# Visible Light Communications through Diffusive Illumination of Sculptures in a Real Museum
## 4 Results and Discussion
### Bordoni's Font: Polar Snt and Per Maps  
The polar amplitude SNR and PER measurements for the _Holy Water Font_ (Figure 5a) have been performed by scanning the horizontal angular position of the RX stage around the artwork, for two different radial distances (2 m and 4 m), and in an angular range of [\(-90^{\circ}\), \(+90^{\circ}\)] with respect to the frontal position (Figure 3a). The photodetector is always oriented towards the artwork, and during the horizontal angular scan, the vertical angle is fixed in such a way that the RX stage always receives the maximum diffused light from the artwork. This configuration is referred to as _Optimal_ in the following. Both horizontal and  
Figure 4: (**a**) Example of measured PER as a function of SNR on the received amplitude. Green circles correspond to experimental data, with horizontal error bars showing the uncertainty in the amplitude measurement. The vertical error bars are due to the variance on the number of wrong packets on consecutive measurements and are smaller than the symbol size. The red line is the best fit curve by the model given in the text. The shaded area represents the error-free region (higher limit) given by the maximum number of packets used in the tests. (**b**,**c**) Heat maps showing eye-diagrams for PER \(>\) 0.1 with SNR = 11 dB (**b**), and error-free communication achieved with SNR = 22 dB (**c**). Three traces are shown: TX, RX, and the (digitized) signal after comparator. The horizontal scale is 4 \(\upmu\)s. The acquisition is self-triggered on the RX signal, so that the horizontal spread of the digitized signal corresponds to a jitter of 1-4 \(\upmu\)s in the reconstructed VLC digital signal, depending on the SNR level. vertical angles are measured by the two orthogonal precision rotation stages equipping the RX stage (Section 2). SNR and PER maps are reported in Figure 5b,c, respectively. Noticeably, at the regular observation distance of 2 m, error-free communication is obtained in the whole horizontal range [\(-90^{\circ}\),\(+90^{\circ}\) ], with SNR always exceeding 25 dB. As expected, both SNR and PER values are worse at 4 m, as a consequence of the decreased diffused intensity. Noticeably enough, PER \(\leq 10^{-1}\) is still achieved at 4 m in the range [\(-90^{\circ}\),\(+90^{\circ}\) ] (minimum SNR = 3.5), and error-free communications are achieved at \(0^{\circ}\)  radial angle (named _frontal_ configuration hereinafter). We note that the analyzed sculpture has a much smaller frontal extension with respect to the planar paintings reported in [22]. In the former case, therefore, one could intuitively expect a globally lower SNR value recorded at a given distance and for the same illumination level as compared to the latter, as the global amount of diffused light collected by RX is in general proportional to the size of the illuminated surface. Furthermore, the cylindrical diffusive surface tends to disfavor back diffusion of light at small angles, which is instead favoured by a plane configuration, due to the Lambertian shape of the diffusion lobe [31]. Following the pristine geometric considerations above, one could naively expect reduced SNR values even in the _frontal_ configuration with respect to the two-dimensional paintings case. However, our data show a non-trivial behaviour of the SNR maps for the three dimensional sculpture. To get more insight in the differences yielded by two- and three-dimensional artworks in VLC performances, we report in Figure 6 a direct comparison between our PER characterization for the sculpture, along with a similar characterization for the case of the plane mural painting _Holy Trinity_ by [PERSON] [22], both performed at 4 m distance. Panel a) reports the PER data (circles), along with a spline interpolation for both painting and sculpture case, whereas panel (b) highlights specific PER values corresponding to relevant angular positions in tabular form.