The Photoplethysmography (PPG) also known as Blood Volume Pulse (BVP) is a non-invasive, low-cost biosignal sensor predominantly used in medical and psychophysiological settings to measure the heart rate. In the former case, it is used as a transportable (mobile or wearable) alternative to Electrocardiograms (ECG) outside of clinical settings, whereas in the latter it is often used in biofeedback, a technique to gain awareness and control over physiological functions that affect mental or neurobiological conditions. Furthermore, the Heart Rate Variable (HRV) derived from PPG signal can be effectively used as an indicator for emotion recognition most effectively alongside other biosignals such as EDA, EMG, or respiration sensing. Alongside other biosignal sensing devices, PPG has also been used in artistic audiovisual installations, typically collecting sensor data from an audience to create an evolving installation environment.

Background The term photoplethysmography was first introduced by Alrick Herzman and colleagues from the Department of Physiology at St. Louis University School of Medicine in the 1930s as “photoelectric plethysmography” based on the principle of light absorption by transilluminated tissue which varies based on the blood content. In 1938, Hertzman performed a validation of his PPG method comparing blood volume changes with measurements taken simultaneously by mechanical plethysmography. He then contended that a good skin contact for this method is required but without a pressure that can cause blanching of the site of measurement. He further concluded that sensor displacement can affect the measurement accuracy. But it was not until the development of pulse oximeter in the 1970s to measure patients’ blood oxygen saturation that a major advancement in clinical use of PPG was introduced which led to developments in computer-based digital signal processing and analysis.

The principle of operation of PPG is based on a non-invasive optical contact sensor that emits an infrared or green LED to measure the heart rate based on the volume of blood that passes through the tissues in a specific body site with the pads of the fingers or the earlobes being the most common. The light emitted to the tissue is then reflected back to the photodetector of the sensor which indicates the blood volume as the hemoglobin in the blood selectively absorbs the light reflected by other tissues. In this manner, the sensor directly measures the changes in blood volume in the arteries and capillaries of the tissues that it is in contact with which corresponds to changes in the heart rate and blood flow. While infrared is used to measure deep blood concentration in body parts such as muscles, green light often measures the oxygen absorption in oxygenated and unoxygenated blood. Green LED is considered to offer more accurate measurement in some literature as it is less prone to motion artifacts when compared to infrared. The PPG Waveform is constituted by plotting the variation in intensity of the reflected light over time.

There are two types of sensor arrangements for PPG signals: transmission mode and reflectance mode. In transmission mode the light and the detector are put on either side of the tissue while in reflectance mode they are positioned on the same side of the tissue. It is noteworthy that in transmission mode too much pressure on the tissue can affect the blood flow which in turn will affect the measurement. Common sites for transmission mode are fingers and earlobe whereas forehead, wrist, forearm, torso, and ankle are often cited for reflectance mode.

Using PPG, several measures can be derived, such as HRV, indicating changes in heart rate, peripheral blood flow, tissue engorgement, blood volume amplitude and elasticity of the vascular bed which is the range over which vasoconstriction (narrowing) and vasodilation (widening) of the blood vessels occurs.

Although PPG and BVP are considered to be very similar signals, BVP is the signal obtained when highpass filtering the PPG signal. Changes in PPG baseline indicates the overall blood volume. While an increase in BVP amplitude indicates the parasympathetic response via the vasodilatation of the capillaries, a decrease in the amplitude points to sympathetic response via vasoconstriction.

There are several factors that can alter the PPG recordings that can be classified as sensing architecture, biological, and cardiovascular factors outlined in the table below.