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Integrating LED-based sensors in activity trackers

Posted: 07 Jan 2015     Print Version  Bookmark and Share

Keywords:wearable gadgets  Optical sensors  LED  smartwatch  photoplethysmography 

More and more people are monitoring their physical fitness through wearable gadgets and appropriate applications. Optical sensors are suitable for measuring pulse rates and oxygen saturation in blood. The technology has long been established in the medical sector and can now be transferred to consumer applications thanks to modern LED technologies.

It all started with armbands that could record the number of paces a person took. Now, many activity trackers such as fitness armbands and smartwatches can also measure heart rate and other biometric values or monitor sleep quality. The new opportunities for tracking your own fitness levels have been enthusiastically embraced by many people, leading to a growing "Quantified Self" movement. Major players such as Samsung, Apple and Google are now entering this growing market with appropriate apps, smartwatches and smartphones.

Whereas pace counters use acceleration sensors, optical methods traditionally used in the medical sector for pulse and blood oxygen measurements are now finding their way into the consumer market. In the hospital environment, the sensors are mostly installed in ear or finger clips. In 2013, the Mio Alpha smartwatch was the first armband that could measure the pulse rate at the wrist using an optical sensor – a significant advance compared with the chest belt worn by athletes, which no one would enjoy wearing all day. Smartphones are also able to measure pulse rates on a finger. The first fitness armbands are now coming onto the market that enable you to measure the oxygen saturation in your blood simply by placing your finger on a screen. This feature is useful, for example, for people engaged in activities at high altitudes, such as mountain climbers, hang-gliders and glider pilots, and also for people with heart or lung problems.

Optical measurement methods
The principle on which the sensors measure pulse rate and oxygen saturation is called photoplethysmography (PPG), in other words the optical measurement of changes in the volume of blood in the blood vessels. The method makes use of the fact that the volume of blood transported in the arteries changes as the heart goes through its pumping cycle. The heart rhythmically expels blood (systole) and draws it in (diastole) in a constant cycle. This means that more blood flows through the arteries in the systolic phase, and less during the diastolic phase. By measuring the change in the volume of blood at a particular location on the body it is possible to derive the pulse rate from the periodicity of the measured signal.

Figure 1: The principle of reflective optical pulse measurements. Light emitted by the sensor penetrates the skin and tissue and is absorbed or reflected back to the detector. Because the volume of blood in the arteries changes with every beat of the heart, the amount of light absorbed and therefore the strength of the detector signal also changes. Green light provides the best results at the wrist, whereas red and infrared light is generally used on fingers.

Measurement of the volume of blood is based on the ability of the haemoglobin in the blood to absorb light (figure 1). The sensor, consisting of a light source and a detector arranged next to one another, is placed directly on the skin. The light emitted penetrates the skin, tissue and blood vessels and is absorbed, transmitted and reflected. The amount of reflected light recorded by the detector changes according to the changing amount of blood flowing through the arteries (figure 2). The appropriate wavelength for this measurement depends on the part of the body where the measurement is taken. Green light provides the best results at the wrist, whereas red and infrared light is generally used on fingers.

Figure 2: Production of the detector signal in a PPG measurement. The light directed at the skin (I0) is absorbed by venous blood and arterial blood or reflected back to the detector. The changing component of the signal corresponds to the amount of blood in the arteries pulsing in time with the heartbeat. The periodicity of this signal indicates the pulse rate. The ratio of the minimum and maximum detector signal values (photocurrent Imin/Imax) provides the basis for determining oxygen saturation of the blood.

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