Heart rate monitors often use green light because hemoglobin in oxygenated blood absorbs green light particularly well, allowing the monitor to detect changes in blood volume.
How Green Light Works in Heart Rate Monitoring
Heart rate monitors, particularly those using photoplethysmography (PPG), rely on shining light into the skin and measuring how much of that light is absorbed. The principle behind using green light is based on its absorption properties relative to blood:
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Absorption by Hemoglobin: Oxygenated hemoglobin, the form of hemoglobin carrying oxygen in the blood, absorbs green light (wavelengths around 530 nm) more effectively than other colors. Deoxygenated hemoglobin also absorbs green light but to a lesser extent.
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Measuring Blood Volume Changes: As the heart beats, the volume of blood in the capillaries fluctuates. By shining green light onto the skin and measuring the amount of light reflected or transmitted, the monitor can detect these changes in blood volume. The intensity of the reflected green light varies depending on the amount of blood present.
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Bayer Filter Advantage: Many devices, including smartphones and wearable sensors, utilize a Bayer filter. The green filter in a Bayer filter readily allows light around 530nm to pass through, making it easy to measure the intensity of the reflected green light and thus, the blood volume.
Advantages of Using Green Light
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Effective Absorption: Green light's high absorption rate by hemoglobin makes it a sensitive indicator of blood volume changes.
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Compatibility with Existing Technology: The green filter on the Bayer filter of many devices is optimized for these wavelengths, making the signal easy to measure.
In Summary
Green light is used in heart rate monitors because it is readily absorbed by hemoglobin in blood. By monitoring the reflection of green light, these devices can detect changes in blood volume associated with each heartbeat, providing a measurement of heart rate.
