Photoplethysmography
This activity allows students to measure their heart rate using the smartphone's colorimeter. It establishes a link between photometric sensor technology and physiology.
Niveau :
Titre 4
Auteur :
Titre 4
Learning objectives :
This activity allows students to measure their heart rate using the smartphone's colorimeter. It establishes a link between photometric sensor technology and physiology.
Concepts covered
Photoplethysmography; Light absorption; Cardiac cycle; Systole and diastole; Hemoglobin and oxygenation
What students will do :
The student uses the FizziQ colorimeter to detect variations in the transparency of his finger caused by blood flow. By placing their finger on the camera illuminated by the LED of the smartphone, the student observes and records the fluctuations in the green light intensity which vary with each heartbeat, thus making it possible to calculate their heart rate and analyze the shape of the pulse wave.
What is required :
Smartphone with FizziQ app and functional LED flash; A quiet environment for measurement; FizziQ experience notebook
Scientific background :
Photoplethysmography (PPG) is a non-invasive technique that detects changes in blood volume in tissues by measuring changes in light absorption or reflection. In this experiment, the smartphone's LED emits light that partially passes through the finger, and the camera measures the transmitted light intensity. The principle is based on the optical properties of blood: hemoglobin strongly absorbs green light (wavelength ~525-535 nm), while surrounding tissues absorb it less. With each heartbeat (systole), the heart pumps blood through the arteries, temporarily increasing the blood volume in the capillaries of the finger. This increase increases light absorption, reducing the detected light intensity. During the cardiac relaxation phase (diastole), blood volume decreases and light intensity increases. This cycle creates a characteristic pulse wave whose frequency directly corresponds to the heartbeat. Green light is preferentially used because it offers the best compromise between tissue penetration and absorption by hemoglobin. Connected watches actually use green LEDs for the same reason. The asymmetry observed in the curves (rising faster than falling) reflects cardiac physiology: ventricular contraction (systole) is a rapid and powerful event, while filling (diastole) is more gradual. This technique not only measures heart rate, but also analyzes pulse waveform, which can reveal information about arterial elasticity and cardiovascular health. PPG is now widely used in medical devices and connected objects, perfectly illustrating how a simple physical principle (differential light absorption) can be exploited for everyday health applications.