# Unobtrusive Vital Sign Monitoring in Automotive Environments--A Review
## 5 Optical Methods  
Radiation has the advantage of unnoticeable transport of information from a driver or passenger without contact. Hence, as long as the optical pathways are free, optical modalities potentially offer  
Figure 6: Potential BCG sensor locations in a car seat. Note that the lumbar and especially the thoracic sensor location in the backrest are likely to have contact problems. By contrast, sensors in the seating area as well as in the safety belts will face vibrations coupled from the vehicle body.  
unobtrusive sensing from a distance that is attractive for automotive applications, especially since, in modern cars, cameras are already used for external obstacle detection, etc.  
In the recent past, there has been a boom in camera-based-technologies and applications for vital-sign monitoring. We note that, according to the utilized frequency ranges, the individual camera technologies differ with respect to materials, prices and size. Figure 7 highlights the frequency spectra suitable for cameras.  
From Figure 7, we can conclude that there are three frequency ranges suitable for driver status monitoring: visible light (VIS, wavelength 350-740 nm), near-infrared light (NIR, wavelength 740-1000 nm) and far-infrared light (FIR, wavelength > 1 \(\upmu\)m). Table 3 compares specific features of the different camera concepts and designs reported in the literature to date.  
In the VIS and the NIR range, light can interact with living tissues. Due to conservation of energy, there are three principal modes of interaction: absorption, reflection and transmission. Transmitted light may travel through the tissue on a straight pathway (as in transmissive PPG), but may also be scattered. In fact, in skin tissue, the dominating effect of tissue-photon interaction is scattering (about 50 times stronger than absorption).  
As for any optical method, a camera-based driver or passenger monitoring inside a car requires free line of sight. As the face offers uncovered skin, looking at the face of drivers and passengers easily comes to mind. Since current cockpit design requires free line of sight for the driver, in addition, the following camera locations have been explored (see Figure 8).  
If the angle of attack \(\rho>0\), the nostrils may become visible, which is interesting for FIR respiratory monitoring. We note that FIR imaging is often referred to as _infrared thermography_ (IRT).  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Name** & **Freq. Range** & **Abb.** & **Price** & **Energy** & **Special Properties** \\ \hline Visible Light (Vis) & 350 nm–740 nm & VIS & cheap & ambient light (passive) or & ambient light may fluctuate, \\  & & & active illumination & passive mode does not work at night \\ \hline Near Infrared (NIR) & 740 nm-1 \(\upmu\)m & NIR & cheap & active illumination & works at night, long exposure may exhaust the retina \\ Far Infrared (FIR) & 3 \(\upmu\)m–5 \(\upmu\)m & MWIR & expensive & passive & works at night, but glasses are not transparent \\ Far Infrared (FIR) & 8 \(\upmu\)m–14 \(\upmu\)m & LWIR & expensive & passive & works at night, but glasses are not transparent \\ \hline \hline \end{tabular}
\end{table}
Table 3: Comparison of different camera-based monitoring systems.  
Figure 7: Frequency ranges (VIS, NIR, FIR) usable for optical monitoring techniques.