Photoplethysmography (PPG) is an optical technique that provides a non-invasive method for monitoring changes in blood volume within the microvascular bed of tissue. This technology uses light to detect fluctuations in the amount of blood passing through the skin’s surface, offering valuable insights into the cardiovascular system. PPG sensors have become a standard feature in many consumer electronics, including smartwatches and fitness trackers, to collect physiological data continuously. A PPG sensor pairs a light source, typically an LED, with a photodetector to measure how much light is absorbed or reflected by the body’s tissues. The resulting waveform, or “pleth,” allows devices to track blood flow dynamics and derive several health metrics.
The Underlying Science of Photoplethysmography
The core mechanism of a PPG sensor relies on the principle that blood absorbs light more strongly than the surrounding soft tissue, bone, or skin pigments. An LED emits light—often green for wrist-worn devices or infrared for deeper measurements—directly into the skin. As this light penetrates the tissue, it is scattered and absorbed, and the photodetector measures the amount of light that returns to the surface.
The captured signal has two primary elements: the Direct Current (DC) and Alternating Current (AC) components. The DC component represents the constant absorption of light by non-pulsatile elements, including stationary tissue, bone, and venous blood, as well as the minimum arterial blood volume between heartbeats. This baseline absorption remains steady over time.
Superimposed on the stable DC baseline is the dynamic AC component, which is synchronized with the cardiac cycle. This AC signal results from the pulsatile change in arterial blood volume when the heart ejects blood into the peripheral vessels. With each heartbeat, arteries momentarily expand, increasing blood volume and light absorption before contracting again. The sensor detects this rhythmic change, which corresponds to the pressure wave traveling through the arteries, allowing for the extraction of physiological data.
Essential Data Points Derived from PPG
The raw plethysmographic waveform is processed using algorithms to extract meaningful physiological metrics, primarily by analyzing the frequency and characteristics of the AC component. Heart Rate (HR) is the most straightforward measurement, determined by identifying the frequency of the peaks in the AC signal. Since each peak represents the maximum blood volume reached during a single heartbeat, counting the number of peaks over a specific time period allows the device to calculate the rate in beats per minute (BPM).
A more nuanced measurement derived from the heart rate data is Heart Rate Variability (HRV), which is the measurement of the subtle fluctuations in the time interval between successive heartbeats. Rather than calculating a single rate, HRV measures the beat-to-beat differences, known as the R-R intervals, even when the overall heart rate remains stable. This variance is used as an indicator of the activity of the autonomic nervous system, providing insights into stress levels, recovery status, and overall physiological resilience.
Measuring Blood Oxygen Saturation ($\text{SpO}_2$) requires a complex approach using two distinct wavelengths of light: red (around 660 nm) and infrared (around 940 nm). This dual-wavelength system works because oxygenated hemoglobin ($\text{HbO}_2$) and deoxygenated hemoglobin (Hb) absorb these wavelengths differently. Deoxygenated hemoglobin absorbs more red light, while oxygenated hemoglobin absorbs more infrared light.
To calculate $\text{SpO}_2$, the sensor measures the AC and DC components for both the red and infrared signals. Data processing computes the ratio of AC to DC for each wavelength, isolating the pulsatile arterial signal from background absorption. The final calculation determines a “ratio of ratios” (red light measurement to infrared light measurement), which is then converted into the percentage of oxygen saturation using a calibration curve.
Integration and Use Cases in Modern Technology
The compact nature and non-invasive function of PPG sensors have made them integral to modern health monitoring devices. In consumer wearables like smartwatches and fitness bands, the sensor is typically placed on the wrist and often uses green light. Green light is effectively absorbed by blood vessels close to the skin’s surface, providing a strong signal for heart rate tracking. These devices utilize the captured data to monitor sleep quality, track physical activity intensity, and screen for potential heart rhythm irregularities.
The same technology is found in medical devices, often used in a transmissive mode, such as a fingertip clip, which shines light through the tissue. Clinical pulse oximeters commonly use the fingertip or the earlobe because these locations have a high density of capillaries. This allows medical professionals to continuously monitor a patient’s oxygen levels during surgery, recovery, or respiratory compromise.
Beyond established applications, current research is exploring the use of PPG to estimate blood pressure without a traditional inflatable cuff. Algorithms analyze characteristics of the pulse wave, such as the time it takes for the pressure wave to travel through the body. This analysis correlates changes in the PPG waveform with changes in blood pressure, aiming to provide continuous, convenient, and cuffless blood pressure monitoring.