## 4 Discussion  
The obtained study results suggest that multispectral image processing is a useful non-invasive tool to estimate the vegetative growth parameters of roots and leaves of aeroponic lettuce plants inside a greenhouse. The main contribution of this research is the proposal of a non-invasive approach for computing the morphometric and thermal parameters for characterizing the vegetative growth of aeroponic lettuce. The proposed approach is based on processing multispectral images (VIS, FIR, NIR) captured along an aeroponic lettuce-growing period. The project development implied the design and implementation of a multispectral vision system as well as image processing algorithms to fulfill the application specifications. The implemented multispectral vision system was designed for capturing homogenous images in the defined spectral range. An important design aspect to be considered in a reliable vision system for variable measuring is the calibration. In the case of computing the lettuce morphometric parameters, the calibration was conducted using an object with a known length in centimeters. In the same way, the calibration for measuring the thermal parameter was done through a commercial infrared thermometer. There are critical variables for the proper functioning of an aeroponics system. In this work, adequate fertigation parameters were established to achieve good development in the aeroponic crop inside the greenhouse. The implemented fertigation system was designed to ensure an easy and efficient nutrient solution absorption by the lettuce root in order to achieve good leaf growth [51]. The lettuce root growing response in the substrate, hydroponics and aeroponics was reported in previous study [52]. One of the conclusions was that aeroponics roots obtained the best characteristics compared with the other two crop techniques. However, the  
Figure 12: Leaf thermal image capture samples (IR): (**a**) two days before water stress, (**b**) on the day when water stress occurred and (**c**) two days after the water stress. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Average Temperature of** & **Two Days before Water** & \multirow{2}{*}{**Water Stress Day**} & **Two Days after Water** \\ (\({}^{\circ}\)**C**) & **Stress Condition** & & **Stress Condition** \\ \hline Leaf & 12.11 & 25.3 & 16.67 \\ Root & 12.71 & 23.2 & 16.99 \\ Ambient & 11.5 & 22.1 & 15.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Average temperatures obtained during the water stress period. achieved foliar development was a fresh leaf mass between 37.8 g and 50.9 g, which was lower than the results with the hydroponic system. These results partially agree with those reported in this paper for the three crops 102.5 g, 185.5 g and 184.3 g, where a better foliar development is inferred. However, the results are not completely comparable because the study in [46] was conducted with green and red lettuce (_Lactuca sativa_ L.), a different crop aeroponic system was used and the lettuce-growing period was 45 days after germination. The foliar area and fresh mass parameters achieved in the described study are better than previous studies [6]. According to the Tukey test results, no significant differences were found, neither in the root morphometric parameters (length, area and perimeter) nor leaf area in the three crops. In addition, significant differences were found for leaf perimeter in crops 1 and 2, which indicate that the vegetative growth of the aeroponic crop lettuce can be characterized by the analysis of all the root morphometric parameters and by the leaf area. The thermal root monitoring could be a more efficient method for detecting the plant status in an aeroponic crop, but foliage monitoring is a simpler method [54]. The results described in this paper are similar to those reported in previous studies, using analysis parameters and methodology similar to those employed in this work; for instance, Refs.