## 4 Evaluation of the Logarithmic Transimpedance Circuit
Nevertheless, with the proper optical filters, this irradiance can be reduced to values similar to the ones that were considered in this test. Furthermore, the cost of such filters would not influence significantly the overall cost of the system. Consequently, this test confirms that the logarithmic transimpedance based VLC sensor is not saturated by background light whose power is very close to the optical power that could be incident on a VLC sensor which is directly facing the sun. Experimental Evaluation of the Logarithmic Transimpedance Circuit: the Comparison with the Linear Circuit  
This section presents an experimental evaluation of the logarithmic transimpedance circuit providing the comparison to a classical linear transimpedance circuit. For these tests, a VLC emitter based on a standard LED traffic light is placed 18 m away from the two transimpedance solutions and their output signals are analyzed. The parameters of the LED traffic light based VLC emitter are presented in Table 6, whereas the measurement equipment used during the testing procedure remains the one from Table 5.  
Figure 4: Experimental evaluation of the logarithmic transimpedance circuit ability to mitigate photoelement saturation. The graphic illustrates the circuit’s response to an increasing incident light. One can see that although the incident irradiance reached 15,000 uW/cm\({}^{2}\), the circuit still responds to changes in light intensity. \begin{table}
\begin{tabular}{c c} \hline \hline
**Equipment Type** & **Equipment** \\ \hline Spectral analyzer & Sekonic C800 \\ Irradiance meter & Delta Ohm HD 2302.0 with LP 471 RAD Probe \\ Oscilloscope & Tektronix TBS 1064 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Equipment used during the tests. At a distance of 18 m, the two VLC transimpedance circuits receive a signal having an irradiance of approximately 0.56 \(\upmu\)W/cm\({}^{2}\). In the absence of any additional parasitic light, the amplitude of the logarithmic transimpedance output signal is higher than the one of the linear transimpedance solution (see Figure 6a). The second tests have been performed in diffuse parasitic natural lighting conditions (i.e., natural light having an irradiance of about 200 \(\upmu\)W/cm\({}^{2}\)). In these circumstances, the logarithmic transimpedance decreases its sensitivity and thus, the amplitude of its output signal decreased 25 times, whereas the signal provided by the linear circuit remains at the same amplitude (see Figure 6b). Next, in order to further evaluate the robustness to noise of the two solutions, a perturbing LED noise source is orientated toward the two circuits. In these conditions, the logarithmic transimpedance circuit further reduces its sensibility, and so, the amplitude of its output signal is reduced at only 40 mV, whereas the amplitude of the classical circuit is still at 3.8V (Figure 6c). At this point, the noise effects are visible on the two signals. One can see that the LED light source is generating in addition to a strong DC component, a rather strong high frequency shot noise component affecting the SNR of both circuits. So, even if the amplitude of the logarithmic transimpedance circuit is significantly lower, the SNR of the two signals is very similar. This confirms that in strong parasitic lighting conditions only the amplitude of the logarithmic transimpedance circuit is affected, whereas the SNR is similar to the one of the linear transimpedance solution. On the other hand, although the amplitude of the logarithmic transimpedance circuit output signal is significantly lower than the one of the linear transimpedance circuit, this fact does not affect the sensor' overall performances as this effect can be compensated with the help of the automatic gain control (AGC) block. Next, the irradiance of the incident parasitic light is further increased to about 4500 \(\upmu\)W/cm\({}^{2}\). From this point, the linear transimpedance circuit is saturated and it is no longer able to receive any data.