## 1 Introduction  
The monitoring of human vital signs, for example respiratory rate (RR), blood oxygen saturation (SpO\({}_{2}\)), heart rate (HR), heart rate variability (HRV) and blood pressure (BP) plays a significant role in modern clinical care of patients in hospitals and at home [1]. Applications include medical diagnosis, training programs, fitness assessment, lie detection and stress measurement [2]. There are various instruments for measuring these vital signs, such as electrocardiograms (ECGs), pulse oximeters, nasal thermocouples, respiratory belt transducers and piezoelectric transducers [3]. These instruments require direct physical contact with the human body as they use contact-based, sensor modalities, straps, probes or electrodes [4]. These instruments may cause skin infection, injury, or harmful reactions on patients especially premature babies, aged people or burns victims who have fragile skin [5]. Moreover, there is a risk of entanglement or strangulation of infants who are attached to monitors by means of wires and leads [6]. These instruments are also not appropriate for long term monitoring as they may cause discomfort, irritation and a cumulative risk of fungal and bacterial infection [7]. In addition, a reduced amplitude of chest wall expansion can affect the respiratory rate (RR) input signal from the impedance lead [8]. Furthermore, cost is an important issue as the monitoring electrodes and leads are only intended and certified for a single use, followed by disposal [9]. Placement of the sensors with self-adhesive pads leads to difficulties with wet, oily, dirty or hairy subjects which is a limitation of these technologies in emergency situations [10]. Accuracy is another issue with conventional contact methods since they are sensitive to artefacts produced by the subject's movement [11]. Therefore, to minimise these limitations, there is a need for an alternative method where vital signs can be measured without any physical contact. As presented in Figure 1, there are several noncontact means based on magnetic induction, the Doppler effect, thermal imaging and video camera imaging which can be an effective alternative means of monitoring vital signs with acceptable reliability and accuracy [12]. These methods depend on the observation of physical and physiological variations including skin colour, temperature, impedance changes, head motion, arterial pulse motion, and importantly, thoracic and abdominal motion due to the activity of both the respiratory and cardiovascular systems. Magnetic induction-based methods can detect the impedance changes caused by blood and air volume variations due to the mechanical action of the heart, diaphragm and thorax. The basic principle is to induce eddy currents in the tissue and to calculate the re-inducted magnetic field externally; the impedance changes can then be observed remotely to extract vital signs [13]. The method uses a simple arrangement based on multiple coils [14] or a single coil [15] integrated into a mattress [16], bed [17] or seat [18]. However, the method is highly susceptible to relative movements between coil and body. The Doppler effect is an active noncontact method that can detect subtle chest movements due to cardiorespiratory activity. In this method, vital signs are extracted using a Doppler radar [19; 20], or laser sensors [21] as well as digital signal processing (DSP) techniques [22] where the phase shift between the transmitted waves and the reflected received waves from a region of interest (ROI) are calculated. There are three types of Doppler-based methods--Doppler with electromagnetics [23; 24], lasers [25; 26], and ultrasonics [27; 28; 29]. Thermal imaging [30; 31] is a passive noncontact method that can detect the radiation emitted from particular parts of the human body in the infrared (IR) range of the electromagnetic spectrum to measure the physiological signal using a thermal camera [32; 33; 34].