624074/07 F-55
The principle of pulse oximetry is based on the difference in red
(R) and infrared (IR) light absorption characteristics of oxy- and
deoxyhaemoglobin. Oxyhaemoglobin absorbs infrared light
better than red light, whereas deoxyhaemoglobin absorbs red
light more then infrared light (Figure F-22).
Figure F-22. Light absorption spectrum of oxy- (HbO
2
)
and deoxyhaemoglobin (Hb)
A pulse oximeter consists of a light emitter (with red and infra-
red LED light sources) and a light detector. The emitted light
crosses a reasonably translucent and well perfused site (such as
finger or earlobe in adults, palm or finger or foot in neonates)
before it reaches the detector, which measures the light
amount passing through the site (Figure F-23).
Figure F-23. Scheme of a pulse oximeter
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