As was mentioned, a pulse oximeter does this.  Shine an infrared light into the
finger, and either measure the reflection (most popular) or light out on the
other side.  Same thing for the ear, scalp, or any other part of the body where
the blood comes close to the surface.

Calibrate by having someone hold their breath until they pass out (low oxygen)
and by getting someone to hyperventilat until the pass out (high oxygen).
(I'm JOKING!  Don't do this.  I'm sure to get sued, or worse, flamed.  egad....)

-Adam

Vic Lopez wrote:
>
> In the finger.
>         Thanks for your help. Vic Lopez

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