How a Heart Rate Monitor Reads Your Pulse Through the Skin

What Happens Inside a Wearable Device When It Touches Your Wrist

A small wearable device sits against the skin. Under the outer shell, a few parts work together. A light source shines into the wrist. A light sensor sits next to it, waiting for the light to bounce back. A battery powers both. A small chip processes what the sensor sees.

The device does not feel for a pulse the way fingers do. Fingers press down and feel the blood vessel expand. The device sends light instead of pressure. Light travels through the outer layer of skin and into the tissue below. Some of that light bounces back to the sensor. The amount of light returning changes with each heartbeat.

Each time the heart pushes blood forward, the blood vessels below the skin expand slightly. More blood fills the space. That extra blood absorbs more light. The sensor sees a drop in returning light. Between heartbeats, the vessels relax. Less blood sits in the space. Less light gets absorbed. The sensor sees more returning light.

The chip watches these small rises and falls in light intensity. Each drop followed by a rise marks one heartbeat. The chip counts how many times that pattern happens in one minute. That number becomes the heart rate shown on the screen.

The whole process happens many times per second. A typical device takes two hundred or more readings every second. That constant sampling catches the small changes between heartbeats. A slower sampling rate would miss the peak of each pulse.

How Does Light Travel Through Skin to Reach Blood Vessels

Skin has several layers. The outer layer, called the epidermis, sits on top. Below that lies the dermis. The dermis contains blood vessels, nerves, and hair roots. Deeper still sits a layer of fat. The blood vessels that matter for heart rate reading live in the dermis and the layer just below it.

Light from the device hits the outer skin layer first. Some of that light bounces off the surface right away. That reflected light never reaches any blood vessels. The sensor ignores that part of the signal because it does not change with the heartbeat.

The rest of the light passes through the outer layer. Different tissues absorb and scatter light in different ways. Water absorbs some wavelengths. Skin pigment absorbs others. The spaces between cells scatter light in random directions. A portion of the light eventually reaches the small blood vessels in the dermis.

Inside those blood vessels, red blood cells float in plasma. Red blood cells contain hemoglobin, a molecule that gives blood its red color. Hemoglobin absorbs light very well at certain wavelengths. When light hits a red blood cell, the hemoglobin catches a large portion of that light. The rest passes through or scatters away.

The light that survives this journey scatters back toward the skin surface. Some of it exits the skin and reaches the photodetector. The intensity of that returning light tells a story about how many red blood cells were in the path. More red blood cells mean more absorption and a weaker signal returning. Fewer red blood cells mean less absorption and a stronger signal.

The distance light travels matters. Light that goes too deep reaches large blood vessels or passes through into muscle. That light gives a confusing signal because it sees too many layers at once. Light that stays too shallow never reaches any blood vessels. The ideal depth sits right where the small vessels in the dermis live, around one to two millimeters below the skin surface.

Why Does Blood Absorb Different Wavelengths of Light

Not all colors of light behave the same way when hitting blood. Red light passes through skin easily and reaches deep into the tissue. Blue light gets absorbed near the surface. Green light falls somewhere in the middle. This difference in behavior comes from how hemoglobin interacts with each wavelength.

Hemoglobin absorbs green light more strongly than red or blue light. A beam of green light shining into skin gets absorbed quickly by any red blood cells in its path. The remaining light that bounces back to the sensor carries a strong signal from the blood. Small changes in blood volume cause large changes in the amount of green light returning.

Red light tells a different story. Red light does not get absorbed as strongly by hemoglobin. It penetrates deeper and bounces off tissues below the blood vessels. The returning signal changes less with each heartbeat. Red light works well for measuring blood oxygen because the difference between oxygenated and deoxygenated hemoglobin shows up more clearly in red and infrared light.

For heart rate alone, green light offers a practical choice. The signal changes a lot with each heartbeat, making it easier for the chip to detect the pattern. Green LEDs also fit into a small space and use less battery power than other colors. A device that runs all day needs to save power wherever possible.

Some devices use multiple colors at once. A green LED measures the pulse. A red LED and an infrared LED measure blood oxygen at the same time. The chip separates the signals from each color and processes them separately. The user sees both numbers on the screen without knowing which color did which job.

The choice of wavelength also affects how well the device works on different parts of the body. Green light works well on the wrist where blood vessels sit close to the surface. Red light works better on the finger or earlobe where light can pass all the way through the tissue. A device clamped onto a finger can shine light through from one side to the other. A device on the wrist cannot do that because the wrist is too thick.

How the Sensor Distinguishes a Pulse From Other Movements

A wrist moves constantly during the day. The hand swings while walking. The arm bends while reaching for objects. The device shifts slightly against the skin. All this movement creates noise in the light signal. The sensor sees changes in light that have nothing to do with the heartbeat.

A person walking swings the arm forward and back. Each swing moves the device relative to the skin. The distance between the LED and the blood vessels changes slightly. That change looks like a change in blood volume to the sensor. The light signal rises and falls with the walking motion. A simple counting method would confuse that motion with heartbeats.

The device solves this problem by looking at two signals at once. One signal comes from the light bouncing off the blood vessels. Another signal comes from a small accelerometer inside the device. The accelerometer measures movement of the device itself. The chip compares the two signals. Any pattern that shows up in both signals comes from motion, not from the heart.

The chip filters out the motion signal. Only the remaining pattern gets counted as heartbeats. This filtering happens in real time, many times per second. The user never sees the raw signal. All that appears on the screen is a clean number.

Walking produces a slow, regular motion. A typical walking pace creates one arm swing per two steps. That frequency sits much lower than a resting heart rate. The two signals separate easily. Other activities create more challenging motion signals.

A person running at a fast pace moves the arms at a frequency close to the heart rate. The motion signal and the heart signal overlap. The filtering process becomes harder. The device may occasionally count a motion event as a heartbeat or miss a real heartbeat that happens at the same moment as a motion. A good device handles this by using stronger filtering and by taking more samples per second.

Cycling on bumpy roads creates random high frequency vibrations. Those vibrations shake the device against the skin. The light signal picks up that shaking as noise. The accelerometer sees the same shaking. The chip filters out the shaking, but some noise always remains. The heart rate number may jump around more during cycling than during walking.

What Role Does Green Light Play in Optical Heart Rate Reading

Green light sits in the middle of the visible spectrum. Its wavelength runs shorter than red light but longer than blue light. That middle position gives green light a useful property for heart rate sensing. It penetrates the outer skin layer but does not go so deep that the signal becomes muddy.

When a green LED flashes against the wrist, the light spreads into the dermis. Hemoglobin in the red blood cells catches that green light readily. The absorption rate stays high. A small change in blood volume causes a noticeable change in how much green light returns to the sensor. The signal stands out clearly from background noise.

The frequency of green light also matters for another reason. Skin pigment absorbs green light less than blue light but more than red light. That balanced absorption means green light works across a range of skin tones. A device that uses only red light may struggle on darker skin because the pigment absorbs too much of the signal. A device that uses only blue light may not reach deep enough. Green light offers a compromise.

The LED itself plays a part in the reading quality. A green LED produces light at a specific intensity. Too much intensity causes glare inside the tissue. The light bounces around too many times before returning. The signal becomes washed out. Too little intensity does not reach enough blood vessels. The signal stays weak.

Manufacturers set the LED intensity based on the expected use case. A device worn on the wrist during daily activities uses a moderate intensity. A device worn on the finger during sleep uses a lower intensity because the finger has less tissue for the light to travel through. The user never adjusts this setting. The device handles it automatically.

The timing of the green light flashes also matters. A steady continuous light would drain the battery quickly. The LED flashes in short bursts instead. Each burst lasts only a few milliseconds. The sensor reads the returning light during each burst. Between bursts, the LED stays dark. The battery lasts much longer this way.

Some devices use multiple green LEDs arranged in a circle around the sensor. Each LED flashes in sequence. The sensor reads the returning light from each direction separately. This arrangement helps the device get a clear signal even when the wrist moves. One LED may move away from a blood vessel while another LED moves closer. The chip averages the signals from all directions to get a stable reading.

How the Device Counts Heart Beats From a Changing Light Signal

The raw signal coming from the sensor looks like a wavy line. The line goes up and down with each heartbeat. The chip needs to turn that wavy line into a single number. That conversion happens in several steps.

First, the chip filters out the steady part of the signal. The baseline level of light bouncing off the skin does not change much over a few seconds. That baseline comes from the skin itself, the fat layer, and the tissues that do not pulse. The chip subtracts that baseline from the signal. Only the changing part remains.

Second, the chip amplifies the remaining signal. The changes caused by each heartbeat are small. Without amplification, those small changes would get lost in the electrical noise inside the device. Amplification makes each heartbeat visible to the measuring circuits.

Third, the chip looks for peaks in the amplified signal. A peak marks the moment when the most blood sits in the vessels under the sensor. That moment happens a fraction of a second after the heart contracts. The chip records the time of each peak. The time between peaks is the gap between heartbeats.

Fourth, the chip calculates the heart rate from those time gaps. If the gap between two peaks measures one second, the heart rate sits at sixty beats per minute. If the gap measures half a second, the heart rate sits at one hundred twenty beats per minute. The chip makes this calculation after each new peak appears.

The device does not wait a full minute to show a number. It measures a few gaps and averages them. The number changes every second or two. A person looking at the screen sees a number that updates frequently. That updating gives the feeling of real time monitoring.

Motion creates false peaks in the signal. A sudden movement of the wrist sends a spike through the light reading. The chip sees a spike that looks like a peak. The accelerometer tells the chip that a movement just happened. The chip ignores that spike and does not count it as a heartbeat. The filtering happens quickly enough that the user never sees the false reading.

Why a Snug Fit on the Wrist Changes Reading Accuracy

The device needs to stay in one place relative to the skin. A loose fit lets the device slide around. Every time the device moves, the distance between the LED and the blood vessels changes. That changing distance looks like a changing blood volume to the sensor. The chip sees motion artifacts instead of clean heartbeats.

A snug fit presses the device gently against the skin. The light window touches the skin surface without gaps. No outside light leaks in under the edges. The LED stays at a constant distance from the blood vessels. The only thing changing the light signal is the pulse itself.

Tightness matters as well. A band that wraps too tightly compresses the blood vessels. Less blood flows through the compressed area. The signal becomes weaker because fewer red blood cells pass under the sensor. The device may struggle to find any clear heartbeat at all.

The ideal fit sits somewhere between loose and tight. The device should not slide when the wrist moves. The band should leave a slight mark when removed, but that mark should fade within a minute. A person can adjust the band until the reading becomes stable.

The position of the device on the wrist also affects the signal. The area with the most blood vessels sits on the underside of the wrist, near the palm. A device worn on the top of the wrist sees fewer blood vessels. The signal stays weaker. Wearing the device an inch or two above the wrist bone gives the clearest reading for most people.

Hairy arms create another fitting challenge. Hair lifts the device away from the skin. A small gap opens between the light window and the skin surface. Light leaks out through that gap. The sensor sees less returning light because some of it escapes. Shaving a small patch of skin or moving the device to a less hairy spot solves the problem.

During sleep, the wrist relaxes. The band may loosen overnight. The device shifts position without the person knowing. The heart rate reading may become less accurate during the early morning hours. A device that tightens automatically or one that uses a stretchy fabric band handles this problem better than a rigid plastic band.

What Interferes With Light Passing Through the Skin

Not everything that interferes with light shows up as motion. Some interferences come from the skin itself or from the environment. The device needs to handle these interferences without help from the user.

Sweat changes how light travels through skin. Water absorbs light differently than dry skin does. A layer of sweat between the device and the skin scatters the light in random directions. The sensor sees a drop in signal strength. The chip may interpret that drop as a change in blood volume. The heart rate number can jump around during a sweaty workout.

Tattoos on the wrist block light from reaching the blood vessels. Dark ink absorbs light before it gets deep enough. The sensor sees almost no returning signal. A device cannot read a pulse through a dense tattoo. Lighter colored tattoos cause less trouble but still reduce signal strength. Moving the device to the other wrist or to a different body part offers a solution.

Cold weather constricts blood vessels near the skin surface. The body pulls blood away from the extremities to keep the core warm. Less blood flows through the wrist. The light signal weakens because fewer red blood cells pass under the sensor. The device may show a lower heart rate than the true value or may stop showing any reading at all. Warming up the hands restores blood flow and fixes the reading.

Ambient light from the sun or from indoor lamps can leak into the sensor. The photodetector sees that outside light as extra signal. The chip cannot tell the difference between light from the LED and light from the room. A well designed device has a light seal around the sensor window. That seal blocks outside light from reaching the photodetector.

Some devices use a second photodetector that faces away from the skin. That detector measures ambient light only. The chip subtracts the ambient light reading from the skin reading. The result shows only the light that came from the LED and bounced back through the skin.

How the Device Adjusts to Different Skin Tones and Hair

Skin tone changes how much light gets absorbed before reaching the blood vessels. Darker skin contains more melanin. Melanin absorbs light across all wavelengths. A green LED shining into darker skin loses more of its intensity before reaching the dermis. The returning signal starts weaker than it would in lighter skin.

The device compensates by increasing the LED intensity. A brighter flash pushes more light through the melanin layer. More light reaches the blood vessels. The returning signal becomes strong enough for the sensor to read. The device measures the returning signal during the first few seconds of wear and adjusts the LED brightness automatically.

Hair on the wrist creates a different problem. Hair does not absorb light evenly. A single hair shaft can block a small area of the sensor window. That blocked area sees no light at all. The photodetector receives a patchy signal. The chip interprets those patches as noise.

Some devices use multiple LEDs arranged around the sensor. Each LED illuminates a different area of skin. If one area has hair blocking the light, another area may be clear. The chip averages the signals from all areas. The hair has less effect on the final reading.

The thickness of the outer skin layer varies from person to person. Thicker skin scatters more light before it reaches the blood vessels. The signal becomes diffuse instead of focused. A device that expects a certain amount of scattering may struggle with thicker or thinner skin. Adaptive algorithms that learn the user's skin properties over time handle this variation better than fixed settings.

The device stores a small profile of the user's skin characteristics. That profile updates continuously during wear. When the device first goes on the wrist, it spends a few seconds learning how light behaves on that particular patch of skin. The rest of the reading session uses that profile to interpret the signal correctly. The user never sees this learning process happen.

Where the Signal Goes After the Sensor Captures It

The photodetector turns light into electricity. Each photon that hits the detector creates a small electrical charge. More light means more charge. Less light means less charge. The detector outputs a voltage that changes with the incoming light intensity.

That voltage signal travels to the processing chip. The chip contains an analog to digital converter. The converter measures the voltage and turns it into a number. A high voltage becomes a large number. A low voltage becomes a small number. The chip collects thousands of these numbers every second.

The numbers flow into a small memory area inside the chip. That memory holds the last few seconds of readings. The chip runs filtering algorithms on the numbers stored in memory. The algorithms remove the baseline, amplify the peaks, and separate motion artifacts using data from the accelerometer.

The cleaned signal then goes into the peak detection stage. The chip looks for patterns that match the shape of a typical heartbeat. A real heartbeat has a quick rise, a sharp peak, and a slower fall. The chip compares each potential peak against that expected shape. Only the matches get counted.

The final heart rate number gets sent to the display screen. The number sits in another memory location, waiting for the screen to refresh. The screen reads that number and shows it to the user. The whole journey from light to number takes a small fraction of a second.

Some devices also send the heart rate number to a phone or a computer. A wireless transmitter, usually Bluetooth, packages the number into a data packet. The packet travels through the air to the receiving device. The receiving device shows the number on its own screen or stores it for later review. The original wearable device continues measuring the pulse without interruption.

The chip also saves the heart rate data over time. A small storage area holds readings from the last several hours or days. The user can look back at that history to see how the heart rate changed during sleep, work, or exercise. That historical data helps show patterns that a single reading could never reveal.