# A Survey of Photoplethysmography and Imaging Photoplethysmography Quality Assessment Methods
## 3 Introduction to Signal Quality Index for Photoplethysmographic Signals
### Definition of Quality
#### 3.1.1 Metrological Quality  
Metrological quality focuses on the measurement factors affecting both the photoplethysmographic signal retrieved and the physiological parameters extracted from it. This aspect of the signal quality is subject to perturbations (detailed in Table 1) arising from the environment, the operator (e.g., motion artifacts, poor installation of the sensor for contact PPG), the sensor (e.g., inadequate sampling frequency or resolution), or the mean. Different perturbations can occur and impact the signal with varying intensities, such as different amplitudes of a motion artifact or noise power of a sensor. Thus, we represent the notion of the quality of the signal with a continuous scale (Figure 2). This scale represents how much the quality of the retrieved signal is affected and depends on the method implemented to extract and process the signal, such as the performances of an algorithm of face tracking, signal extraction, and filtering for an iPPG measured from the face. For example, a smartwatch application dedicated to the measurement of photoplethysmographic signals during exercise may be more subject to motion artifact than a pulse oximeter device used at rest. However, the use of motion artifact reduction algorithms [34] allows one to recover a signal from some perturbations. This processed signal can ultimately be labelled as good-quality. Metrological quality focuses on the following question: given a measurement  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Physiological Factor** & **cPPG** & **iPPG** \\ \hline Measurement site & Impact on the shape of the PPG pulse [25] \\ \hline Autonomic nervous system (sympathetic & Impact on the baseline of the \\ and parasympathetic) & signal [26] \\ \cline{2-3}  & Impact on pulse amplitude [7; 27; 28] \\ \hline \multirow{3}{*}{Arterial stiffness} & Impact on the waveform [29; 30] \\ \cline{2-3}  & Impact on pulse wave velocity \\  & [29; 30] \\ \hline Presence of disease (Arrythmia, premature & Pulse to pulse interval [1; 18] & Pulse with multiple peaks (diabetes), incomplete pulses \\ ventricular & PPG waveform [7; 18] & (arrhythmia) [31] \\ \hline \multirow{3}{*}{Respiration} & Baseline wandering & \\ \cline{2-3}  & Modification of the waveform & \\ \cline{2-3}  & shape [25] & \\ \cline{2-3}  & Modification of frequency and & \\ \cline{2-3}  & amplitude of the AC & \\ \cline{2-3}  & component [29] & \\ \hline \multirow{3}{*}{Skin thickness} & Lower PPG signal intensity & \\ \cline{2-3}  & and modification of the & \\ \cline{2-3}  & waveform [29] & \\ \hline \multirow{3}{*}{Skin tone} & Higher absorption of & \\ \cline{2-3}  & lower-wavelength light & \\ \cline{1-1}  & (green) [29] & \\ \hline \multirow{3}{*}{Venous return} & Affects both low-frequency & \\ \cline{1-1}  & components and AC & \\ \cline{1-1}  & components [29] & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Physiological factors affecting a PPG signal.  
with a protocol and method, in a given condition of measurement, how well does the obtained signal reflect the variation in blood volume in the tissue?