## 3 Results and Discussion  
### Data Correction  
The system SNR measured in dB is defined as \(20\log_{10}(m_{S}\ /\ \sigma_{S})\), where \(\sigma_{S}\) is the standard deviation of a data set S with average \(m_{S}\), collected by the system on the static phantom in 20 s. For a total effective acquisition time of 65 ms on 13 SiPMs, 24 LEDs, and 1 dark phase for all the photodetectors,  
Figure 6: Operative range of used silicon photomultipliers with 1 k\(\Omega\) resistor in series, measured under (a) 700 nm and (b) 830 nm irradiation. as above described, the SNR values, reported in Figure 7, are quite remarkable. At 830 nm illumination wavelength, the SNR at 2 cm SDS results in about 70 dB and remains quite high, resulting in 53.4 dB at SDS equal to 10 cm. Though the SNR values are quite promising, the photocurrent signals still show instability and inhomogeneity effects. In Figure 8a, the photocurrents (12 time traces for 830 nm light and 12 time traces for 700 nm light) related to the fourth nearest-neighbor source-detector distance (SDS = 7.21 cm) are reported. Figure 8b reports, in detail, one of the photocurrent time traces related to 830 nm light. It is possible to observe two types of phenomena affecting the measurement:  
1. Differences on the value of the photocurrents related to channels with the same SDS, as shown in Figure 8a. They are due to little displacements of the LEDs or of the SiPMs, small differences in the EQE of the different SiPMs involved, or differences among LEDs' brightness. 2. Drifts over time of the photocurrents, as shown in Figure 8b. To explain the decrease over time of the photocurrents recorded on the static phantom, it is important to consider that we subtract the dark current to determine the SiPM signal. In general, at a constant bias voltage, as SiPM temperature increases, both the photocurrent and the dark current in a SiPM increase while the gain slightly decreases. Moreover, the light output of an LED at a constant current also decreases with the increase of its junction temperature [30]. All this instability effects the sum up giving rise to the overall slow drift effects shown in Figure 8.  
Figure 8: (a) Photocurrents (12 time traces for 830 nm light in red and 12 time traces for 700 nm light in blue) related to the fourth nearest-neighbor source-detector distance (SDS = 7.21 cm). (b) Detail: one of the photocurrent time traces related to 830 nm light. Figure 7: Single channel system signal-to-noise (SNR) vs. SDS measured on the phantom at 830 nm illumination. To correct such issues, we performed a calibration procedure directly on the phantom before starting the metal bar motion experiment. First, considering the 2 cm SDS, each resistive trimmer on the LEDs is regulated to guarantee the output current of the SiPMs being at the upper limit of their linear range (Figure 6), as described above. Second, the photocurrent levels at both wavelengths for each channel are recorded after a stabilization phase, lasting five minutes, with the system in the on state. Such data are then used for the actual data calibration, as explained in the following. After the calibration, the test phase starts. The mobile bar of the phantom starts its motion at constant velocity with the step motor rotating 100 degrees in about 200 s (0.5 deg/s), so that the bar's end travels a linear distance of about 10 cm in 30 s. During the movement, all the SiPMs outputs are recorded while the LEDs are singularly turned on and then turned off sequentially, as explained in the previous section. Off-line, each photocurrent vs. time data is filtered with a moving average method. Then, for each channel, the photocurrent vs. time data recorded in the test conditions are divided by the photocurrents' values measured after the five minutes of stabilization in the calibration phase. That is, the actual signals used for further data processing are the normalized to baseline photocurrents for each channel at different SDS.