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Why is green light used in watches to measure heartbeat ?

Updated: Nov 25, 2022

Many applications allow you to calculate the heart rate using only the camera of a smartphone. On what principles are these applications based? Can we use FizziQ to perform the same measurements? What is the limit of this technology?

1. What is photoplethysmography ?

The physiological phenomenon on which these applications are based was first demonstrated in 1937 by Hertzman and Spealman (5). These two scientists found that they could use a photoelectric cell to measure variations in the transmittance of light through the finger and that these variations made it possible to measure the heart rate.

This is because with each heartbeat, an influx of blood spreads through the vessels during the phase called systole. The blood volume increases rapidly in the capillaries and the tissues become slightly thicker and red with the influx of hemoglobin-laden blood (3). With cardiac reflux, during the diastole phase, the amount of blood in the tissues decreases and they become less opaque. By analyzing variations in opacity or color in sufficiently transparent tissues, such as those of a finger or an earlobe, we can determine the systole and diastole phases and calculate the heart rate (1).

The analysis of blood flows by this optical method is called photoplethysmography, from the Greek "phôtós", light; "plêtusmos", the increase; and "gráphô", to write. This method has gradually become very important in the medical field of monitoring, for example it is used in portable oximeters which have been essential for monitoring patients with Covid -19. It is also used in connected watches to calculate heart rate. Under certain conditions, we can use the camera of our smartphones to conduct simple photoplethysmography analyses. Of course, we will not reach the precision of specialized devices. but one can nevertheless obtain a certain number of interesting intuitions on the operation of the human cardiovascular system. We study in the following two methods to carry out this analysis.

2. Analysis with the light sensor

Our smartphones have light and color analysis capabilities thanks to the camera. The FizziQ application can in particular provide two types of information that will be useful for this analysis: luminance which measures the quantity of light reflected by a surface and colorimetry which makes it possible to measure the quantities of light transmitted by the Bayer filter of the camera. In the following, we will use these two types of measures.

The luxmeter of the FizziQ application allows you to measure the opacity of the tissues of our index finger. Let us select the average luminance which allows a global analysis of the reflected light. We place the fingertip in contact with the lens of the camera by pressing very lightly as shown in the photo.

We ensure that the brightness is between 15% and 40% by illuminating the finger more or less with an external light source. The best place to do this measurement is the extreme fingertip. After a few moments, we see that a regular signal appears on the chart. This signal is weak, and only causes a variation of a few percent of the luminosity. Gradually the finger in contact with the phone heats up and dilates the vessels which improves the signal. You have to find the best position by moving your finger on the lens. Be careful, if the pressure exerted by the finger is too strong, the diameter of the capillaries and their dilation capacity is less important, which reduces the variations in transmittance. On the other hand, the quality of the camera and the speed of the smartphone are decisive elements for making precise measurements. Finally, on very thin fingers, such as those of children, the measurement can prove to be difficult to implement.

To study the signal, the data is recorded for about ten seconds with the REC button and added to the notebook. With the scale buttons you can center and enlarge the scale, and select the study range. Note that you can move the selection button + if it gets in the way.

The rhythm of the peaks is regular and makes it possible to measure what should be the heart rate. The troughs correspond to the systole phases during which the blood is abundant in the vessels, the peaks correspond to the diastole phase. Using the magnifying glass of the experiment book, we measure the time difference between the different peaks (1.05 s) which gives us the heart rate which in this case is 57 beats per minute. This value is checked with a medical device.

3. Analysis with the color sensor

The method that we have just described makes it possible to obtain generally acceptable results, but the measurement can be improved by using the colorimeter (see this blog for the operation of this instrument). Indeed, hemoglobin in its oxygenated form absorbs green radiation between 510 and 560 nm (4). As the green filter of the Bayer filter of our devices allows light rays of wavelengths around 530 nm to pass, we can measure the amount of blood in the tissues by observing the intensity of the green color reflected by them. this. During the systole phases, the green radiation emitted by the light source will be more widely absorbed than during the diastole phase.

It is this method that is also used by connected watches: they emit a green light at regular intervals and measure the intensity reflected by the fabrics.

Our smartphones cannot emit green light, but we can nevertheless do the same analysis by measuring with the colorimeter the intensity of the long wavelengths of green in a fabric illuminated by the white light of the torch of our smartphone, used as a source. steady light. To turn it on during the measurement, select the "LED for the colorimeter" option in the Application menu of the Settings tab.

We select the "Intensity" measurement of the Colorimeter and we measure this intensity in the wavelengths of the color green (530 nm). We will move the finger so that the average measurement is at least 10%. The graph we obtain is usually more precise than that obtained with the luminance measurement and allows us to obtain more intuitions about the phenomenon.

For example we note on the graph that the variations of the intensity are not symmetrical. In other words, the rise in blood pressure pressure is rapid (decrease in intensity), and the pressure drop phase (increase in intensity) is slow. Intuition tells us that the blood pressure wave generated during the contraction should rather be symmetrical, how to explain this phenomenon? The large vessels that start from the heart are elastic (aorta, large arteries) and deform under the pressure generated by the stroke volume. The pulse wave propagates rapidly with a speed of 8-10 m/s, but quickly encounters obstacles due to the progressive reduction in the diameter of the arteries of the blood distribution network. These small vessels are also not elastic. The wave will therefore be reflected and will leave in the opposite direction (2, 7).

This second wave (dicrote wave) is superimposed on the first with an offset and allows the blood pressure during the relaxation phase of the heart to decrease more gradually (2). This phenomenon is very important because it allows to optimize the coronary perfusion pressure.

4. Conclusion

Can we do other types of analyzes on the physiology of the cardiovascular system? It is likely that to go further, smartphones must integrate other sensors or electronic components. Oximeters, for example, calculate the level of oxygen in the blood by comparing the intensity reflected when a tissue is illuminated with two different wavelengths, red and infrared. The use of techniques such as artificial intelligence also makes it possible to make better use of sensors. For example, recent research has shown that it is possible to analyze the heart rate by studying videos of the face (5). The use of smartphones to prevent diseases has made significant progress over the past ten years and there is no doubt that with the development of new sensors and the use of even more efficient analysis methods, new applications will see the light of day to help people identify diseases even more quickly and participate in proposing treatments (6).

The current technology present on most of our smartphones cannot transform them into medical devices, but for those interested in better understanding the physiology of our cardiovascular system, it provides relevant and objective information from which to conduct investigative approaches. quite interesting.

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Images :

Figure 2 : © Bernard Valeur

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