# Measurement of Visible Radiation through a _Sansevieria cylindrica_-Based \"Living Sensor\"
## 3 Discussion
### Metrological Characterization and Discussions  
This section focuses on the characterization of the living sensor; in particular, in order to validate the working principle presented in Section 2, the first study regarded the evolution of the output voltage as a function of various distances, d, between the source and the sensor. It is worth noting that Figure 8 shows the response of the _Sansevieria cylindrica_ as a function of time for five distances, from 40 cm to 60 cm, which correspond to about 2395 lux to about 1438 lux, respectively. The graph evinces a decrement of the voltage correlated with a decrement of the distance. \begin{table}
\begin{tabular}{c c c c} \hline
**T [\({}^{\circ}\)C]** & **V\({}_{\mathbf{1}}\) [V]** & **V\({}_{\mathbf{D}}\) [V]** & **\(|\Delta\mathbf{V}|\) [mV]** \\ \hline
5 & 0.9336 & 0.9374 & 3.8 \\
15 & 0.9289 & 0.9330 & 4.1 \\
25 & 0.9183 & 0.9252 & 6.9 \\
35 & 0.9080 & 0.9127 & 4.7 \\
45 & 0.8958 & 0.8908 & 5.0 \\
55 & 0.8802 & 0.8699 & 10.3 \\
65 & 0.8599 & 0.8599 & 0 \\ \hline \end{tabular}
\end{table} 
Table 2: Temperature influence on the output voltage. \begin{table}
\begin{tabular}{c c c c} \hline
**RH [\%]** & **V\({}_{\mathbf{1}}\) [mV]** & **V\({}_{\mathbf{D}}\) [mV]** & **\(|\Delta\mathbf{V}|\) [mV]** \\ \hline
30 & \(-\)0.5393 & 0.1679 & 0.7 \\
40 & 0.0071 & 0.7214 & 0.7 \\
50 & \(-\)0.3464 & \(-\)0.6250 & 0.3 \\
60 & 1.5000 & \(-\)0.6714 & 2.2 \\
70 & \(-\)0.0536 & \(-\)0.1179 & 0.1 \\
80 & 0.1929 & 0.1357 & 0.1 \\
90 & \(-\)0.7607 & 0.3893 & 1.2 \\ \hline \end{tabular}
\end{table} 
Table 3: Relative humidity influence on the output voltage. Figure 7: Relative humidity effects on the output voltage; the graph includes the increment and decrement of the influence applied to the plant. The y = 0 line is included as a visual guide. Figure 9 shows the output voltage as a function of the measurand considering an irradiation level of 1400-2400 lux. The graph includes the mean value of measurements corresponding to each value of irradiance and the linear interpolation. The following transduction function between the measurand (I) and output voltage (\(V_{out}\)) can be observed:  
\[V_{out}=-2.9\times 10^{-5}.\mathrm{I}+1 \tag{2}\]  
Figure 10 shows the calibration diagram of the _Sansevieria cylindrica_ as a function of the applied Vis-light. The graph includes the linear interpolation and each point represents the difference between the measurement and the absence of the measurand in terms of the dark level for the living sensor (\(\Delta V_{out}\)). It includes the uncertainty, expressed as an error bar, and the mean value obtained from 10 repeated measurements of the data of interest. Furthermore, the following sensitivity (\(S\)) can be estimated:  
\[S=\frac{\Delta V_{out}}{I}=2.9\times 10^{-5}\frac{V}{lux} \tag{3}\]  
Figure 8: Output voltage across the electrodes as a function of the time considering various distances between plant and Vis-light source. Figure 9: Output voltage as a function of the irradiation level. Considering that, during the characterization, a noise level of 3.8 mV was measured, and taking into the account the sensitivity coming from the slope of the transduction function, by inverting the model (3), the resolution of the sensor which corresponds to about 131 lux can be estimated. It is worth mentioning that, considering the selectivity study presented in the previous section, assuming a temperature working condition of 25 \({}^{\circ}\)C and hypothesizing a variation of ambient temperature between 15 \({}^{\circ}\)C to 35 \({}^{\circ}\)C, an output voltage variation, considering the worst case of the hysteresis presented in Figure 6, of about 10.6 mV (decreasing at 15 \({}^{\circ}\)C) and 12.5 mV (increasing at 35 \({}^{\circ}\)C) can be observed. These variations could affect the resolution by a factor of about 3.3.