PPG foundations
Learn photoplethysmography.
Start with the light. Follow it through tissue, into a waveform, and toward a better understanding of optical biosignals.
*Infrared is invisible; violet is false color. Abstract, separated layers explain the light paths; real tissue is continuous. Paths are not to scale. Sampling depths overlap and vary with tissue and sensor geometry.
FIELD NOTES / 001
More than
skin deep.
Light scatters. Blood absorbs. A detector catches a small part of the returning light—and its subtle changes with each pulse.
Explore the anatomy behind the waveform, from the blood-free epidermis to the vessels beneath it.
Read the PPG primer
What is SpO2 in Photoplethysmography?
SpO2 (peripheral oxygen saturation) measures the percentage of hemoglobin in arterial blood saturated with oxygen. PPG-based pulse oximeters estimate SpO2 by comparing light absorption at red (~660 nm) and infrared (~940 nm) wavelengths, exploiting the different absorption spectra of oxygenated and deoxygenated hemoglobin.
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What Is an Interbeat Interval?
An interbeat interval is the time from one heartbeat to the next, usually in milliseconds. IBI is the generic name. On an ECG the same gap is an R-R interval. On a PPG recording it is the time between matching points on two pulses, which Garmin calls a beat-to-beat interval (BBI) and some papers call a pulse rate interval (PRI). The point you mark, and any beat you miss, both land in the HRV math.
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Perfusion Index in PPG Signal Processing
Perfusion Index (PI) is the ratio of the pulsatile (AC) component to the non-pulsatile (DC) component of the PPG signal, expressed as a percentage. It quantifies peripheral blood flow strength and indicates signal quality — higher PI values generally indicate better perfusion and more reliable physiological measurements.
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What Is HRV (Heart Rate Variability)?
Heart rate variability (HRV) is the variation in time between one heartbeat and the next. Even at a steady 60 BPM, the gaps between beats are not all exactly one second. HRV is measured in milliseconds, mostly reflects the parasympathetic ('rest and digest') side of the nervous system, and is highly individual: it falls with age and differs a lot between healthy people.
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What Is RMSSD?
RMSSD is the root mean square of successive differences between heartbeat intervals. Square each gap-to-gap change, average those squares, then take the square root. Watches usually print it in milliseconds. The conventional minimum recording in an open review is 5 minutes, and a nightly wearable average is a different object from that lab window.
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What Is Actigraphy?
Actigraphy is a movement record from an accelerometer on the wrist, ankle, or waist. Software turns that record into rest, activity, and an estimate of sleep versus wake. Clinical units use a piezoelectric sensor or a MEMS accelerometer, and they store activity counts in epochs or, on newer devices, the raw acceleration. Stage labels come from polysomnography. A PPG sensor in the same case times the pulse. The accelerometer still supplies the movement.
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AC/DC Ratio in PPG: Pulsatile vs. Static Components
In a PPG signal, the AC component represents the pulsatile variation in optical absorption caused by the cardiac-driven blood volume pulse, while the DC component represents the relatively static absorption from tissues, venous blood, and non-pulsatile arterial blood. Their ratio is the basis for perfusion index and SpO2 calculation.
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Signal Quality Index (SQI) in PPG Analysis
A Signal Quality Index (SQI) is a numerical metric that automatically quantifies the reliability of a PPG segment for physiological analysis. SQIs typically range from 0 (unacceptable) to 1 (clean signal) and are used to gate downstream algorithms — only high-quality segments are processed for SpO2, HRV, or other metrics.
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Pulse Transit Time (PTT) from PPG
Pulse Transit Time (PTT) is the time for the arterial pressure pulse to travel between two points in the vascular tree, typically measured between the ECG R-wave and the PPG pulse arrival at a peripheral site. PTT is inversely correlated with blood pressure and forms the basis for cuffless continuous blood pressure estimation.
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Estimating Pulse Wave Velocity from PPG
Pulse Wave Velocity (PWV) is the speed at which the arterial pressure wave travels along a vessel, measured in m/s. It is the gold-standard non-invasive marker of arterial stiffness, a major independent predictor of cardiovascular events. PWV can be estimated from the ratio of vascular path length to PTT.
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Augmentation Index (AIx) from PPG Waveform Analysis
The Augmentation Index (AIx) quantifies the contribution of the reflected arterial pressure wave to the central aortic pulse waveform, expressed as a percentage of pulse pressure. In PPG, AIx is estimated from the ratio of the late systolic augmentation pressure to the total pulse pressure, serving as an index of arterial stiffness and wave reflection.
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PPG Waveform Morphology: Features and Analysis
PPG morphology analysis examines the shape characteristics of individual pulse waveforms to extract cardiovascular information beyond heart rate. Key morphological features include systolic peak amplitude and timing, dicrotic notch position, diastolic peak, pulse width at half-maximum, and features from the first and second time derivatives.
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