# Unobtrusive Vital Sign Monitoring in Automotive Environments--A Review
## 5 Optical Methods
### PPG Imaging
Of course, the fascination of these visualizations results from the fact that the human eye can not see these subtle color changes directly. This explains why the corresponding videos (available on the author's websites and on YouTube) have attracted signifcant attention. Unfortunately, by now, a variety of names referring to remote PPG imaging exist in the literature (see Table 4). While currently the terms _remote_ PPG and sometimes _remote_ PPG imaging/_remote_ PPG are becoming more and more popular, the term _PPG imaging_ (PPGI) is actually much older and was originally coined at RWTH Aachen University in Germany (see [81; 82; 83]). From the authors perspective, this name is quite descriptive and clearly refers to the underlying methodology of using a camera for extracting physiological information. To end the Babylonian confusion on the name, it is suggested to have a naming debate in the scientific community. In any case, the abbreviation \"rPPG\" is an unlucky choice, as traditionally rPPG and tPPG refer to _reflective_ and _transmissive_ PPG, respectively. All examples mentioned so far do not directly relate to monitoring in a car. Hence, the following subsection will look at literature with explicit automotive applications. PPGI Vital Sign Monitoring in the Car  
In 2014 and 2017, respectively, [PERSON] et al. from the Shenzhen Institutes of Advanced Technology and the Chinese University of Hong Kong, China [117; 118] presented their results on PPGI monitoring of heart and respiratory rate by RGB image processing and independent component analysis (ICA) in the visible light frequency range. While the earlier publication focused on laboratory experiments, the later publication of this group included pilot measurements on one subject while driving in a real car. In 2015, [PERSON] et al. [102] from Monash University in Melbourne, Victoria, Australia, reported on their results using imaging PPG. In their study, 10 participants completed an on-road driving task. A commercial heart rate monitor was used as a gold standard. As reported, IPPG produced a valid heart rate estimation for only 4 out of 10 participants. In cooperation with Volvo (Volvo Car Corporation, Torslanda, Gothenburg, Sweden), in 2015 [PERSON] and colleagues from Malardalen University in Sweden published a comprehensive review paper on the state-of-the-art in driver heart rate monitoring using cameras for the visible light range [119]. In the following year, this group suggested using a webcam for real-time heart rate monitoring [120]. While the paper focused on algorithms and lab experiments, the suggested applications included driver monitoring. In 2016/2017, a similiar approach to heart rate detection in the car was presented by [PERSON] et al. [107] from the National Chiao Tung University, Hsinchu, Taiwan. The authors also used RGB cameras and presented their results on lab experiments as well as test drives employing empirical mode decomposition (EMD) for filtering motion artifacts. Similarly, in 2017, [PERSON] et al. [97] from Karlsruhe Institute of Technology presented a camera-based online heart rate monitoring system for in-vehicle applications using such an off-the-shelf webcam as well. Their system reached a sensitivity of 93% under laboratory conditions (constant lightning and resting conditions). Their system was also tested under real-world driving conditions and showed encouraging results. However, due to a insufficient number of test persons (two), an additional evaluation on a larger test cohort is required to obtain statistically relevant results. Employing cameras with active illumination in the NIR frequency has been proposed as well, but not too many reports exist thus far. In 2015, [PERSON] proposed to integrate a near infrared (NIR) camera with active LED illumination into the rear mirror and employ it for heart beat detection during driving [121].