# Passive UHF RFID Tag for Multispectral Assessment
## 1 Introduction  
In its simplest form, a radio frequency identification (RFID) tag is composed of an antenna and an integrated circuit (IC). Most popular RFID tags are low-cost passive tags that draw energy from the electromagnetic field radiating from the reader when they are within range of its interrogation zone [1]. In this regard, printed electronics are a good solution to manufacture thin, wearable, flexible, lightweight, and cost-effective electronic devices [2]. There are several examples of RFID tags produced with printing techniques such as screen printing, gravure, flexography, and inkjet printing, among others [3; 4; 5; 6; 7].  
There is growing interest in including sensing capabilities in RFID tags in order to expand their features, adopting different solutions to achieve this aim. On one hand, some authors have associated the tag's analog response (read range, resonance frequency, backscattered power, etc.) with a variation in the monitored magnitude [8; 9; 10; 11; 12; 13; 14]. However, although quite valuable results have been achieved, several factors could interfere in link performance, making it difficult to distinguish whether the change in the tag's analog response is in fact due to a variation in magnitude or to some other interference. Moreover, the RFID reader generally needs to include additional circuitry to detect a tag response change. On the other hand, there are some other approaches to design the sensor tag based on passive, semi-passive or active operation modes proposed for packaging surveillance applications [15; 16]. Some of these designs combine microcontroller architectures with RFID chips to incorporate different environmental sensors together: temperature, light, and acceleration [17]; temperature, humidity, and gases [18; 19; 20]; ion or gas concentration [21; 22; 23]; pressure [24; 25; 26]; temperature with surface acousticwave (SAW) sensors [27; 28]; or general RFID platforms [29; 30; 31; 32; 33]. Other single-chip sensing architectures without a microcontroller unit have also been reported [34; 35; 36]. These strategies have the advantage of both processing the sensor data directly in the RFID tag and reducing costs. In addition, chipless RFID tags do not require any IC in the transponder as they encode the data in the frequency or the time domain [37; 38]. This chipless approach has been used to develop tags sensing temperature [39], humidity [40; 41], surface cracks [42], gases [43; 44], and visible radiation [45].  
In this work, we present the design, fabrication, characterization, and validation of a passive printed RFID tag with multiple optical sensing capabilities. The main novelty is the ability to measure up to five magnitudes from different sensors with a unique, fully passive printed tag. The tag, directly powered from the electromagnetic field that the RFID reader radiates, includes five photodiodes to cover a wide spectral range: infrared (IR), visible region (blue (B), green (G), and red ((R) components), and ultraviolet (UV). These sensors provide useful information to detect spectral data, and their practical applications can be extended to areas such as colourimetry and optical chemical sensing [46]. The tag has been printed on a flexible substrate using screen-printing technology, making it adaptable to different shapes and useful in smart packaging applications. The tag architecture boosts the sensing capabilities of the RFID chip, allowing the use of five external sensors with a single chip, as explained in Section 2. In Section 3, two possible applications are tested, both in the context of reflective colour sensing. The first application consists of a simple colour checker of plain-coloured cardboards. In the second application, the tag is used for identifying the stage of banana ripening, from immature to overripe.