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General information for researchers and clinic or lab staff. It is not medical advice and does not describe any clinical procedure.
Short answer
Photoplethysmography (PPG) is a non-invasive optical method that measures blood-volume changes in the microvascular bed of the skin. A light source shines on tissue, a photodetector records the light that is transmitted or reflected, and the pulsing component of that signal tracks each heartbeat. Pulse oximeters and most wrist wearables use it.
Definition and scope
Charlton and colleagues define a PPG signal as one that carries information on arterial blood volume, obtained from light reflected from or transmitted through tissue (Proc IEEE 2022). Park and colleagues describe it as a method for measuring blood volume changes in a microvascular bed of the skin (Front Physiol 2022). The word plethysmograph refers to measuring volume change, which is why the body plethysmograph and inductance plethysmography share the root but measure different things with different hardware. PPG is optical and measures blood volume in skin tissue.
How a PPG sensor works
The detected signal has two parts. The AC component follows the cardiac cycle: the detector receives the least light during systole, when more blood is in the tissue, and the most during diastole. The DC component reflects the tissue baseline and slower influences such as respiration. Absorption follows the Beer-Lambert law. Peak-to-peak timing of the AC component gives pulse rate, and the ratio of AC to DC amplitudes at two wavelengths underlies oxygen saturation (SpO2) estimation, covered in the pulse oximeter guide.
Wavelength matters because longer wavelengths penetrate deeper. Red (about 640 to 660 nm) and infrared (about 880 to 940 nm) are the most common choices in pulse oximetry; green is often used in reflectance-mode wearables, where higher-quality signals have been reported.
Transmission versus reflectance mode
| Feature | Transmission | Reflectance |
|---|---|---|
| Light path | Light passes through the tissue to a detector on the opposite side | Light source and detector sit side by side and light returns from the tissue |
| Typical sites | Finger, toe, earlobe | Forehead, wrist, carotid; also arm, chest, face and finger rings in wearables |
| Signal | Generally more stable | Weaker, degrades more easily |
| Typical devices | Pulse oximeters | Wrist wearables |
| Typical wavelengths | Red and infrared | Green often used |
Sources: Charlton et al. 2022 and Park et al. 2022 (both listed below). Wrist and finger recordings also differ in waveform shape and amplitude, so values from one site are not interchangeable with another.
What is the purpose of a PPG sensor?
A PPG sensor provides a continuous, non-invasive pulse waveform. Applications described in the literature include pulse oximetry, perfusion index, heart rate and consumer sleep and stress tracking, and research estimates of respiratory rate, arterial stiffness and blood pressure from pulse transit time, as well as screening research on atrial fibrillation and sleep apnoea. Whether any one of these is valid depends on the device, the site and the population studied.
What does a finger plethysmograph measure?
A finger PPG probe records the pulsatile change in blood volume in the fingertip, in transmission mode. From that waveform software derives pulse rate and, with two wavelengths, an SpO2 estimate. It does not measure lung volume; that is the role of the body plethysmograph.
What is a good PPG signal?
A good signal is a clean, repeatable pulse waveform with its cardiac-cycle pulses clearly resolved from noise. Research groups quantify this with a signal quality index (SQI), computed by rule-based methods, template matching or machine learning. Low-quality segments cause false alarms, so published pipelines grade or reject them before analysis. No single universal pass mark is given in the sources used here, so a study should state the SQI method it applied.
Artefact checklist for research use
| Artefact | What it looks like | Reported mitigation |
|---|---|---|
| Motion | Spectrum of 0.01 to 10 Hz overlaps the PPG band | Filters, adaptive cancellation using an accelerometer, independent component analysis |
| Baseline wander | Slow drift, 0.15 to 0.5 Hz, linked to respiration, sympathetic activity and thermoregulation | Not specified in the sources |
| Hypoperfusion | Weak pulsatile signal in hypothermia, vasoconstriction or low cardiac output | Not specified in the sources; report the condition |
| Ambient light | External light reaching the detector | Not specified in the sources |
| Skin pigmentation | Affects light absorption and signal quality | Validate across groups (Charlton et al.) |
| Contact pressure and alignment | Signal amplitude and shape change with fit | Good skin contact is required (Charlton et al.) |
Frequency bands and artefact types are from Park et al. 2022; where a cell says not specified, the sources name no remedy. Charlton et al. 2022 add that the signal needs good skin contact and that conventional pulse oximeter SpO2 has been reported as less accurate and less reliable in Black patients than in white patients, which is why validation across skin tones belongs in a study design.
Where it matters for labs and clinics
For research wearables, the choices that drive results are measurement site, wavelength, mode, sampling and artefact handling. Document each in the methods section, and record the device class (see wearable digital health technologies). Monitoring devices that report pulse or SpO2, such as an apnea monitor, rely on the same signal quality considerations.
Common misunderstandings
- PPG is not ECG. It records blood volume in tissue optically, not the heart’s electrical activity.
- Different sites are not interchangeable: wrist and finger waveforms differ in shape and amplitude.
- A consumer reading is not automatically a validated measurement; green-light heart rate accuracy in darker skin tones is discussed as an open issue in the literature.
- More light is not always better: wavelength choice sets how deep the light penetrates.
FAQ
How much does a PPG sensor cost?
This guide does not give prices. Cost depends on the device class and the vendor, and none of the sources reviewed state a price.
What is the relationship between PPG and SpO2?
SpO2 is derived from the AC and DC amplitudes of PPG signals at two wavelengths, typically red and infrared.
Sources
- Charlton PH, Kyriacou PA, Mant J, Marozas V, Chowienczyk P, Alastruey J. Wearable Photoplethysmography for Cardiovascular Monitoring. Proc IEEE 2022;110(3):355-381. doi:10.1109/JPROC.2022.3149785. PMC7612541. Read 2026-11-07.
- Park J, Seok HS, Kim S-S, Shin H. Photoplethysmogram Analysis and Applications: An Integrative Review. Front Physiol 2022;12:808451. doi:10.3389/fphys.2021.808451. PMC8920970. Read 2026-11-07.
