Photoplethysmography (PPG)

Photoplethysmography (PPG) is a well-established, non-invasive optical technique used since 1942 for measuring physiological parameters by detecting volumetric changes in blood under the skin. It works by emitting light into the skin and measuring how much is absorbed or reflected by the blood vessels. PPG is commonly found in oximeters, wearable devices, and health monitors to track indicators such as heart rate, oxygen saturation, and blood pressure.

This low-cost, non-invasive method provides valuable insights into the cardiovascular system by detecting changes in blood volume with each heartbeat. By analysing these fluctuations in light absorption, PPG can assess various physiological characteristics, making it a key component in modern health and fitness monitoring technologies.

The primary difference between PPG (Photoplethysmography) and rPPG (remote Photoplethysmography) lies in how they detect and measure physiological signals:

1. PPG (Photoplethysmography):
  • Contact-Based: PPG typically requires direct contact with the skin. Sensors (often found in wearable devices, fingertip pulse oximeters, or medical devices) emit light into the skin and measure the amount of light absorbed or reflected by the blood vessels beneath the surface. The pulsatile changes in blood volume, caused by the heartbeat, are detected through this interaction with light.
  • Use Cases: PPG is commonly used for measuring heart rate, blood oxygen levels (SpO2), and sometimes blood pressure or cardiac output in a controlled or clinical environment.
2. rPPG (remote Photoplethysmography):
  • Non-Contact/Remote: rPPG works without direct contact with the skin. Instead, it uses cameras to capture video footage of the skin (usually the face). By analyzing subtle color changes on the skin caused by blood flow, rPPG algorithms extract physiological signals. These signals are often processed using image processing techniques and machine learning algorithms to estimate heart rate and other metrics.
  • Use Cases: rPPG is typically used in applications where non-invasive, non-contact monitoring is needed, such as in mobile health apps, remote monitoring, or wellness checks via webcams or smartphone cameras.