1. Application of Optical Heart Rate Monitoring
學號：
學生：林彥樺

2. Measurement Principles
PPG technique relies on illuminating a living tissue with a light beam, capturing a portion of the light that has propagated through the living tissue, and analyzing said captured light, depicting functional or structural information on the tissue.
Assuming the intensity of an injected monochromatic light beam being I0, one expects the intensity of the transmitted light through the medium to be
I=I0 e- α ι

3. Measurement Principles
However, the blood and other biological tissues are not homogeneous, quite contrary, and therefore absorption of light passing through is not simply proportional to the concentration of hemoglobin and to the optical path length. Blood is an inhomogeneous liquid exhibiting a nonlinear absorbance of light.
Absorbance and scattering varies during the cardiac cycle with respect to the orientation of red blood cells during the ontraction and relaxation periods of the heart. Absorbance is increased because of light reflection at the skin surface and multiple scattering effects, causing the deviation of the light beam from its initial direction.

4. Measurement Principles
The main factors affecting the DC fluctuations are respiratory and vasomotor activities, and thermoregulation (as described in a later section).
The transmitted light captured by the photodetector might come from two different modes or pathways, as shown in Figure 1
Fig.1

5. Factors Affecting the Quality of Signal
As depicted by Figure 2, the PPG signal derived from the PPG phenomenon is determined by three families of factors: sensing, cardiovascular, and biological factors.
PPG measurements are highly determined by the implemented sensing setup: the amount and nature of emitted light, the coupling between the skin, tissue, and the optical probe, and the response of the photodetector will influence the measured PPG signal.
Sensor geometry and ergonomics constitute an important factor that will also highly determine the so-called optical shunting effect, which is the amount of direct light traveling from the light emitter toward the photodetector without penetrating the biological tissue.

6. Factors Affecting the Quality of Signal
Perturbations due to ambient light on PPG easurements are to be minimized as well by an adequate sensor design. Optimal distance between light emitters and receivers is to be chosen as a trade-off between the desired depths of tissue penetration and the achievable light intensity to be injected into the skin.
Fig.2

7. APPLICATIONS
Monitoring HR during exercise is especially useful in endurance training, professional training planning, or fitness workout. Maximal oxygen uptake and energy expenditure can be accurately estimated from HR measurements .
Today, ECG-based chest straps are widely used for monitoring HR during sports, and their annual sales exceed 10 million pieces worldwide. Chest- strap-based HR monitors provide relatively accurate monitoring of HR, but they suffer from reduced comfort from the chest strap, especially for female users. In addition, their reliability may be compromised with dry skin, dirty electrodes, or poor strap placement.

8. APPLICATIONS
Optical monitoring of HR has been recently introduced as an alternative to overcome especially the usability and user acceptance-related challenges in chest-strap HR monitors.
Optical monitoring of HR during sports may be done from different body positions,including ear, forearm, and wrist . Most of the commercialized devices use a green light source (one or several LEDs) combined with a single photodetector.

9. CONCLUSION
Because of the complex interactions between mechanical sensor parts, optical properties of the sensors, and particularities of living tissues, the design of optical HR monitors should be driven by a thorough analysis of the specific PPG configuration to be implemented.
In conclusion, optical HR monitors have the potential to become one of the central technologies to support the development of the twenty-first century’s health and well-being assessment revolution. The technology is there; the implementation is in your field.