## 1 Introduction
Thermal imaging-based methods extract vital signs by measuring temperature changes around the nostril area [35; 36; 37; 38] as well as heat differences due to pulsating blood flow in the main superficial arteries at various regions such as the carotid artery in the neck [39; 40] and temporal artery in the forehead [41; 42]. However, both Doppler- and thermal imaging-based approaches are susceptible to noise and motion artefacts and constrain the movement of the subjects due to the high cost of the sensor, preventing saturation sampling of the environment. Their relatively low resolution limits the detection range and specificity to one subject. Moreover, these methods need exposed ROI and specialized hardware, making them costly [4]. They are also constrained to short-term monitoring and monitoring a single subject at a time. Additionally, Doppler-based methods may have biological effects on humans [43] with unknown future population risks if broadly adopted. Digital cameras offer high resolution, in spatial (number of pixels per degree), temporal (number of frames per second), intensity (number of bits per pixel) and in spectrum (at least 3 visible channels, with  
Figure 1: Contactless measuring methods. (a) The Doppler effect; (b) thermal imaging; (c) video camera imaging. hyperspectral options increasingly common), all due to consumer market demand. Furthermore, a large base of research assets exist for processing imagery, much of it free for use, for example; OpenCV [44]. Flexibility with visible light optical design, offering panoramic, microscopic and telescopic solutions in well integrated commercial product families allows diverse measurement scenarios. Tailored fields of view allow analysis of multiple ROIs in parallel, or in series based on availability. The mass market has led to low cost [4; 12] and affordable optics that can be used in almost any conceivable application scenario [9]. Video camera imaging is a passive contactless method where video cameras are used to extract different physiological signals from several regions of the human body, exploiting two principles. The first principle relies on skin colour variations due to cardiorespiratory activity, photoplethysmography (PPG). Vital signs are measured by exploiting variations in the reflectance properties of human skin from video, which causes variation in brightness values in sequences of images. The second principle depends on cyclic body motion owing to cardiorespiratory activity in techniques that can be broadly characterised as motion-based methods. The motion in the regions of the head, arterial pulse, and thoracic and abdominal region are included in this method. For noncontact physiological assessments, camera imaging based methods seem to be a promising approach since they are robust, reliable, safe, cost-effective, suitable for long distance and long-term monitoring as well as multiple person detection simultaneously [12]. Camera imaging-based methods have been attracting increasing attention in the literature. This paper aims to explore the progress of video camera imaging-based technology from controlled clinical scenarios with fixed monitoring installations and controlled lighting, towards uncontrolled environments, crowds and moving sensor platforms. We focus on the diversity of applications and scenarios being studied in this topic. We emphasise visible light sensing since these cameras represent the largest installed base, the lowest costs, the highest rate of improvement and the greatest opportunity to insert new capability into existing devices. First, we discuss studies of motion and colour-based methods. Then, we discuss the considerations and scenarios appropriate to colour-based methods, for example, in the presence of motion artefacts, illumination variation, different sensors, different subjects, different vital signs, multiple ROIs, long distance and multiple persons. Additionally, potential application of iPPG in both clinical and non-clinical sectors are described.