## 4 Measurement Results of HR and HRV with Optical Homodyne and Heterodyne Technique
### Measurement of HRV  
The time difference between two successive R peaks is calculated throughout the ECG wave. The mean value of this time difference is called the R period. Similarly, the time difference between two successive \(V_{1}\) peaks is calculated, and this time difference is called the V period. The R period and V period provide the time duration for each heart activity. The difference between two consecutive R period intervals is calculated as the R-R variability, while the difference between two consecutive V period intervals is calculated as the V-V variability [45]. In this experiment, different subjects with different ages and BMI are considered. Table 4 shows an example of the measurement of the heart period and \(HRV\) by means of ECG and VCG waveforms. For this measurement, the test subject was in a state of rest. In order to measure the \(HRV\), the periodic characteristic related to the heartbeat needs to be identified [46].  
From Table 4, the mean value of the R-R interval is calculated to be 101.056 ms with a standard deviation of 48.8 ms and a standard error value of 6 ms, while the mean value of the V-V interval is calculated to be 101.257 ms with a standard deviation of 36.3 ms and a standard error of 4 ms. The UU variability and VV variability are equal, as they are part of the same signal. Therefore, for further analysis of \(HRV\), only one of UU or VV variability is considered.  
Figure 28 shows the column chart representation of variability in the R-R interval, U-U interval, and V-V interval, simultaneously. The heart period for each heartbeat count for both the ECG and VCG signal is represented as a vertical column line. For the VCG signal, the heart interval is measured for both the U-U and V-V wave, while the R-R wave is considered for the ECG [47]. From Figure 26, it is clear that the measurement of \(HRV\) from the ECG and VCG plots shows similar results, with a less than 1% deviation value. Figures 29 and 30 show the R-R and V-V variability plot for the test subject under sleep. A \(HR\) of 40 bpm is measured with the test subject under deep sleep. Figure 31 shows \(HRV\) analysis of the test subject under an active state, and a \(HR\) of 72 bpm is measured. Figures 32 and 33 show \(HRV\) analysis of the test subject under excessive stress and physical exertion, respectively. A \(HR\) of 96 bpm and 108 bpm are recorded for test subjects under excessive stress and physical exertion, respectively. All results shown here are obtained on the chest wall. Figure 34 shows the \(HRV\) in the ECG and VCG for different test subjects. The  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Hearbeat Count** & **R Period (s)** & **R-R Variability (s)** & **U Period (s)** & **V Period (s)** & **V-V Variability (s)** \\ \hline
1 & 0.256836 & - & 0.23564 & 0.27564 & - \\
2 & 1.283203 & 1.026367 &  
Figure 31: Comparison of R-R and V-V variability from ECG and VCG for _HR_ 72 bpm.  
Figure 32: Comparison of R-R and V-V variability from ECG and VCG for _HR_ 96 bpm.  
Figure 33: Comparison of R-R and V-V variability from ECG and VCG for _HR_ 108 bpm.  
Table 5 shows a complete summary of \(HR\) and \(HRV\) for different test subjects under deep sleep, active state, and excessive exertion. It is clear that the ECG recording and VCG recording show similar results for \(HRV\) and \(HR\). For non-contact type measurement, both homodyne and heterodyne detection techniques are used. The \(HR\) and \(HRV\) values obtained from homodyne and heterodyne detection show much less deviation from the ECG values.