# Unobtrusive Vital Sign Monitoring in Automotive Environments--A Review
## 5 Optical Methods
### PPG Imaging  
Using a camera and a proper light source (VIS and/or NIR, either ambient or artificial illumination), the reflective PPG principle can be expanded to remote camera-based monitoring from a distance. Figure 10 explains the principle. Note that both the light source and the camera can now operate remotely from a distance but require a free line-of-sight. Figure 10: Principle of PPG imaging (PPGI). Figure 9: LED and PD placement for transmissive and reflective photoplethysmography. In reflective mode, on average only 50 out of \(10^{6}\) photons leave the tissue to reach the PD. (**a**) transmissive PPG (tPPG); (**b**) reflective PPG (rPPG); (**c**) typical banana-shaped pathways of scattered photons in rPPG. Camera-based PPG methods have recently attracted increasing scientific attention. The principle of PPG imaging is a simple one: as the heart beats, the apparent colour of the skin changes due to blood pulsation. In principle, remote PPG imaging allows unobtrusive measurements of heart rate and derived parameters like heart rate variability (HRV) or pulse transit time (PTT) [80], for example when different locations are related to each other or when combined with ECG. To our knowledge, the earliest PPG applications were published by [PERSON] and colleagues from RWTH Aachen University starting in 2000 and centered around heart rate and perfusion measurements at the leg [81; 82; 83]. In 2005, [PERSON] et al. [84] reported on first steps towards SpO\({}_{2}\) imaging using a multi-wavelength approach. Employing a time-lapse image acquired from a CCD camera, [PERSON] and [PERSON] [85] developed a non-contact and non-invasive device in 2007, which was able to measure both the respiratory and pulse rate simultaneously. In the same year, [PERSON] and [PERSON] reported [86] that by using LEDs at three wavelengths (VIS and NIR), they were able to extract a PPG signal from skin with a CCD USB camera. In 2008, [PERSON] et al. [87] from the University of California at Irvine and the Norwegian University of Science and Technology at Trondheim were among the first to demonstrate that simple low-cost digital video cameras and ambient daylight illumination might be sufficient to capture a remote reflectance photoplethysmography (rPPG) waveform in controlled scenarios. The authors reported that they were able to extract both heart rate and respiratory rate. As mentioned in [PERSON]'s Ph.D. thesis [88], Philips Research started camera-based work on what they call the _Vital Signs Camera_ as early as 2004 [88]. Starting in 2012, Philips Research has been offering this software as an app for download to a smartphone for several years. While this applet was mainly targeting private customers for fun applications, Philips Intellectual Property and Standards department recently began licensing the Vital Signs Camera for commercial customers. Intended applications include wellness products and automotive applications (see [[https://www.ip.philips.com/licensing/program/115](https://www.ip.philips.com/licensing/program/115)]([https://www.ip.philips.com/licensing/program/115](https://www.ip.philips.com/licensing/program/115))). Significant work on camera-based monitoring has been performed by [PERSON]'s group from the University of Oxford, U.K. (see, e.g., [89]). As an example, in 2014, [PERSON] et al. from that group reported on heart rate estimation in neonates using CCD cameras and visible light [90]. Similarily, [PERSON] and coworkers published their results on SpO\({}_{2}\) imaging using RGB cameras in 2015 [91]. In order to estimate blood pressure from local pulse transit time signals, [PERSON] presented the results of their clinical studies involving volunteers and dialysis patients in 2016 [80]. While not directly refering to the term \"PPG\" as the underlying measurement principle, in 2012, [PERSON] and colleagues [92] have reported that, by employing Eulerian magnification, the subtle colour changes in the skin due to perfusion can nicely be visualized in relaxed subjects.