# Unobtrusive Respiratory Flow Monitoring Using a Thermopile Array: A Feasibility Study
## 1 Introduction  
Continuous respiratory monitoring can be used to predict potentially serious clinical events like cardiopulmonary arrest [1; 2], and it is particularly relevant for the detection of specific clinical conditions such as apnea of prematurity, Sleep Disordered Breathing (SDB), or Sudden Infant Death Syndrome (SIDS) [3]. Respiratory activity can be monitored with a variety of techniques including flow sensors, chest wall motion sensors, and indirect extraction of the respiration signal [2]. However, in some applications, flow sensors are necessary. In particular, obstructive apnea is a type of apnea characterized by the presence of chest wall motion, but absence of flow [4]. Various sensors can be used to detect respiration flow such as pneumotachographs, or-nasal thermistors, or nasal pressure transducers. These, however, need to be positioned close to the nostrils, fixed on the upper lip, or integrated in a mask and result in being particularly uncomfortable for patients [4]. Many remote sensing technologies have been used to monitor respiration, e.g., RGB and near-infrared cameras or radars. Their detection is, however, based on respiration motion, not flow. An unobtrusive way to monitor respiration flow is based on thermal imaging, also called InfraRed Thermography (IRT) [5; 6]. Most of the work on IRT uses the nostrils and/or the mouth area as the Region Of Interest (ROI) to evaluate the thermal induced respiratory variations. The ROI can be determined manually or automatically by using anatomical characteristics, e.g., medial canthus region, combined with tracking algorithms [7; 8; 9]. In real applications, e.g., sleep monitoring, multiple cameras would be necessary to monitor the patient continuously and cover all possible orientations. This would undoubtedly raise the cost of thesolution, making it even more important to find an inexpensive sensor solution to detect respiration (and the absence of it). [PERSON] et al. [10], for example, used FLIR ONE to detect respiration flow in outdoor and indoor settings. The work focused on the robustness to environmental thermal dynamics. Whereas, [PERSON] et al. [11] proposed a sensor fusion of FLIR Lepton and RGB cameras. Both works were based on facial landmark detection and used relatively small microbolometer-based thermal cameras. Microbolometers are the most common sensors for uncooled infrared imaging: the infrared radiation absorbed changes the resistance of the material. Thermopile arrays, instead, consist of multiple thermopile sensors. These sensors are constituted by thermocouples in series, which are based on the Seebeck effect, i.e., they generate a voltage proportional to the infrared radiation absorbed [12]. They have been widely used for non-contact thermometers [13], fall detection [14], human detection [15; 16; 17], and fever screening [18]. Thermal cameras based on microbolometers started having smaller dimensions and cost, as FLIR ONE or FLIR Lepton. However, the main difference between thermopiles and microbolometers lies in the production cost; microbolometer-based infrared imagers require several costly processes (e.g., know-how of micromachining, vacuum packaging, and materials' preparation) that significantly raise the price [19]. It would therefore be of interest to understand if a thermopile array could be used as a respiratory flow detector, yielding a further reduction in cost. Moreover, the detection of facial landmarks is not straightforward in real environments [20], especially when the resolution would be further reduced. Recent work from [PERSON] et al. [20] proved the usability of a thermal camera to detect the respiration signal without the use of anatomical features, both on healthy adults and infants in the Neonatal Intensive Care Unit (NICU). The method uses a black-box approach, wherein the resolution is reduced by averaging multiple pixels. Moreover, an empirical Signal Quality Index (SQI) is used to analyze the spectrum of each cell, and the ROI containing respiratory information is chosen based on the SQI value.