## 4 Evaluation of the Logarithmic Transimpedance Circuit
Moreover, even in the presence of strong light, the circuit still responds to low light intensity changes, meaning that the data has the potential to be extracted (Figure 2). Nevertheless, we must admit that in such conditions, the voltage amplitude generated by the low light intensity changes (i.e., by the data signal) is very low (i.e., as low as few mV), and thus the SNR may be significantly affected. Yet, the important thing is that the circuit is not saturated, and so if proper signal processing is implemented, the data can be extracted. Another aspect which must be pointed out is that the overall amplitude response of the logarithmic transimpedance circuit in the 10-15.000 lux interval is lower than the amplitude response of the linear circuit (see Figure 2), and so an AGC circuit must be introduced to compensate the highly variable output voltage. In low lighting conditions (i.e., few lux and lower), the sensitivity and the amplitude response of the logarithmic transimpedance circuit increases, and thus its output voltage is quite similar to the one of a sensitive linear circuit. Therefore, considering an environment where the light oscillates on 5 decades, the logarithmic transimpedance circuit provides a fair response, it has a built it flexibility, providing the sensitivity of a highly sensitive linear circuit and the robustness of a noise resilient circuit. Experimental Evaluation in Strong Lighting Conditions: Confirming the Logarithmic Transimpedance Circuit Ability to Prevent the Saturation of the Photosensitive Element  
Before proceeding, the first set of experiments is meant to analyze the behavior of the logarithmic transimpedance circuit and to confirm its ability to prevent photodiode saturation. For this test, the circuit is orientated toward a light source with no data transmission capability and its output voltage is analyzed as the incident irradiance is gradually increased. Tables 3-5 show the parameters of the photosensitive element, of the LED source and the measurement equipment used during the testing procedure. Figure 4 presents the results of these tests and it confirms the logarithmic transimpedance circuit's ability to mitigate the photoelectron saturation. In this case, one of the curves represents the experimental data and the other curve represents the theoretical data for a perfectly logarithmic response of the system. This plot is obtained in accordance with Equations (1) and (2). \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Feature/Value** \\ \hline Light source & High power white LED \\ LED power & 5 W \\ LED Correlated color temperature & 5128 K \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary of the light source parameters. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Feature/Value** \\ \hline Transimpedance circuit & Logarithmic \\ Photodiode type & BPX 61 \\ Photodiode Capacity & 72 pF \\ Photodiode Photocurrent & 70 nA/lux \\ Photodiode surface & 7.02 mm\({}^{2}\) \\ Photodiode Spectral range & 400–1100 nm \\ Photodiode switching time & 20 ns \\ Photodiode Sensitive area & 7.02 mm\({}^{2}\) \\ Photodiode Half angle & \(\pm 55^{\circ}\) \\ Photodiode Spectral sensitivity & 0.62 A/W \\ Incident irradiance & 0–15.000 \(\mu\)W/cm\({}^{2}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Summary of the VLC receiver parameters during the tests. One can see that even at 15,000 uW/cm\({}^{2}\) the circuit is not saturated and that it is still able to respond to light variations. Considering the differences between the experimental results and the theoretical ones, they are due to the circuit nonlinearities. Nevertheless, these slight variations do not affect the form of the digital signal. Another observation regarding this experiment is related to the irradiance of the incident light. It is true that when the sun is in direct line of sight with the VLC receiver, the irradiance of the incident light can be even greater than 15,000 uW/cm\({}^{2}\).