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timidhermit

macrumors 6502
Original poster
Jul 6, 2011
366
3
From my research, I know that Apple Watch measures heart rate using a method called plethysmography. During active measurements, the green LED light is activated. However, I read somewhere that Apple Watch can also measure PASSIVELY heart rate WITHOUT the green LED light being activated by using infrared light. Is this true?

Are there other devices, like Fitbit Alta HR, that can measure heart rate PASSIVELY without having the bright green LED light on?
 

Lennyvalentin

macrumors 65816
Apr 25, 2011
1,431
793
However, I read somewhere that Apple Watch can also measure PASSIVELY heart rate WITHOUT the green LED light being activated by using infrared light.
Apple watch's pulse sensor has both green and IR LEDs according to information I've seen, so it's not really passive monitoring. IR LEDs are more power efficient than visible light LEDs, so therefore less of a drain on the battery.
 

timidhermit

macrumors 6502
Original poster
Jul 6, 2011
366
3
Does this random auto monitoring of heart rate ALSO works for first-generation Apple Watch? Or is this feature only available for S1 and higher? I presume that this is DIFFERENT from the continuous heart rate monitoring that is only available in higher models, correct?

Apple watch's pulse sensor has both green and IR LEDs according to information I've seen, so it's not really passive monitoring. IR LEDs are more power efficient than visible light LEDs, so therefore less of a drain on the battery.
 

Lennyvalentin

macrumors 65816
Apr 25, 2011
1,431
793
Does this random auto monitoring of heart rate ALSO works for first-generation Apple Watch?
I'm not sure if wOS supports any form of auto monitoring on OG watches. wOS 1 (the original release that came at launch and only lasted until autumn 2016) did brief 10 second or so periodic background monitoring while you were moving, and not running the exercise app IIRC, and which disappeared in wOS 2 as I also seem to recall. Not sure if it was ever re-implemented for OG watches in later wOS versions.

As you seem to be aware, S1 watches and onwards has some form of background monitoring, which I assume uses the less power hungry IR LEDs of the pulse sensor. Exactly how persistent it is I'm not sure; I haven't actually seen any sort of comprehensive breakdown of how that feature really works. I assume OG watches still has no form of background heart rate monitoring, to keep that feature unique to more recent watch models. Also, OG watch's battery really doesn't need the added strain! :p
 
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