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.

Every time your heart beats, a pressure wave of blood pulses through your arteries, momentarily increasing the blood volume in your fingertip. This tiny change in blood volume alters the amount of light that passes through or reflects from your finger, creating a pulsating optical signal that can be detected by a simple light sensor. This principle, called photoplethysmography (PPG), is the technology behind the pulse oximeters clipped to patients' fingers in every hospital and the heart rate monitors in modern smartwatches. The key lies in the optical properties of hemoglobin: oxygenated blood strongly absorbs green light (around 525-535 nm wavelength), so when the blood volume in the fingertip increases during systole, more green light is absorbed and less reaches the detector. The smartphone's camera, combined with its LED flash as a light source, provides everything needed to replicate this medical measurement. By placing a finger over the camera and flash, students can visualize their own heartbeat as a fluctuating green light signal and calculate their heart rate with clinical-level accuracy.

Activity overview:

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.

Level:

Middle and high school

FizziQ

Author:

Duration (minutes) :

25

What students will do :

- Detect cardiac pulsations using the smartphone camera as a photoplethysmography sensor
- Measure the green light intensity fluctuations caused by blood volume changes in the fingertip
- Calculate heart rate from the period of the PPG signal
- Understand the optical principles behind pulse oximetry
- Compare the PPG-derived heart rate with manual pulse counting

Scientific concepts:

- Photoplethysmography
- Light absorption
- Cardiac cycle
- Systole and diastole
- Hemoglobin and oxygenation

Sensors:

- Camera (used as colorimeter / light intensity sensor)
- LED flash (light source for transillumination)

Material needed:

- Smartphone with FizziQ app and functional LED flash
- A quiet environment for measurement
- FizziQ experience notebook

Experimental procedure:

Open FizziQ and select the Colorimeter tool or the green channel intensity from the camera sensor.
Turn on the smartphone's LED flash to provide a constant light source. (Some phones do this automatically in FizziQ.)
Place your index finger gently over both the camera lens and the LED flash, covering them completely but without pressing too hard.
Start recording. You should see a pulsating signal in the green channel intensity, fluctuating with each heartbeat.
If the signal is not visible, adjust your finger pressure: too hard blocks all light; too light lets ambient light in. Find the sweet spot where the green signal oscillates clearly.
Record for at least 30 seconds to capture 30-50 heartbeat cycles.
Stop recording and examine the graph. Count the number of peaks (one per heartbeat) over a 20-second interval.
Calculate your heart rate: HR (bpm) = (number of peaks / time interval in seconds) × 60.
Verify your result by counting your pulse manually at the wrist for 15 seconds and multiplying by 4.
Now perform 2 minutes of light exercise (jumping jacks or jogging in place), then immediately repeat the measurement.
Compare your resting and post-exercise heart rates. The post-exercise rate should be significantly higher.
Try to identify the dicrotic notch in the PPG waveform: a small secondary dip that corresponds to the aortic valve closing.

Expected results:

The green channel intensity should show clear, regular oscillations with a period of 0.6-1.0 seconds (corresponding to heart rates of 60-100 bpm at rest). The amplitude of the oscillations is typically 1-5% of the mean green intensity. After exercise, the heart rate should increase by 30-80% (to 90-160 bpm), and the signal may also show larger amplitude pulsations. The PPG-derived heart rate should agree with manual pulse counting within ±2-3 bpm. The dicrotic notch may or may not be visible depending on the phone's camera quality and sampling rate. Some phones produce cleaner PPG signals than others due to differences in camera sensitivity and LED brightness.

Scientific questions:

- Why is the green light channel most sensitive to blood volume changes?
- How does a hospital pulse oximeter measure blood oxygen saturation in addition to heart rate?
- Why does the PPG signal decrease (more absorption) when the blood volume in the fingertip increases?
- What factors other than heart rate could affect the shape of the PPG waveform?
- How do smartwatches measure heart rate continuously using the same principle?
- Could you use this technique to detect irregular heart rhythms (arrhythmias)?

Scientific explanations:

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.

Extension activities:

Frequently asked questions:

Q: I cannot see any pulsating signal in the colorimeter reading. What am I doing wrong?
R: The most common issues are finger positioning and pressure. Cover both the camera and LED completely with the flat part of your fingertip (not the tip). Press gently but firmly. Ensure the LED flash is on. Try different fingers. Some phone cases may need to be removed.

Q: The signal is very noisy and irregular. How can I improve it?
R: Keep your hand and finger completely still during the recording. Ambient light leaking past your finger adds noise. Ensure you are in a dimly lit environment and that your finger completely covers the camera.

Q: My heart rate measurement differs from my manual pulse count. Which is more accurate?
R: Both methods have uncertainties. The PPG method is typically accurate to ±2-3 bpm when the signal is clean. Manual pulse counting has a resolution of about ±4 bpm when counting over 15 seconds. Average multiple measurements for the best estimate.

Q: Why does the signal use the green channel rather than red or blue?
R: Hemoglobin has a strong absorption peak in the green wavelength range (525-535 nm), making the green channel most sensitive to blood volume changes. Red light penetrates deeper into tissue and is used in hospital pulse oximeters for oxygen saturation measurement, but green gives a stronger pulsatile signal for surface measurements.