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Contact PPG

Photoplethysmography (PPG) shines light into the skin and measures how much comes back. As blood volume pulses with each heartbeat, the reflected light changes. It’s the same principle as rPPG, but with a sensor touching the skin, which gives a much stronger signal. Smartwatches use it, and so do some headbands, such as Muse S. The Elata SDK’s ppg-web package reads a headband’s optical sensor over Web Bluetooth and estimates heart rate and heart-rate variability on the device.

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Heart-rate variability (HRV)

Your heart doesn’t beat like a metronome. The time between beats varies slightly from beat to beat, and that variation is heart-rate variability. Higher HRV at rest is often linked with recovery and relaxation, and lower HRV with stress or fatigue, though it also varies with age, fitness, breathing, and time of day. The SDK reports two common time-domain measures: Strictly, variability measured from PPG is pulse-rate variability. It closely tracks ECG-based HRV at rest, but the two aren’t always interchangeable.

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What makes HRV meaningful

The SDK withholds HRV values (returns no value) when it can’t detect beats reliably, instead of reporting a misleading number.

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What it can’t claim

PPG-based heart rate is widely used and generally accurate at rest. It does not diagnose arrhythmias or other heart conditions: published reviews note that ECG confirmation remains necessary for diagnosis.

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Sources

Peer-reviewed literature, retrieved via PubMed:
  • Georgiou K, et al. Can wearable devices accurately measure heart rate variability? A systematic review. Folia Medica, 2018. doi:10.2478/folmed-2018-0012
  • Hejjel L, Béres S. Comment on “Pulse rate variability in cardiovascular health: a review on its applications and relationship with heart rate variability”. Physiological Measurement, 2021. doi:10.1088/1361-6579/abd332
  • De Wever M, et al. Photoplethysmography for heart rate and rhythm monitoring: current evidence and clinical applications. JACC: Clinical Electrophysiology, 2026. doi:10.1016/j.jacep.2026.06.034
SDK implementation: ppg-web (public SDK repository).