PPG stands for photoplethysmography, a light-based sensing technology used in fitness trackers to measure heart rate, SpO2, blood oxygen, HRV, and blood pressure. The sensor shines LED light (green, red, and infrared wavelengths) into the skin. Blood absorbs light differently depending on how much is flowing through your capillaries with each heartbeat. The sensor detects these light absorption changes, and algorithms convert the signal into health metrics. Sensor quality and algorithm maturity are the two primary factors that determine how accurate your fitness tracker's readings are.
If you've ever wondered why two fitness trackers with nearly identical spec lists can give completely different readings for the same person, PPG sensor quality and algorithm development are almost always the answer.
Understanding PPG doesn't require a biology degree. Once you get the core concept, you'll immediately understand why some trackers are worth the money and why others are, politely, educated guesses in wristband form.
The Basic Principle: Light Through Skin
Your skin contains millions of tiny capillaries, blood vessels close enough to the surface that light can interact with them. When your heart beats, it pushes a surge of blood through these capillaries. Between beats, blood volume in the capillaries drops slightly before rising again with the next beat.
Blood absorbs light. More blood in the capillaries means more light is absorbed and less bounces back. Less blood means less absorption and more light returns to the sensor. A PPG sensor tracks this cycle of light absorption by shining LED light into your wrist and measuring how much comes back thousands of times per second.
The resulting waveform, a graph of light absorption over time, contains a remarkable amount of health information. The frequency of peaks tells you your heart rate. The shape of each peak reveals your HRV. The ratio of red to infrared light absorption tells you your SpO2. The speed and morphology of the pulse wave give algorithms enough information to estimate blood pressure.
One sensor. Four metrics. That's the elegance of PPG.
The Three LED Colours (And Why They Matter)
Basic fitness trackers use green LEDs; only green light is absorbed particularly strongly by haemoglobin (the oxygen-carrying protein in red blood cells), making it efficient for heart rate measurement. Most budget step counters with heart rate use green PPG only.
More advanced trackers, including the Qorfit Pulse, use multiple LED wavelengths:
- Green LED (520-530nm): Heart rate monitoring. Strong haemoglobin absorption, good for continuous HR tracking with lower power consumption.
- Red LED (660nm): SpO2 measurement. Oxygenated haemoglobin absorbs red light differently from deoxygenated haemoglobin; this contrast is what enables blood oxygen percentage calculation.
- Infrared LED (880-940nm): Also used for SpO2 and for pulse wave analysis used in blood pressure estimation. Infrared penetrates deeper into tissue, reaching larger blood vessels.
A tracker with only a green LED genuinely cannot measure SpO2 or estimate blood pressure accurately. If you see these features on a device with only a green sensor, be sceptical of the readings.
HOW TO CHECK YOUR TRACKER'S SENSORS
Flip your fitness tracker over and look at the sensor cluster on the back. A single dot of LED light (usually green when active) = basic HR only. Multiple LEDs in different colours, green, red, and a larger infrared emitter, indicate a multi-wavelength PPG setup capable of SpO2 and BP estimation. The Qorfit Pulse uses a multi-wavelength PPG array for this reason.
Why the Algorithm Matters as Much as the Hardware
Here's something the specs pages never tell you: two trackers can have identical hardware sensors and produce wildly different accuracy levels. The algorithm, the software that interprets the raw PPG signal, is often more important than the sensor itself.
Building a good PPG algorithm means having access to large, diverse datasets of paired wristband PPG readings and reference measurements (ECG for HR/HRV, clinical oximeters for SpO2, validated blood pressure cuffs for BP) across different skin tones, ages, fitness levels, and activity states. This R&D is expensive and time-consuming.
Budget tracker manufacturers often skip this step or license generic algorithms that haven't been validated for their specific hardware. The result looks like a health metric on your app, but it's essentially a number that hasn't been properly tested.
This is why established health tracker brands that invest in clinical validation testing their readings against reference devices across diverse user populations consistently outperform cheaper alternatives that share the same sensor component but not the algorithm behind it.
What Affects PPG Accuracy in Daily Use
Even a high-quality PPG sensor and algorithm combination can give inaccurate readings in certain conditions. These are the most common:
- Movement artifact: Motion during exercise creates noise in the PPG signal. This is the biggest challenge for wrist-based trackers and why high-intensity interval training readings are less accurate than resting or steady-state readings.
- Fit: The sensor must be in consistent contact with your skin. A loose band introduces air gaps and dramatically degrades signal quality. Wear the tracker snug, two fingers above your wrist bone, firm but not tight.
- Skin tone: Melanin absorbs some of the LED light before it reaches the capillaries, which can slightly affect sensor performance. Quality algorithms account for this through calibration.
- Cold skin: Cold causes vasoconstriction; your capillaries narrow, reducing blood flow near the surface. PPG accuracy in very cold conditions (or when your hands are cold) can drop. This is especially relevant in North India's winters or high-altitude cold environments.
- Tattoos: Dark tattoo ink on the wrist can absorb LED light and significantly interfere with PPG readings. If you have wrist tattoos, consider a different placement or be aware that readings may be inconsistent.
PPG vs ECG: Understanding the Difference
ECG (electrocardiogram) directly measures the electrical signals that trigger each heartbeat. It's what a hospital cardiac monitor uses, and what some premium smartwatches now include. It's more accurate for detecting arrhythmias like atrial fibrillation.
PPG measures the blood flow response to each heartbeat as a secondary, optical signal. It's excellent for continuous monitoring of heart rate, SpO2, HRV, and BP trends. It cannot detect the specific electrical abnormalities that an ECG catches.
For general health monitoring, understanding your resting heart rate trends, recovery through HRV, blood oxygen during sleep, and blood pressure patterns, PPG is more than adequate and has the advantage of being passive and continuous. ECG requires active measurement (pressing two fingers on sensors) and is better suited for specific cardiac screening.
The Qorfit Pulse uses a multi-wavelength PPG sensor for accurate heart rate, SpO2, HRV, and BP monitoring. All-day. All-night. → qorfit.in
Read: Best Fitness Tracker Under Rs 9,000 in India | Best Fitness Tracker for Beginners in India (Complete Guide) | Best Health Trackers in India
