I think they work on the intensity of the light being modulated by
the blood as it passes through the finger. I saw one of these just
recently on TV, probably on "ER", and it used visible red light
(looked like that nice pinky-red colour that you get from some
really super-bright LEDs). The persons finger tip was lit up like
rudolfs nose! Main advantage I guess is that you get a visible
indication that the equipment is in fact working, before deciding to
throw the patient away.

Brent Brown
Electronic Design Solutions
16 English Street
Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile: 025 334 069
eMail:  brent.brown@clear.net.nz