## 2 Video Camera Imaging Based Method
### Colour-Based Method  
Photoplethysmography (PPG) is a non-invasive, low-cost, passive, optical technique first proposed by [PERSON] et. al. in 1937 [PERSON] et al. (1937). It can monitor three vital signs: HR, RR and SpO\({}_{2}\). PPG is used extensively in biomedical, clinical and non-clinical fields due to its simple design and relatively low cost. PPG measures variations in the optical properties of transmitted or reflected light from the human skin caused by blood volume changes during cardiorespiratory activity. Blood absorbs light more than the adjacent tissue which causes subtle optical property variations of the skin because of the haemoglobin in blood. Generally, PPG can function either in a transmissive or a reflective mode; however, the first mode is limited to regions such as the ear lobes and fingertips. In the PPG technique, a dedicated light source is required to illuminate a part of the body and an optical sensor or photodetector connected to the skin is needed to sense the optical properties of the skin. Imaging photoplethysmography (iPPG) is basically remote or non-contact PPG; whereas for skin-contact, a PPG sensor and illuminator replace the camera. To detect PPG signals remotely, many researchers have considered a video camera as an optical sensor. Some studies have used a dedicated light source and others have considered just ambient light as a source of light. Nevertheless, a dedicated light source increases the hardware setup of experiments as well as starts to become invasive to the participant, particularly infants. As shown in Figure 6, the iPPG reflection model mainly consists of three parts such as a light source, a patch of human skin containing pulsating blood and a video camera. The light source can be a dedicated light source or ambient light. When a light source illuminates human skin, subtle colour changes are observed from the videos captured by the camera. Without illumination variations and motion artefacts, colour changes denote the blood volume changes in the microvascular tissue  
Figure 5: Block diagram of the contactless vital sign monitoring system using a Microsoft Kinect sensor [64]. bed under the skin because pulsatile blood flows vary each cardiac cycle. Nevertheless, illumination variations and motion artefacts can also be a reason for the intensity and spectral composition variations. So, the skin area observed by the camera has a colour variation because of motion-induced and illumination-induced intensity/specular changes and pulse-induced subtle colour changes. It is assumed that the spectral composition of the light is fixed. The variation of light intensities is dependent on the distance from light source to camera and to the skin, and the specific geometry of the situation. Based on the dichromatic model [67], the light reflected from the skin can be presented as follows  
\[P_{k}(t) = I(t)\;(R_{s}(t)\;+\;R_{d}(t)\;)\;+R_{n}(t) \tag{4}\]  
where \(P_{k}(t)\) represents the RGB channels of the \(k\)th skin pixel; \(I(t)\) denotes the illumination intensity level; \(R_{s}(t)\) signifies the specular reflection and \(R_{d}(t)\) represents the diffuse reflection. \(I(t)\) is modulated by both specular and diffuse reflection. \(R_{n}(t)\) signifies the quantization noise of the camera sensor. The specular reflection of light from the skin surface is like light reflected from a mirror. As specularly reflected light does not penetrate skin tissues, it does not carry any information about the physiological signal. Additionally, the spectral composition of specular reflected light is identical to the light source [67]. On the other hand, diffuse reflected light penetrates the skin, it is absorbed and scattered inside skin tissues and then reflected. Useful information about the physiological signal can be extracted from the diffuse reflected light. Even though the slight colour variations of the skin are invisible to human eyes, they can be detected by video cameras.