From: dlzc1@cox.net   
      
   Dear RonTheGuy:   
      
   On Thursday, April 1, 2021 at 11:22:44 PM UTC-7, RonTheGuy wrote:   
   > On Feb 16, 2021, Andy Burns wrote   
   > > Shine a red light and an infrared light through* a blood   
   >> vessel onto an opto sensor.   
   >   
   > Why red?   
      
   Most efficient.   
   1) Red LEDs are very efficient and low voltage,   
   2) If you put a "white light" flashlight up against the palm of your hand the   
   only light that makes it through is red, and   
   3) Red blood cells change color based on the amount of oxygen (or carbon   
   monoxide) they contain.   
      
   Ride the horse in the direction it is going.   
      
   David A. Smith   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)