# Passive UHF RFID Tag for Multispectral Assessment
## 4 Conclusions  
This work describes a passive printed UHF RFID tag with multiple optical sensor capabilities. Five photodiodes have been integrated on the tag, covering a wide spectral range from ultraviolet to infrared regions. A microcontroller-based switch circuit has been designed and tested to allow the multiplexing of several photodiodes to one SFE input. Furthermore, the chosen RFID chip has a built-in temperature sensor. Therefore, six different magnitudes can be measured in every reading in a fully passive mode. The tag has been manufactured by screen-printing on a flexible polymeric substrate. This system boasts a very high sensing capability, in this case up to six different magnitudes in the same tag. The implemented switch array expands the capabilities of the sensor-enabled RFID chip, which includes at most two inputs for external sensors. As the switching process is sequential, this interface solution is scalable, the number of available MCU I/O pins being the only limitation to the number of sensors that can be interfaced. The operation of this passive tag has been successfully demonstrated. First, using several standard illuminants, a procedure was defined to obtain the spectral fingerprint of the target illuminant. Therefore, these types of RFID tags would be very useful to detect different light conditions. Subsequently, tag performance was validated through reflective colour-sensing measurements. In this regard, the spectral fingerprints obtained with our RFID tag were modified by the target surface colour, using a cool white LED as an illuminant and different plain-coloured cardboards as targets. As the main limitation, we have detected some influence of the label/illuminant source position on the tag results that should be minimized in later refinements of this design. Also, the measured read range of the tag is limited to 1.1 m. Beyond this range there is no longer sufficient power to drive the SFE of the RFID chip and power up the MCU, whose measured power consumption in operation mode is \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Target**} & \multicolumn{5}{c}{**Spectral Fingerprint Coordinates**} \\ \cline{2-6}  & **UV** & **B** & **G** & **R** & **IR** \\ \hline White cardboard & 0.007 \(\pm\) 0.003 & 0.37 \(\pm\) 0.04 & 0.56 \(\pm\) 0.05 & 0.194 \(\pm\) 0.018 & 0.0027 \(\pm\) 0.0006 \\ Red cardboard & 0.006 \(\pm\) 0.004 & 0.36 \(\pm\) 0.04 & 0.51 \(\pm\) 0.07 & 0.38 \(\pm\) 0.05 & 0.0042 \(\pm\) 0.0017 \\ Green cardboard & 0.005 \(\pm\) 0.005 & 0.32 \(\pm\) 0.03 & 0.80 \(\pm\) 0.06 & 0.154 \(\pm\) 0.011 & 0.0036 \(\pm\) 0.0013 \\ Blue cardboard & 0.007 \(\pm\) 0.003 & 0.34 \(\pm\) 0.05 & 0.52 \(\pm\) 0.08 & 0.140 \(\pm\) 0.020 & 0.0018 \(\pm\) 0.0007 \\ Yellow cardboard & 0.005 \(\pm\) 0.005 & 0.265 \(\pm\) 0.017 & 0.98 \(\pm\) 0.06 & 0.314 \(\pm\) 0.018 & 0.0051 \(\pm\) 0.0014 \\ Orange cardboard & 0.005 \(\pm\) 0.004 & 0.34 \(\pm\) 0.07 & 0.69 \(\pm\) 0.14 & 0.42 \(\pm\) 0.08 & 0.0058 \(\pm\) 0.0021 \\ Brown cardboard & 0.006 \(\pm\) 0.004 & 0.45 \(\pm\) 0.04 & 0.85 \(\pm\) 0.07 & 0.29 \(\pm\) 0.03 & 0.0050 \(\pm\) 0.0023 \\ Immature banana & 0.0011 \(\pm\) 0.0003 & 0.26 \(\pm\) 0.05 & 0.90 \(\pm\) 0.07 & 0.30 \(\pm\) 0.06 & 0.00026 \(\pm\) 0.00008 \\ Ripe banana & 0.00084 \(\pm\) 0.00007 & 0.31 \(\pm\) 0.03 & 0.968 \(\pm\) 0.011 & 0.41 \(\pm\) 0.04 & 0.00021 \(\pm\) 0.00018 \\ Overripe banana & 0.008 \(\pm\) 0.006 & 0.43 \(\pm\) 0.04 & 0.62 \(\pm\) 0.05 & 0.2198 \(\pm\) 0.0021 & 0.0012 \(\pm\) 0.0011 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Spectral fingerprint coordinates obtained with the RFID tag for the reflective colour-sensing experiments with the plain-coloured cardboards and the banana ripening stages. Figure 8: **(a)** Normalized spectral responses measured with the spectrometer and spectral fingerprints obtained with the RFID tag for the progress of banana ripeness from immature to overripe; **(b)** Photographs of the banana tested in its different ripening stages: immature, ripe, and overripe. 0.42 mW.