Why does PPG (photoplethysmography) Use Infrared and Green Both Light

[jaus tail]

Hi Everyone,
I'm doing research in Implanted Devices in Human Body (Sensors, Hearing Aid, Pacemaker) for my college course.

I've started with FitBit that uses the PPG Principle. But I'm struggling to understand why the FitBit uses both Green and IR LED whereas the sensor at the receiving end is only for IR.

All the theory I read suggested that there is a Green (or Red) LED and an IR Sensor. So if there is an IR Sensor, why not use only an IR LED instead of Green (or Red) plus IR LED.

Any insight would be grateful...

 

[berkeman]

Looks like using both improves accuracy, especially at low temperatures...

https://www.ncbi.nlm.nih.gov/pubmed/19163152

 

[jaus tail]

Thanks for the quick n great reply n link. Very helpful.
Not sure hoemw to ask but can you please tell me what did you type in Google to get that paper?

 

[berkeman]

Thanks for the quick n great reply n link. Very helpful.
Not sure hoemw to ask but can you please tell me what did you type in Google to get that paper?

Happy to. I didn't get directly to it on my first try. First Google search was to find out what "PPG" stood for in your title (I then added the parenthetical bit to help others know what it was). I clicked into a few articles about PPG to see if they talked about the different wavelengths of light, but none seemed to. There were a couple good papers comparing EKG and PPG for accuracy and information content, but I couldn't find anything on the two light modalities.

So I got frustrated and tried a more directed search, in case I got lucky with some hits. I searched: PPG IR versus Green Light, and the link above was the 1st hit on the hit list. I prefer to use references to the NIH website when I can, and clicking into the abstract seemed to be a pretty good match for what you were looking for. Sometimes we get lucky!

https://www.google.com/search?q=PPG+IR+versus+Green+Light&ie=utf-8&oe=utf-8&client=firefox-b-1

 

[jaus tail]

Thanks again.