What is a PPG sensor? How your watch reads your pulse with light

Short answer

A PPG sensor shines light into your skin and measures how much comes back. PPG stands for photoplethysmography: each heartbeat pushes a small wave of blood into the tiny vessels under the skin, the blood absorbs light, and the sensor sees that change as a rhythmic signal (1). The green flashing light on the back of a smartwatch, the sensor inside a ring and a fingertip on your phone camera all use this idea.

From that signal, devices estimate heart rate, heart rate variability (HRV), blood oxygen and breathing rate — but PPG reads your pulse, not your heart's electrical activity. That is the job of an ECG. Studies find wrist PPG reasonably accurate at rest, with more error during activity: in a study of six wearables, absolute error during activity was on average 30% higher than at rest (3).

If your watch has ever shown a strange number and you wondered whether you'd worn it wrong, you're not imagining things — optical sensors are sensitive to fit, movement and cold, and a bad reading is usually about the signal, not about you.

Note: this article is for general education and is not medical advice. A wearable or app reading cannot rule out a heart problem. Chest pain, fainting, severe shortness of breath, or a racing or irregular heartbeat with dizziness are emergencies: call 911 or your local emergency number. This guide is published by Welltory, which makes one of the apps mentioned. Welltory is a general wellness product: it helps you track and understand your data and does not diagnose or treat any condition.

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What does PPG mean?

PPG is short for photoplethysmography — "photo" for light, "plethysmo" for changes in volume, "graphy" for recording. A PPG sensor optically detects changes in blood volume in the microvascular bed of the tissue (1). The recorded curve is called a photoplethysmogram, or simply the PPG signal.

You'll see the term in three places:

• Smartwatches, fitness bands and rings. Most call it an "optical heart rate sensor." Apple, for example, states that the optical heart sensor in Apple Watch uses photoplethysmography (2).

• Pulse oximeters. The clip you put on your finger at the doctor's office is a PPG device that uses two colors of light to estimate oxygen saturation (1).

• Phone camera apps. When you cover the camera and flash with your fingertip, the flash is the light source and the camera is the detector (8).

If you searched "PPG meaning" and landed on paint companies or basketball stats (points per game), that's the same three letters in a different world. In health tech, PPG always means the light-based pulse sensor.

How does a PPG sensor work?

The principle fits in four steps:

1. A light source shines into the skin. In watches and rings it's one or more small LEDs. On a phone, it's the flash.

2. Blood absorbs some of that light. Hemoglobin in your blood absorbs light more strongly than the surrounding tissue.

3. The amount of blood changes with each heartbeat. When your heart contracts, a pressure wave fills the small arteries a little more; between beats they hold a little less.

4. A photodetector measures the light that comes back. More blood means less light returns. The rhythmic rise and fall of that light is your pulse.

Engineers split the signal into two parts. The steady DC component reflects the average volume of tissue and blood and drifts slowly, for example with breathing. The pulsing AC component reflects the beat-to-beat change in blood volume, and its rhythm tracks your heart rate (1).

Apple describes the practical version: green LED lights paired with light-sensitive photodiodes detect the amount of blood flowing through your wrist, and by flashing the LEDs hundreds of times per second, the watch can calculate how many times your heart beats each minute (2).

Why green light on the wrist, red and infrared on the finger

Wrist wearables mostly use green light. Green, roughly 500–600 nm, shows the largest change from pulsing blood near the skin surface and is strongly absorbed by hemoglobin, which gives a better signal-to-noise ratio for superficial blood flow (1).

Red and infrared light travel deeper. Pulse oximeters compare how red and infrared light are absorbed to estimate blood oxygen (1). Some watches switch to infrared at certain times; Apple notes that its watch uses infrared LEDs during your sleep schedule or when a sleep focus is on (2).

Reflectance vs. transmission

There are two ways to place the light and the detector (1):

Your phone's camera reading is a reflectance measurement: the flash and the lens sit side by side, and your fingertip covers both.

What can a PPG sensor measure?

The same pulse wave carries more than one piece of information. Researchers list heart rate, oxygen saturation, respiration-related trends, relative blood perfusion and indicators of arterial stiffness among the parameters PPG can provide (1). In consumer devices, that translates to:

As of November 2026, cuffless blood pressure features on watches also start from optical pulse data, but they need calibration with a cuff and are offered as wellness features — Samsung, for example, states that its blood pressure monitoring "is for wellness only" (7). We cover that separately in can a smartwatch measure blood pressure?

PPG vs. ECG: what's the difference?

People often ask whether the sensor on the back of their watch is "an ECG." It isn't, and the difference matters.

The two work together on some watches. Samsung's irregular heart rhythm notification, for example, analyzes pulse rate data to flag rhythms that may suggest atrial fibrillation and then prompts you to record an ECG (6). In Samsung's U.S. validation study of 810 adults, the notification picked up 68.0% of people who had atrial fibrillation episodes lasting an hour or more, with a specificity of 98.8% (6). In plain terms: when it alerts, it's usually worth acting on, but no alert doesn't mean no problem. The same document states that the feature does not notify on every episode and that a missing notification does not rule out disease (6).

For more on what a clinical ECG shows, see our guide what is an EKG (ECG)?

Is HRV from a PPG sensor real HRV?

Strictly speaking, a PPG device measures pulse rate variability (PRV) — the variation between pulse peaks at the wrist or fingertip — not the variation between electrical heartbeats. The pulse arrives a fraction of a second after the electrical beat, and that delay itself changes slightly with blood pressure, vessel stiffness and breathing.

A review in Physiological Measurement concluded that the relationship between PRV and HRV is "not entirely understood yet" and that pulse rate variability "must not be considered as a valid surrogate of heart rate variability in all scenarios" (4). The authors also note that PRV may carry its own information about vascular responses (4).

What this means for you:

• At rest, sitting or lying still, a good PPG reading gives HRV numbers you can track as a trend.

• During movement, standing up or exercise, the gap between PRV and ECG-based HRV can widen. That's why most devices measure HRV while you're still or asleep — Apple notes that HRV is sampled while you're still (2).

• Compare like with like. A watch's overnight HRV, a ring's overnight HRV and a 1-minute morning camera reading are three different measurements. Compare each one only with itself over time.

We dig into the camera case in how accurate is HRV from a phone camera? and explain what the number itself means in heart rate variability meaning.

What makes a PPG reading wrong?

Because PPG depends on a tiny change in reflected light, anything that weakens the pulse signal or adds a fake rhythm can throw it off. Research on wearable sensors names these main sources of error (1, 3):

1. Motion. Arm swing, typing or running add their own rhythm. If your steps happen at a cadence close to your heart rate, the sensor can lock onto the wrong signal. In the six-device study, error during activity was about 30% higher than at rest (3).

2. Contact pressure. Too loose and light leaks in; too tight and you squeeze the blood out of the very vessels being measured. Both shrink the pulse signal (1).

3. Cold. In the cold, blood flow to the fingers and skin drops, so there's less pulsing blood to see. Fingertip and earlobe sites are especially affected by low temperatures (1).

4. Ambient light. Bright sunlight leaking under a loose band, or a fingertip that doesn't fully cover the camera, adds light that has nothing to do with your pulse.

5. Skin and what's on it. For heart rate, the six-device study found no statistically significant difference in accuracy across skin tones (3). For blood oxygen, the picture is different: the FDA lists skin pigmentation among factors that can affect pulse oximeter accuracy (5). Tattoos on the wrist can also block light.

6. Irregular rhythms. If the heart beats irregularly, some beats push out too little blood to show up as a clear pulse wave. That's one reason PPG-based heart rate and HRV can be misleading in arrhythmias, and why an ECG is the test that classifies rhythm.

7. Device differences. Accuracy varied significantly between devices in the same study (3). Two watches on two wrists won't show identical numbers.

How to get a better PPG reading

Whether you use a watch, ring or phone camera, the same habits help:

1. Sit or lie still for a minute or two first. Don't measure right after climbing stairs or rushing for your phone.

2. Fit the watch snugly, a finger's width above the wrist bone. Tight enough that it doesn't slide, loose enough that it doesn't leave a deep mark. Tighten it before a workout, loosen it after.

3. Warm up cold hands. Rub them together or hold a warm mug for a minute before a fingertip reading.

4. Cover the camera and flash fully, with light pressure. Pressing hard flattens the pulse.

5. Breathe normally and don't talk. Talking and deep sighs change the signal.

6. Keep the sensor clean. Sweat, lotion and dust on the sensor window scatter light.

7. Measure at the same time and in the same position each day. For HRV in particular, consistency matters more than any single number. The Welltory Help Center suggests measuring in the morning, after waking, before coffee or exercise, in the same position, for one to two weeks to build a baseline (10).

Watch, ring, chest strap or phone camera?

All of these except the chest strap use PPG. They differ in where they sit and when they measure.

• Smartwatch — Sensor: Reflectance PPG (green, often red/infrared too) on the wrist; some add an ECG electrode; When it measures: Day and night; Best for: Heart rate trends, sleep, background alerts

• Smart ring — Sensor: Reflectance PPG on the finger; When it measures: Mostly during sleep and rest; Best for: Overnight HRV and resting heart rate

• Chest strap — Sensor: ECG electrodes, not PPG; When it measures: During workouts or deliberate readings; Best for: Beat-to-beat accuracy during exercise

• Phone camera — Sensor: Flash plus camera on a fingertip; When it measures: Only when you start a reading; Best for: A spot reading with no extra device

• Finger pulse oximeter — Sensor: Transmission PPG, red and infrared; When it measures: When you clip it on; Best for: Spot SpO2 and pulse

Our guide to measuring HRV without a wearable explains the phone-only route, and our roundup of the best Android smartwatches for heart health compares watches.

Living with a PPG sensor: the questions people ask next

Why does my watch flash green at night?

Many watches use their optical sensor to sample heart rate and HRV during sleep. Depending on the model, they may use green light or switch to infrared, which you can't see. Apple, for example, uses infrared LEDs during the sleep schedule (2).

Is the PPG light safe?

Wearables use small LEDs of the same kind found in everyday electronics, and they are sold for all-day wear. If a band irritates your skin, check fit, sweat and the strap material first. Take it off and ask a doctor if a rash doesn't clear, and follow the maker's guidance if you have a light-sensitive skin condition.

Why does my heart rate jump during weightlifting but not running?

Gripping a bar tenses your forearm muscles and changes blood flow under the sensor, and the movement isn't rhythmic, so the algorithm has a harder time picking out your pulse. Steady running gives a cleaner signal. For lifting or interval training, a chest strap is more reliable.

Can a phone camera really read my pulse?

Yes. Covering the camera and flash with a fingertip turns the phone into a reflectance PPG sensor: the flash lights the skin and the camera records a short video of the color changes with each beat (8). It's a spot reading you start on purpose, not continuous tracking.

Does skin tone affect my watch?

For heart rate, one study of six wearables across the full range of skin tones found no statistically significant difference in accuracy (3). For blood oxygen estimates, the FDA lists skin pigmentation as a factor that can affect pulse oximeter accuracy, so treat SpO2 numbers with more caution (5).

Before you blame yourself

When a sleep score drops or the watch says your HRV "crashed," it's easy to assume you did something wrong — slept badly, trained too hard, failed at recovery. Before you blame yourself, look at your data and ask a simpler question first: was the signal good?

• The feeling: "My numbers are terrible today."

• The cause you can see: a loose band after a shower, cold hands on a winter morning, a reading taken while walking, a late drink, a short night.

• One lever for one to two weeks: pick one — same time and position for every reading, a snug fit at night, or warming your hands before a camera reading. Then compare week to week, not day to day.

Most "bad days" on a PPG device are a mix of real physiology and measurement noise. Fixing the noise first makes the real changes easier to see.

When to see a doctor

A PPG reading can prompt a conversation with your doctor but can't settle it. See a doctor if:

• Your watch or app shows an irregular rhythm alert, especially more than once.

• Your resting heart rate is often above 100 or below 50 and you feel dizzy, faint, short of breath or unusually tired.

• You feel palpitations — fluttering, pounding or skipped beats — that keep coming back.

• Your blood oxygen readings are repeatedly low and you also feel unwell. The FDA advises not to rely only on a pulse oximeter and to contact a health care provider if readings concern you (5).

Call 911 or your local emergency number if you have chest pain or pressure, shortness of breath, fainting, or signs of a heart attack. Samsung's own instructions for its heart features say to seek emergency care in these situations rather than rely on the watch (6). If you're worried about an irregular heartbeat, our guide to atrial fibrillation explains the symptoms.

How to bring this up with your doctor

What to ask for, by name. "Could you check my heart rhythm with a 12-lead ECG?" If symptoms come and go: "Would a heart monitor I wear for a few days or weeks help catch it?"

What to bring. Screenshots or an export of the readings that worried you, with dates and times; any watch ECG recordings as PDFs; a list of medicines, supplements, caffeine and alcohol; notes on how you felt.

What to say, out loud. "My watch's optical sensor flagged [an irregular rhythm / a resting heart rate of X] on [dates], and at the time I felt [symptom]. I know it's not a diagnosis — can we check it properly?"

If you're told wearables aren't accurate. That's partly true, and it's a fair point. Ask: "Then can we do the test that is accurate, so we can rule a rhythm problem in or out?"

How Welltory helps — and what it cannot do

The limits first. Welltory is a general wellness product, not a medical device. It does not diagnose arrhythmias or any other condition, and its camera reading is a PPG measurement, not an ECG. The Help Center notes that camera readings are unreliable with arrhythmias and may be inaccurate if you're under 18 or pregnant (9).

What it can do is use the PPG sensors you already have and help you read the results against your own baseline.

A camera PPG reading. You cover the camera and flash with a fingertip and get a spot reading of heart rate and HRV — a deliberate check, not 24/7 monitoring (9). Not every phone works: the Help Center lists models where the flash sits too far from the camera. On the free plan you get one camera or heart rate monitor measurement a day; Premium, a paid subscription that renews automatically until you cancel, removes that limit (9).

Your watch or strap as the sensor. If you'd rather not use the camera, you can measure with an Apple Watch, a Samsung Galaxy Watch (Series 4 and later) or a compatible chest strap, and connect other sources such as Apple Health or Health Connect (12).

A routine that makes the signal comparable. The Help Center suggests measuring every morning for one to two weeks — after waking, before coffee, exercise or a shower, in the same position, breathing normally (10).

A record for your doctor. From the web app you can export your data as CSV: Dashboard → choose a chart → pick the time range → Export (11).

How we made it

We based this article on a review of wearable photoplethysmographic sensors in Electronics, a study of sources of inaccuracy in wearable optical heart rate sensors in npj Digital Medicine, a review of pulse rate variability in Physiological Measurement, the FDA's page on pulse oximeters, Apple's support page on heart rate, and Samsung's U.S. instructions for use for its irregular heart rhythm notification. Device details are as of November 2026. Search questions came from what people in the United States most often ask about PPG sensors. For the product section, we checked every function against the Welltory Help Center on November 6, 2026.

Made with AI tools, then edited and fact-checked by the Welltory team. See our Editorial & AI policy.

Jane Smorodnikova, co-founder of Welltory, built the methodology behind how we read physiological data.

Written by Tatsiana Yashyna.

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