How Optical Heart Rate Sensors Work: Green Light, PPG and What Affects Accuracy

Every COLMI watch and ring has a small cluster of LEDs on its underside that light up green whenever it's actively tracking your heart rate throughout the day and night. It's easy to take this for granted, but the science behind it — and its genuine limitations — is worth understanding if you want to know how much to trust the numbers on your wrist or finger.

The Basic Principle: Photoplethysmography

The technology behind optical heart rate sensing is called photoplethysmography, usually shortened to PPG. It works on a simple principle: blood absorbs more light than the surrounding tissue, and the amount of blood in your capillaries changes very slightly with every heartbeat as blood is pumped through. By shining light into the skin and measuring how much bounces back to a photodetector, the sensor can detect these tiny, rhythmic changes in light absorption — and each rhythmic pulse corresponds to one heartbeat.

Why Green Light Specifically

You've probably noticed the LED under your COLMI watch or ring glows green during heart rate tracking, and this isn't an arbitrary design choice. Green light is absorbed particularly well by haemoglobin, the oxygen-carrying protein in blood, which creates a stronger, clearer signal than other wavelengths for continuous heart rate monitoring. Red and infrared light, which penetrate deeper into tissue, are typically used alongside green light specifically for blood oxygen (SpO2) measurement, since that calculation relies on comparing how differently oxygenated and deoxygenated blood absorb these different wavelengths.

From Light Signal to a Number on Your Screen

The raw light signal picked up by the photodetector is genuinely noisy and doesn't look anything like a clean, obvious heartbeat pattern on its own without significant processing. COLMI's firmware processes this signal, filtering out noise and identifying the regular peaks that correspond to each heartbeat, then calculates beats per minute from the timing between those peaks. This processing happens continuously and quickly, which is how your watch can show a live, updating heart rate reading during exercise rather than only providing a delayed calculation.

Wrist vs Finger: Why Placement Matters

Fingers generally have significantly better blood perfusion (blood flow close to the skin's surface) than wrists, which is part of why COLMI's smart rings often produce notably stable readings, particularly at rest and overnight. Wrist-based sensors, used in COLMI's watches, rely more heavily on a snug, consistent fit against the skin to get a clean reading, since the signal available at the wrist is inherently a bit weaker and more affected by positioning than at the finger.

Movement: The Single Biggest Accuracy Challenge

Motion is the dominant, single biggest source of inaccurate readings for any optical heart rate sensor on the market today, COLMI's included, regardless of price point or brand reputation. When your wrist or finger moves, the sensor picks up light fluctuations from that movement alongside the genuine pulse signal, and separating the two is genuinely difficult, especially during high-intensity or erratic movement like interval training or activities involving a lot of wrist rotation. COLMI's watches use accelerometer data alongside the optical signal to help filter out motion-related noise, a technique used across the wearable industry, but this compensation is never perfect, and accuracy during intense, fast-changing exercise remains a genuine limitation compared to a chest strap, which measures the heart's electrical signal directly rather than inferring it from blood flow.

Skin Tone and Sensor Accuracy: An Honest Note

This is a documented, industry-wide limitation worth addressing directly and honestly rather than glossing over or avoiding entirely in the way some wearable marketing tends to. Research on PPG-based sensors, including studies on both fitness wearables and clinical pulse oximeters, has found that melanin in skin can affect how much light is absorbed versus reflected, which can reduce accuracy for some users with darker skin tones compared to lighter skin tones under certain conditions. This isn't a COLMI-specific flaw — it reflects a genuine limitation of the underlying PPG technology used across essentially the entire consumer wearable industry, including premium brands. If you have any reason to be cautious about the accuracy of your readings, cross-checking against a manual pulse count occasionally is a reasonable, free way to build confidence in your specific device's accuracy for you personally.

Other Factors That Affect Accuracy

Beyond movement and skin tone, several other everyday factors influence reading quality. Cold hands or wrists reduce peripheral blood flow, weakening the signal available to the sensor. A loose-fitting watch or ring allows light to leak in from outside or the sensor to shift position against skin, both of which degrade signal quality. Certain tattoos, particularly dark or dense ink directly over the sensor area, can interfere with light absorption and reflection in ways that reduce accuracy. And direct, bright ambient light — strong sunlight hitting the sensor area, for instance — can occasionally interfere with the photodetector's ability to isolate the genuine pulse signal.

Continuous Monitoring vs Spot Checks

Your COLMI device's continuous background heart rate tracking (checking periodically throughout the day) and an active, real-time reading during exercise use the same underlying sensor technology, but continuous monitoring at rest tends to be more consistently accurate simply because you're generally still, warm, and not generating the same motion artifacts that challenge the sensor during exercise. If you want the most accurate possible spot-check reading, sitting still for 30 seconds to a minute with a snug, correctly positioned device gives the sensor its best chance at a clean signal.

How This Compares to ECG

As covered in more detail in our cardiovascular health guide, ECG (electrocardiogram) measures the heart's actual electrical activity directly, making it a fundamentally different and more clinically reliable technology than optical PPG sensing. No COLMI device includes ECG. Optical sensing is best understood as a genuinely useful estimate for everyday fitness and wellness tracking, not a clinical-grade measurement, and this distinction matters most if you're using heart rate data for anything beyond general fitness and trend awareness.

Tips for Getting the Most Accurate Readings

A few practical, easy-to-adopt habits genuinely improve reading quality day to day, without requiring any change of hardware or spending any additional money at all. Wear your watch snugly, roughly a finger-width above your wrist bone, rather than loosely — a common mistake that meaningfully degrades signal quality. For rings, make sure you've got the correct size, since a loose ring shifts position and produces noisier data than a properly fitted one. Keep the sensor area clean, since dirt or residue buildup can interfere with light transmission over time. And if you're checking heart rate specifically during intense exercise where precision matters most, understand that a chest strap paired with a compatible device will outperform any wrist or finger optical sensor, COLMI's included.

Why Optical Sensors Remain the Industry Standard Despite Limitations

Given these limitations, it's fair to ask why optical sensing remains standard across the wearable industry rather than ECG. The honest answer is practicality: ECG requires two points of contact (typically both wrists, or a chest strap), making continuous all-day tracking through a single wrist-worn or finger-worn device impossible with ECG alone. PPG, despite its accuracy trade-offs, enables the continuous, all-day, single-point tracking that makes wearables genuinely convenient for everyday use — a reasonable trade-off for most people's actual needs, even though it's worth understanding the limitations rather than assuming clinical-grade precision.

A Brief History of the Technology

PPG isn't a new invention built specifically for smartwatches — the underlying principle dates back to the 1930s, and clinical pulse oximeters using similar light-absorption techniques have been standard equipment in hospitals for decades, primarily for measuring blood oxygen during surgery and critical care. What's changed is miniaturisation: the same fundamental technology that once required bulky clinical equipment now fits into a chip small enough for a smart ring costing under £30, alongside considerably more sophisticated signal-processing algorithms than early clinical devices used.

How Your Watch Handles a Poor Quality Signal

When your COLMI device detects that the incoming optical signal is too noisy or weak to produce a reliable reading — due to movement, poor fit, or low blood flow — well-designed firmware should ideally either flag lower confidence in the reading or avoid displaying a number at all rather than presenting an unreliable figure as if it were solid data. If you notice your watch occasionally shows no reading or a brief gap in your heart rate graph, this is often the sensor correctly recognising it doesn't have a clean enough signal to report, rather than a fault worth being concerned about.

Battery Trade-offs of Continuous Monitoring

Running the optical sensor continuously throughout the day and night, as most COLMI devices do by default, does consume meaningfully more battery than checking heart rate only periodically. If battery life matters more to you than uninterrupted continuous data, reducing the frequency of automatic heart rate checks in Da Fit's settings — from continuous to, say, every 10 or 30 minutes — can extend battery life somewhat, at the cost of slightly less granular data for spotting brief heart rate spikes between check intervals.

How COLMI's Approach Compares to Other Brands

Optical PPG heart rate sensing is genuinely standard across almost the entire wearable industry at every price point, from budget trackers to premium smartwatches from major brands. The core technology and its limitations are broadly similar across manufacturers, since they're all working with the same fundamental physics of light absorption in blood. Where brands do differentiate is in the sophistication of their signal-processing algorithms and motion-compensation techniques, which affects how well each specific device handles challenging conditions like intense exercise — an area where more expensive devices sometimes, though not always, have a genuine edge through more refined algorithms built on larger training datasets.

What Happens During SpO2 Measurement Specifically

Blood oxygen measurement uses a related but distinct process from heart rate tracking, comparing how differently oxygenated and deoxygenated blood absorb red versus infrared light, then calculating a ratio that corresponds to blood oxygen saturation percentage. This measurement is generally more sensitive to movement and poor positioning than heart rate alone, which is why SpO2 readings are typically taken while still, often overnight or during a dedicated measurement rather than continuously throughout an active day.

A Note on Trusting Your Own Data Over Time

Regardless of any individual reading's precision, the most valuable thing you can do with your own heart rate data is get familiar with your own personal baseline over weeks and months of regular use. Once you know roughly what your normal resting heart rate looks like, genuine deviations from that personal baseline become far more meaningful and easier to notice than trying to judge any single reading against a generic population average, which was never really the point of continuous personal tracking in the first place.

Frequently Asked Questions

Why does my heart rate reading jump around during exercise?

Movement is the primary cause of unstable optical heart rate readings during exercise. Fast, erratic movement is particularly challenging for any wrist or finger-based optical sensor to filter out accurately.

Does skin tone really affect heart rate sensor accuracy?

Yes, this is a documented, industry-wide limitation of PPG technology generally, not specific to COLMI. Melanin in skin can affect light absorption in ways that occasionally reduce accuracy for some users.

Why does my ring give more stable readings than my watch?

Fingers generally have better blood perfusion than wrists, giving ring-based sensors a stronger, more consistent signal to work with, particularly at rest.

Should I trust my COLMI device's heart rate over a chest strap during a workout?

For precise, real-time accuracy during intense exercise, a chest strap is generally more reliable, since it measures electrical activity directly rather than inferring pulse from blood flow changes.

Why does my watch sometimes show no heart rate reading at all?

This often means the sensor has detected too much noise or too weak a signal to produce a reliable number, and is correctly withholding a reading rather than displaying unreliable data. Checking your fit and trying again after a moment of stillness usually resolves it.

Does a tattoo over my wrist affect heart rate tracking?

Dark or dense ink directly over the sensor area can interfere with light absorption and reduce accuracy. If possible, positioning the sensor slightly away from heavily tattooed skin can help.