What is the best color for a PPG signal? Red, green, blue or infra-red?
I'm asking because commercial oximeter sensors usually use a pair of red and infra-red LEDs. But recently, I've seen a lot of sensors using green LEDs.
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The reason red and infra-red are used is the difference in absorbance at those two frequencies between oxygenated and unoxygenated hemoglobin. Using the difference in these two frequencies allows the concentration of oxyhemoglobin to be calculated. (The red LED is at the frequency where oxyhemoglobin and hemoglobin have identical absorbance).
As far as I know, there's no reason why a green LED wouldn't work. You would only be able to measure the PPG signal and not the oximetry data (unless you used two different LEDs and did some math).
Found the difference in colors :
Longer wavelenght penetrate deeper.
because main artifacts are caused by changes in the blood volume at the measurement site. During movement, the changes in venous blood volume are larger than those in arterial one. While the penetration depth of light is dependent on its wavelength, and green light do not penetrate as deeply as infrared light. Therefore, the green PPG would include less information from various non-pulsatile sources than the infrared PPG.
I also looked up multi-spectral ppg and from what I read they all give you an exploitable PPG signal, it really depends from what you want to do. For oximeter you need red and ifr light source. but if ou just want a pulse meter shorter wavelenght are better.