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Photoplethysmography (PPG)

Discover how this optical technique enables precise and efficient blood flow monitoring.

Photoplethysmography (PPG) is an optical technique used to detect volumetric changes in blood within the peripheral circulation. This non-invasive and cost-effective method measures blood flow variations at the skin’s surface.

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Providing valuable insights into cardiovascular health, PPG has gained renewed interest due to recent technological advancements. It is widely utilized in clinical settings for physiological measurement and monitoring.

Principle of PPG

PPG utilizes low-intensity infrared (IR) light to measure blood flow. As light passes through biological tissues, it is absorbed by bones, skin pigments, and both venous and arterial blood.

Since blood absorbs more light than surrounding tissues, PPG sensors detect blood flow variations by measuring changes in light intensity. The resulting voltage signal is proportional to the volume of blood flowing through the vessels. Although this method is highly sensitive to even small fluctuations in blood volume, it does not provide a direct measurement of blood quantity.

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A PPG signal comprises multiple components, including arterial blood volume changes linked to cardiac activity, venous blood variations that influence the signal, a DC component reflecting tissue optical properties, and subtle energy shifts within the body.

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Key factors affecting PPG recordings include the measurement site and the contact force between the sensor and the skin. Notably, blood flow variations primarily occur in the arteries rather than the veins.

The PPG Waveform

PPG shows the blood flow changes as a waveform with the help of a bar or a graph. The waveform has an alternating current (AC) component and a direct current (DC) component.

The AC component corresponds to variations in blood volume in syncronization with the heart beat. The DC component arises from the optical signals reflected or transmitted by the tissues and is determined by the tissue structure as well as venous and arterial blood volumes.

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The DC component shows minor changes with respiration. The basic frequency of the AC component varies with the heart rate and is superimposed on the DC baseline.

Uses of PPG

Medical devices based on PPG technology are widely used in various applications in the clinical set up.

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Specific applications include the following:

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  • Clinical physiological monitoring

  • Blood oxygen saturation

  • Blood pressure

  • Cardiac output

  • Heart rate

  • Respiration

  • Vascular assessment

  • Arterial disease

  • Arterial compliance and ageing

  • Venous assessment

  • Endothelial function

  • Microvascular blood flow

  • Vasospastic conditions

  • Autonomic function monitoring

  • Vasomotor function and thermoregulation

  • Blood pressure and heart rate variability

  • Orthostasis

  • Other cardiovascular variability assessments

Wearable Devices

This technology has enabled the development of wearable pulse rate monitors—compact, cost-effective devices equipped with high-intensity green LEDs and photodetectors. These components ensure reliable, non-invasive pulse rate monitoring.

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Key design considerations for these systems include miniaturization, durability, and ease of use.

 

The sensors in these devices detect subtle changes in light intensity as it is transmitted through or reflected from the tissue. These variations correspond to fluctuations in blood flow, providing essential cardiovascular data, including pulse rate.

Contactless photoplethysmography (cPPG)

At WaveLight, we utilize Contactless PPG (cPPG), a modern adaptation of traditional PPG that provides essential benefits in both the medical field and other computing applications.

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Smartphones are the most convenient devices available today. By using videos of fingertips captured with a smartphone camera, it is possible to estimate vital signs such as pulse rate (PR), blood pressure (BP), or arterial oxygen saturation (SpO2) using the photoplethysmography (PPG) technique. This method is based on tracking subtle color changes on the skin due to cardiovascular activities. These color changes are invisible to the human eye but can be detected by digital cameras.

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The method is divided into three main stages: first, reading and processing video frames to obtain PPG data; second, segmenting the waveform; and finally, estimating the requested vital signs from this signal. The pixel color intensity of the skin is utilized, with filters applied to eliminate noise and retain only the relevant pulses. The extracted signal is then subjected to our detection algorithms (wavemods), which generate the requested estimate.​​

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