# Non-Contact SpO2 Prediction System Based on a Digital Camera
## 1 Introduction  
Eighteen months after the emergence of COVID-19, the number of cases worldwide amounts to more than 152 million people, with more than 3.2 million deaths. In Iraq, the COVID-19 pandemic has infected more than 1 million people and caused more than 15,000 total deaths [1]. COVID-19 causes several symptoms, including fever, cough, fatigue and a respiratory illness whereby the patient often suffers from shortness of breath which causes a decrease in blood oxygen saturation level [2, 3]. As cases rise, the terms 'oxygen saturation' and 'pulse oximetry' regarding COVID-19 infection have been extensively utilised. Pulse oximetry is a non-invasive optical technique based on a photoplethysmography (PPG) signal obtained from the wavelength illumination transmitted or reflected by a tissue that is used to measure blood oxygen saturation (SpO2) level in both hospital and home environments [4]. With the urgent need for this device in hospitals or during home quarantine, it has become a significant concern for researchers to develop it and find suitable alternatives at the lowest possible cost. Extracting SpO2 is often essential to maintain patients' health because of its primary role in diagnosing respiratory diseases and in patient monitoring, especially during this global pandemic. Pulse oximetry relies on direct contact, most commonly with a fingertip, which restricts the patient and may become uncomfortable [5]. In addition, it may causeskin damage in subjects with a very fragile skin, especially infants in neonatal care, if it is left attached to the skin for long periods [6]. Moreover, pulse oximetry devices do not always fit well with people's fingertips, depending on patient's age and fingers' size, which may lead to distorted readings [7]. In addition, direct contact with the device exposes people to the risk of infection and skin irritation [8; 9]. Other reported limitations that may introduce errors include motion artefacts, and are linked to anaemia, venous pulsations, skin pigmentation, low perfusion, light interference and presence of nail polish [10; 11; 12; 13]. Therefore, there is a need to monitor SpO2 when contact with the skin is either undesirable or unsafe. With the rapid growth of remote-imaging technologies, non-contact and unobtrusive SpO2 monitoring has become feasible. Colour-based techniques using cameras in a variety of forms (such as digital cameras, thermal cameras, IR cameras, Webcams, drone and Kinect), sometimes called imaging photoplethysmography (iPPG), have been widely used in healthcare for monitoring applications, especially to evaluate vital signs, including heart rate (HR) [14; 15; 16; 17], heart rate variability (HRV) [18; 19; 20], breathing rate (BR) [21; 22; 23] and blood pressure (BP) [24; 25; 26]. The principle of iPPG is the non-contact monitoring of human cardiac synchronous variations by detecting optical features passing through or reflecting from the epidermis captured by a camera sensor. Experiments regarding such techniques, however, were performed under extremely controlled conditions in terms of subject movement, region of interest (ROI), distance and illumination conditions. The advantages and disadvantage of iPGG have been intensively discussed [9; 27; 28]. Conventional contact pulse oximetry mainly utilises two separate wavelength illuminations, the red light wavelength (660-700 nm) and the infrared (IR) light wavelength (800-950 nm) [29]. SpO2 can be determined as a linear function by considering the 'ratio of ratios' (RRs) relationship between the PPG signal in the red over IR wavelengths [29; 30]. The disadvantages of contact pulse oximetry have motivated research to use iPPG based on camera sensors (visible light and/or IR cameras) as an alternative. For example, the red-IR wavelengths were used in several studies [31; 32; 33] to predict SpO2. The possibility of estimating SpO2 using only visible-light camera channels as reference wavelengths instead of IR have been investigated in different studies.