The Physics on Your Wrist: How Smartwatches Use Light to Read Vitals (Heart Rate & SpO2)

Have you ever wondered about the small, flashing green lights on the underside of your smartwatch? It’s a feature so common on modern wearables, like the RUXINGX G53 and countless others, that we often take it for granted. These devices promise to be our personal health dashboards, constantly monitoring our heart rate and even blood oxygen levels. But how can a simple flash of light possibly peer inside our bodies and pull out such vital information? It’s not magic; it’s a fascinating field of science called photoplethysmography, or PPG, and it essentially turns your wrist into a miniature physics laboratory.

The Flashlight and the Finger: A Simple Introduction to PPG

To understand PPG, let’s start with a simple experiment you’ve likely done as a child: shining a flashlight through your fingertip in a dark room. You see a glowing red hue. What you’re witnessing is the core principle of PPG. The light from the flashlight enters your finger, and as it passes through, some of it is absorbed by your bones, tissue, and blood, while the rest passes through to your eye.

Photoplethysmography works on a similar principle, but instead of light passing through, it measures the light that is reflected. Your smartwatch contains a set of small light-emitting diodes (LEDs) that shine light onto the skin of your wrist. Right next to these LEDs is a photodiode, which is a tiny sensor that measures how much of that light bounces back. The key to everything is this: the amount of light that gets reflected changes in a predictable, rhythmic way with every beat of your heart.

The Secret of the Green Light: Solving for Heart Rate

So, why the persistent green light? The choice is deliberate and based on the properties of our blood. As your heart pumps, it pushes a pulse of blood through the arteries and capillaries in your wrist. This blood is red because it’s full of hemoglobin, a protein that’s excellent at absorbing green light.

Here’s how it works in a cycle:
1. Between Heartbeats (Diastole): There’s less blood in the capillaries under the sensor. More of the green light from the watch’s LEDs is reflected and less is absorbed. The photodiode sensor detects a brighter reflection.
2. During a Heartbeat (Systole): A pressure wave pushes a greater volume of blood into the capillaries. This surge of blood absorbs more of the green light. The photodiode sensor detects a dimmer reflection.

This constant fluctuation between brighter and dimmer reflections creates a wave pattern. The watch’s software analyzes this wave, and the frequency of its peaks and valleys directly corresponds to your heart rate. If the peaks are happening 70 times a minute, your heart rate is 70 beats per minute (BPM). Green light is preferred for this task because it is absorbed more strongly by blood than other colors and is less susceptible to interference from ambient light, making it ideal for tracking your pulse during daily activities and workouts.

A Deeper Look: How Red and Infrared Light Uncover Blood Oxygen

But what if we want to know more than just how fast our blood is pumping? What if we want to know how efficiently it’s carrying oxygen? To answer that, the tiny lab on our wrist needs to turn on a different set of lights. This is where blood oxygen (SpO2) monitoring comes in, and it relies on the same core principle but with an added layer of complexity, loosely based on a scientific concept known as the Beer-Lambert Law, which states that the amount of light absorbed is related to the concentration of the substance it’s passing through.

Our blood contains two main types of hemoglobin: oxygenated hemoglobin (the one carrying oxygen, which is bright red) and deoxygenated hemoglobin (the one that has dropped off its oxygen, which is darker red). Crucially, these two types of hemoglobin absorb different wavelengths of light differently. * Oxygenated hemoglobin absorbs more infrared light and allows more red light to pass through (or be reflected). * Deoxygenated hemoglobin absorbs more red light and allows more infrared light to pass through.

To measure SpO2, your watch stops flashing the green light and instead flashes a combination of red light (typically around 660nm) and infrared light (around 940nm). By comparing how much of each light wavelength is reflected, the watch’s algorithm can calculate the ratio of oxygenated hemoglobin to total hemoglobin. The result is expressed as a percentage—your SpO2 level. A reading of 97% means that 97% of the hemoglobin in your blood is fully loaded with oxygen.

The Real-World Challenges: When Physics Meets Biology

This system sounds brilliantly simple in theory. However, the journey from a clean physics principle to a reliable reading on your wrist is fraught with real-world challenges. Several factors can interfere with the light and affect the accuracy of the readings.

• Watch Fit: This is the most critical factor. As noted by industry engineers, any gap between the sensor and your skin can allow ambient light to leak in, creating “noise” that can corrupt the signal. A snug (but not tight) fit is essential for accurate readings.
• Skin Tone: Melanin, the pigment that determines skin color, absorbs light. Higher concentrations of melanin in darker skin tones can absorb some of the light from the LEDs before it even reaches the blood vessels. According to a study in the Journal of Clinical Monitoring and Computing, this can sometimes make it more challenging for sensors to get a strong, clear signal, though manufacturers are continuously improving their algorithms to account for this.
• Tattoos: The ink in tattoos, especially dark-colored ones, can block the light from the sensor, making it nearly impossible to get a reading from that area of the skin.
• Motion: During intense exercise, your arm is moving rapidly. This can cause the watch to shift, creating motion artifacts that can be mistaken for a pulse, leading to inaccurate readings. Advanced algorithms use data from the accelerometer (the motion sensor) to try and filter out this noise.

Conclusion: The Tiny Lab on Your Wrist

The next time you see those little green or red lights flashing on your wrist, you’ll know it’s not just a random blink. It’s a sophisticated process of photoplethysmography at work. It’s a system that sends beams of light into your body and carefully analyzes the echo that bounces back, translating those subtle reflections into meaningful data about your cardiovascular system.

Devices like the RUXINGX G53 are a testament to how far this technology has come, democratizing access to health metrics that were once confined to clinical settings. While they are not medical devices and their accuracy can be influenced by many factors, they are powerful tools for awareness. By understanding the elegant physics behind their operation, we can become more informed users, better appreciating both the incredible potential and the inherent limitations of the tiny, powerful laboratory we now wear on our wrists.