Chapter 5: How to record PPG with different devices, including a smartphone

Introduction

In the previous chapters, we described the origin of the electrocardiogram (ECG) and the photoplethysmogram (PPG), as well as the differences and similarities between both technologies. In this chapter, we’ll explore PPG more in depth, go over the effect of the color of light on the recordings, and discuss the different ways of collecting PPG signals.

Key Messages

✔️ Photoplethysmography (PPG) is an innovative non-invasive optical technique used to measure changes in blood volume using a light source and a photodetector.

✔️ The used wavelength of the light source that is used determines the depth of light penetration. Green and (infra)red light are the most commonly used wavelengths for PPG measurements.

✔️ Emitting white light onto the skin and recording the different photodetector colors via a smartphone enables you to collect red-light-based PPG, green-light-based PPG, and blue-light-based PPG.

How can PPG be recorded?

Photoplethysmography (PPG) is an innovative non-invasive optical technique used to measure changes in blood volume in a peripheral vascular bed. This technique only requires two components - a light source and a photodetector. When tissues are illuminated, the blood in the capillaries partially absorb the light beams, and the photodetector records pulsatile changes in the intensity of light that is reflected or transmitted.

A PPG can be recorded in two modes: transmission mode and reflection mode.

In transmission mode, the light source and photodetector are placed on two opposite sides of the tissue. The photodetector quantifies the amount of light that was transmitted through the tissue. An example of this mode is the usage of your fingertip in an oximetry clamp.

In reflection mode, the light source and photodetector are placed on the same side of the tissue. The photodetector, located next to the light source, quantifies the amount of reflected light. An example of this mode is the usage of a smartwatch on the wrist.

The effect of the color of the light source

The wavelength of the light source that is used determines how deep the emitted light will go through the skin layers. Green and (infra)red light are the most commonly used wavelengths for PPG measurements. As red light has a higher signal intensity, it is ideal to capture changes in blood volume in larger blood vessels which can be found deeper in the tissue. Red light is however more sensitive to motion artifacts and may lead to a lower signal-to-noise ratio in some cases. In contrast, green light only penetrates the upper part of the skin, making it less sensitive to motion, and more suitable for active people.

Skin layers	Light penetration through dermal sublayers

How are smartphone-based PPG recordings possible?

Smartphones can also be used to record PPG by emitting "white light" onto the skin. The camera of a phone is a specialized photodetector that can record three different colors: red, green, and blue (RGB). Emitting white light onto the skin and recording the different photodetector colors via the smartphone enables you to collect red-light-based PPG, green-light-based PPG, and blue-light-based PPG. Blue light is not commonly used for PPG as it does not penetrate the skin as deeply as green or (infra)red light.

Sensors of a photo camera	Example of a PPG signal measured with different wavelengths (red, green, blue) compared to a blood pressure waveform

⏩ Learn more about the benefits of the first derivative of the PPG in the next chapter!