# Passive UHF RFID Tag for Multispectral Assessment
## 2 Materials and Methods
### Methodology  
To obtain the optical spectra as reference data in our experimental characterization and validation, we used a high-resolution spectrometer model HR2000+ (Ocean Optics Inc., Dunedin, FL, USA). This device has a spectral response range of 200 to 1100 nm and an optical resolution of 1.10 nm, with a 14-bit ADC module and a fiber light input method. The output of our RFID sensor tag consists of five current values (one for each photodiode) defining a spectral fingerprint. To adapt the spectral fingerprint obtained with our RFID tag, composed of five coordinates, to the spectrum measured by the spectrometer, the following procedure was carried out:  
1. Standard normalization of the response of the five photodiodes according to their spectral sensitivities. The aim is to obtain an equivalent spectral graph with the same area under each photodiode response curve, but with a flat spectral sensitivity. To do that, we divide the area under the spectral sensitivity curve of each photodiode by its bandwidth (see Figure 2). The result of each division is the height of the flat sensitivity whose area is the same as the original spectral sensitivity. Then, we calculate the inverse of these heights. Finally, we normalize them to obtain the normalization factor assigned to each photodiode.
2. Multiplying each current value of the spectral fingerprint (obtained with our RFID tag) by each corresponding normalization factor calculated in Step 1. The result of this step is a set of five coordinates.
3. Assigning each of the five coordinates of the spectral fingerprint to the peak sensitivity wavelength of each photodiode (350, 470, 550, 620, and 950 nm).
4. Considering one of the values of the new spectral fingerprint and its associated wavelength. Then, calculating the factor necessary to get the same value as that obtained with the spectrometer at that particular wavelength.
5. Applying this calculated factor to the five coordinates of the spectral fingerprint.
6. Repeating Steps 4 and 5 for the rest of the coordinates.
7. The final spectral fingerprint is the average of all the values obtained for each photodiode. The error is calculated as the standard deviation of all the values for each photodiode from Steps 5 and 6.  
This procedure was developed and optimized by comparing the spectral coordinates from our system to the spectrometer measurements for the studied illuminants. We used seven standard illuminants: white LED (cool and warm), infrared LED, UV lamp, fluorescent light, tungsten halogen lamp, and daylight. As a result, the calculated normalization factors for the five photodetectors were: 1.00 for the UV; 0.75, 0.57, and 0.30 for the blue, green, and red photodiodes, respectively; and 0.29 for the IR photodetector.  
The next step was to validate this methodology by identifying cards from a colour chart. We carried out reflective colour-sensing measurements using several plain-coloured cardboards as targets: white, red, green, blue, orange, yellow, and brown. A cool white LED model XPGWHT-L1-0000-00H51 (Cree, Inc., Durham, NC, USA) was used as an illuminant. The geometric setup of the illuminant and detector consists of a typical \(45^{\circ}/0^{\circ}\) geometry setup to carry out diffuse reflectance measurements(Figure 4). In this type of reflection, the target surface properties modify the spectrum of the incident light. The RFID reader is placed at the base of the setup to read the output of the photodiodes in the tag. The reader can be moved away from the target surface up to the maximum read range, which will be shown in the next section. To apply our system in a real situation, we then replaced the cardboards with a banana in order to monitor its stage of ripeness. Bananas change colour from green to yellow as they ripen, then becoming dark brown as they get overripe. The illuminant, setup, and procedure used in the banana experiment were the same as those used with the cardboards.