# Visible Light Communications through Diffusive Illumination of Sculptures in a Real Museum
## 2 Experimental VLC Setup  
With reference to Figure 2, our VLC transmitter (TX) is based on a low-cost digital microcontroller encoder, Arduino DUE, featuring an internal clock frequency of 84 MHz. The digital board, allowing for a minimum observed resolution of \(\sim\)1 us in the generation of the Tx signals, supplies and modulates, by means of a customized current driver, a two-lamp high-power white LED source used for artworks lighting (Exenia Museo Mini 2L, with nominal power \(9\)W \(\times\) 2). The driver (see [23] for details) adds a 0-200% AC current modulation on top of the DC nominal 100% value of 0.7 A, required to provide the nominal light intensity on the artworks. This modulation scheme delivers the maximum possible modulation index, preserving the average brightness without appreciably affecting the LEDs lifetime [24; 25].  
The system transmits a continuous stream of 9-byte data packets at a baudrate of 28 kBaud. The packet includes a 3-byte pre-equalization frame, a 2-byte synchronization preamble and 4-byte data payload (see Figure 1d). The digital message is entirely generated by the Arduino board and sent to the high-power current modulator for injection into the LED source. The digital modulation is performed using Non-Return-to-Zero On-Off Keying (NRZ-OOK) scheme with Manchester encoding [3], which ensures a constant average illumination of the artworks.  
The information carried by the light diffused from artworks is collected by an ad-hoc optical receiver (RX), consisting of a high-gain, AC-coupled photo-diode (Thorlabs PDA100A2, active area 75 mm\({}^{2}\)) mounted on a precision two-axis rotation mount for precise angular alignment. The AC filtering stage is embedded in the hardware before the  
Figure 1: (**a**) Overview of VLC experimental setup used in the Basilica of Santa Maria Novella; (**b**) LED light sources used as TX stage and (**c**) custom RX stage, with shield tube for FoV adjustment visible. \(D\) represents the input tube diameter, \(L\) its length. (**d**) Data frame composition (NRZ-OOK with Manchester encoding) used in the experiments (see text).  
first transimpedance amplifier (TIA) stage (see [23] for further technical details), based on a high-speed op-amp with 130 MHz gain-bandwidth product. This effectively suppresses spurious DC stray light components at frequencies below 1 KHz such as sunlight, other artificial light sources, or the DC component of the illumination light source itself, so that the gain of the amplification stage can be increased to high values without incurring in DC saturation.  
The photoreceiver amplified signal is digitised by a single-threshold Schmitt-trigger comparator stage and decoded by an Arduino DUE-based digital RX board. The firmware performs a bit-wise comparison on each received packet, discarding the pre-equalization bits, and comparing the preambles and data payload against a predefined message. PER is calculated by definition as the ratio of received packets featuring at least one wrong bit in the data payload to the total number of packets received showing correct preambles.  
The FoV of the photoreceiver is physically limited at approximately 60\({}^{\circ}\) by a tube of diameter D = 25 mm and length L = 20 mm (Figure 1c), placed ahead of the active area of the photo-diode, aligned with the optical axis of RX. This configuration blocks possible light components impinging on the RX stage with large angles with respect to the optical axis, hence allowing to collect most of the light diffused by a specific artwork, but limiting at the same time the interference effects and crosstalks caused by signals cast by nearby VLC hotspots. As already observed [22], the proper choice of the RX FoV is a key element for future implementations of dedicated services and indoor positioning through diffusive VLC links.