation (log (I/R)) of the inverse of the reflectance method were adopted for proper HSI preprocessing. The study reported in [13] showed that low-order differential processing of the spectrum has a low sensitivity to noise and is more effective in practical applications. Thus, first-order differential processing technology was adopted in this study to remove part of the linear or near-linear background and the influence of the noise spectrum on the target spectrum [2,24]. The logarithmic transformation (log (I/R)) of the inverse of the reflectance method was employed to address the nonlinear problem. The logarithmic transformation of the reciprocal of the spectral reflectance tends to not only enhance the spectral difference in the visible and infrared region, but also reduce the effects of multiplicative factors caused by changes in lighting conditions [11].  
To determine the correlation coefficient between the tomato leaf water status (WC and MC) and individual wavelengths of the raw relative reflection (RAW), the first derivative of relative reflectance (DIFF) and the inverse-logarithm relative reflectance (LOG) were calculated from 900 nm to 1700 nm (Figure 6).  
The maximum negative correlation coefficient between the raw relative reflectance and WC is observed at 1410 nm, where the coefficient value is \(r=-0.3334\). Regarding the relationship between the raw relative reflectance and MC, the maximum negative correlation coefficient is obtained at 1410 nm with the coefficient value of \(r=-0.6803\). The maximum positive correlation coefficient between the inverse-logarithm relative reflectance and WC is observed at 1420 nm with a low coefficient value of \(r=0.2795\). With regard to the relationship between the inverse-logarithm relative reflectance and MC, the maximum positive correlation coefficient is obtained at 1420 nm, and the coefficient value is \(r=0.5745\). For the relationship between the first derivative of relative reflectance and WC, the maximum positive correlation coefficient value is \(r=0.4694\) at 1430 nm. Moreover, the maximum negative correlation coefficient is obtained at 1140 nm with \(r=-0.7298\), and the second largest correlation coefficient value is obtained at 1430 nm with \(r=0.6457\).  
It should be noted that, as indicated by Pu [2], compared to other plant biochemical parameters, water has several obvious absorption bands at 970, 1200, 1400, and 1940 nm. Of these bands, 1400 nm  
Figure 6: Correlation coefficient between the leaf water status and individual wavelength. (**a**) Leaf WC; (**b**) equivalent water thickness (g/dm\({}^{2}\)). RAW: raw relative reflectance; DIFF: first derivative of relative reflectance; LOG: inverse-logarithm relative reflectance (log(I/R)).  
and 1900 nm are considered the main bands that characterize water absorption [9]. The maximum negative or positive correlation coefficient in this experiment was thus concentrated in the range from 1410 nm to 1430 nm owing to the high water absorption in this region.  
Recent studies have revealed that reflectance is related to changes in MC rather than to changes in WC. It is considered that WC depends on two independent leaf variables: the leaf equivalent water thickness and leaf dry mass area, both of which affect the leaf optical properties [6; 7; 27]. The correlation value between MC and wavelength was always observed to be higher than that between WC and wavelength in the present experiment.