An optical heart rate monitor (OHR) works by shining a light into your skin and then measuring how much of that light is reflected back. Here's a more detailed breakdown:
The Basic Principle: Photoplethysmography (PPG)
Optical heart rate monitors use a technique called photoplethysmography (PPG). PPG relies on the fact that blood absorbs light differently than surrounding tissue.
How OHR Measures Heart Rate: A Step-by-Step Guide
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Light Emission: The monitor contains an LED (usually green, but sometimes red or infrared) that shines light into the skin.
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Light Interaction with Blood: This light penetrates the skin and reaches the blood vessels. When your heart beats, more blood flows through these vessels, leading to increased light absorption. Between heartbeats, less blood flows, and less light is absorbed.
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Light Reflection and Detection: The remaining light is reflected back from the skin and surrounding tissues. A photodetector in the monitor measures the intensity of this reflected light.
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Signal Processing: The photodetector generates an electrical signal that corresponds to the amount of light reflected. This signal fluctuates with the changes in blood volume caused by each heartbeat. The monitor then processes this signal to identify the peaks (representing heartbeats) and calculate your heart rate (beats per minute or BPM).
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Data Interpretation: The monitor interprets the varying light signals to determine heart rate. Higher blood volume (systole) absorbs more light, while lower blood volume (diastole) absorbs less, creating a pulsatile pattern.
Factors Affecting Accuracy
Several factors can affect the accuracy of optical heart rate monitors:
- Skin Pigmentation: Darker skin absorbs more light, which can reduce the signal strength. Some monitors use green light because it is less affected by melanin.
- Movement: Excessive movement can create noise in the signal, making it difficult to accurately detect heartbeats.
- Sensor Contact: A loose or poorly positioned sensor can also lead to inaccurate readings.
- Ambient Light: Strong external light sources can interfere with the reflected light signal.
- Temperature: Extremely cold temperatures can restrict blood flow, affecting measurement.
Common Uses:
- Fitness Trackers: Used in wearables to track heart rate during exercise.
- Smartwatches: Provides continuous heart rate monitoring for health insights.
- Medical Devices: In some medical devices for monitoring vital signs.
