## 6 Mobile Thermal IR Imaging  
The relevant spread of thermal IR technology, together with the miniaturization of IR detectors, induced manufacturers to produce portable thermographic systems, i.e., mobile and low-cost thermal IR imaging devices. One of the first companies to commercialize mobile thermal devices was FLIR with systems such as the FLIR ONE Pro, \(160\times 120\) spatial resolution, an approximately 8.7 Hz sampling rate, 0.15 \({}^{\circ}\)C thermal sensitivity, and dimensions of \(68\times 34\times 14\) mm\({}^{3}\), or FLIR Lepton, \(80\times 160\) spatial resolution, an approximately 8.7 Hz sampling rate, \(<\)0.05 \({}^{\circ}\)C thermal sensitivity, and dimensions of \(11.8\times 12.7\times 7.2\) mm\({}^{3}\). FLIR ONE was designed to be integrated on mobile phones. Another mobile thermal system designed to be integrated on a mobile phone is the Therm-App system developed by Opgal manufacturer; it has \(384\times 288\) spatial resolution, an approximately 9 Hz sampling rate, approximately 0.07 \({}^{\circ}\)C thermal sensitivity, and dimensions of \(55\times 65\times 40\) mm\({}^{3}\). Market research has shown that the SmartIR640 mobile thermal system, manufactured by Device aLab (\(640\times 480\) spatial resolution, 30 Hz sampling rate, \(<\)0.05 \({}^{\circ}\)C thermal sensitivity, and dimensions of \(27\times 27\times 18\) mm\({}^{3}\)) is also a valid solution, but it is not yet used for research projects. Despite the relatively low-quality thermal imaging outputs of a mobile thermal system, this technology could help bridge the gap between the findings from highly constrained laboratory environments and wild real-world applications. Indeed, its portability (e.g., small size and low computational resource requirement) allows the camera not only to be easily attached to a mobile phone but also to be integrated in a social robot head. Recent studies have started to explore mobile thermal IR imaging for affect recognition tasks, especially focused on stress monitoring [78; 79; 80; 81]. [PERSON] et al. proposed a system consisting of a smartphone camera-based PhotoPlethysmoGraphy (PPG) and a low-cost thermal camera added to the smartphone, which was designed to continuously monitor the subject's mental stress [78]. By analyzing the nose tip temperature and the blood volume pulse through PPG [82; 83], they were able to classify the stress with an accuracy of 78.33%, which is comparable to the state-of-the-art stress recognition methods. The employed mobile thermal camera was the FLIR ONE. The study was conducted in a quiet laboratory room with no distractions. Another study by the same authors included the mobile thermal camera as a standalone system to monitor mental stress [80]. The authors proposed a novel low-cost non-contact thermal IR imaging-based stress recognition system that relayed on the breathing dynamic patterns analysis. In fact, since breathing is an important vital process controlled by the ANS, its pattern monitoring can be informative of a person's mental and physical condition. Results showed a classification accuracy of 84.59% for a binary classification (i.e., no-stress, stress) and an accuracy of 56.52% for multi-class classification (i.e.,none, low, high-level stress). The breathing signal was recovered by tracking the person's nostril area. Then, the extracted breathing signal was converted in a two-dimensional spectrogram by stacking the Power Spectral Density (PSD) vector of a short-time-window respiration signal over time. Since the PSD function handles the short-time autocorrelation that identifies similarities between neighboring signal patterns, it can be used to examine respiration variations in a short period [65]. The breathing signal was also investigated through the use of a mobile thermal camera by [PERSON] et al., who embedded such a camera in smart glasses [84]. [PERSON] et al. instead used a mobile thermal system for the challenging purpose of classifying personality (psychoticism, extraversion, and neuroticism) [81].