Photoplethysmography (PPG)

Photoplethysmography (PPG) is an optical sensing technology that detects blood volume changes in the microvascular bed of tissue, most commonly measured through the skin on the wrist or fingertip. It is the primary technology used in modern wrist-worn health wearables — including Penng — to continuously measure heart rate, derive heart rate variability (HRV), and track sleep stages without the need for a chest strap or electrode-based monitor.

How PPG works

The principle behind PPG is straightforward:

1. Light emission — The sensor shines LED light (typically green for heart rate, and sometimes red or infrared for SpO2) into the skin.
2. Blood volume fluctuation — With each heartbeat, blood is pumped through the arteries and capillaries, causing a momentary increase in blood volume beneath the skin. Between beats, blood volume decreases.
3. Light absorption — Haemoglobin in the blood absorbs some of the LED light. More blood (during a pulse) means more absorption and less reflected light. Less blood (between pulses) means less absorption and more reflected light.
4. Signal detection — A photodetector captures the reflected or transmitted light. The rhythmic variations in light intensity correspond directly to the pulse wave, from which heart rate is calculated.

What PPG can measure

From the pulse wave signal, a PPG sensor can derive several valuable health metrics:

• Heart rate — The time between successive pulse peaks gives beat-to-beat heart rate.
• Heart rate variability (HRV) — The subtle variation in timing between beats, used as an indicator of autonomic nervous system balance and recovery status.
• Blood oxygen saturation (SpO2) — By using both red and infrared light and comparing their absorption ratios, the sensor estimates the percentage of haemoglobin carrying oxygen.
• Sleep stages — Heart rate patterns, combined with accelerometer data, allow algorithms to classify sleep into light, deep, and REM stages.
• Respiratory rate (some devices) — Derived from cyclical modulations in the PPG signal caused by breathing.

PPG in Penng

Penng uses a PPG optical heart rate sensor as one of its core sensing technologies, alongside an accelerometer and gyroscope. The PPG sensor enables continuous heart rate monitoring, overnight HRV measurement, SpO2 readings, and sleep stage classification — all captured passively through the wrist without requiring any user action.

Accuracy considerations

PPG accuracy depends on several factors:

• Fit — A snug, stable fit against the skin produces the cleanest signal. A loose band allows light leakage and motion artefacts.
• Skin tone — Darker skin absorbs more light, which can reduce signal strength in some sensors, though modern algorithms have improved significantly.
• Motion — Movement during exercise introduces noise into the optical signal. Advanced signal processing filters out most motion artefacts, but wrist-based PPG is generally most accurate at rest and during steady-state activity.
• Tattoos — Dense or dark tattoos over the sensor area can interfere with light transmission.

For most health and fitness tracking purposes, wrist-based PPG provides reliable, clinically correlated data that is more than sufficient for guiding training, recovery, and sleep decisions.

Learn more about your health data — take the free quiz at penng.ai/quiz.