> It's probably cheaper to use just a single IR led and phototransister,
> but I thought the original desings included both red and IR, and you can
> effectively subtract the red (which doesn't change with blood flow,

why is blood red ?
because it reflects red light ;-)
here is a (rather old) document explaining bloodflow/oxigenation
measured with light
  http://www.perfusion.com.au/CCP/Instrumentation/oximetry.pdf
Stef Mientki

>

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