Figure 1: Oxygenated versus deoxygenated blood light absorption of IR and RED
Common body locations for taking pulse-oximeter measurements are through the tissue at the fingertip, earlobe, or foot (normally on infants). LEDs intensity, tissue thickness, skin color, sensor placement, and the oxygenated and deoxygenated blood light absorption all need to be considered during measurement.
The oximeter calculates oxygen saturation by taking the ratio of the absorption of red and infrared light, separating the time invariant parameters (intensity of light, skin color, tissue type, and deoxygenated blood) from the time varying parameters (oxygenated blood). Normal oxygen saturation values for a healthy individual range between 95 and 100 percent. The measured signal is pulsatile in nature due to the arterial blood vessels which expand and contract corresponding to each heartbeat. There are two types of pulse-oximeter measurements: transmissive and reflective. Reflective is normally used on the chest or forehead locations. Transmissive, which is more common, is normally used for finger, earlobe, or infant foot locations.
Placement of the LED light source and the photo-diode, along with separation of the light and detector, differentiate these two device types. The transmissive method projects a light through the tissue and a photodiode measures the light, which makes it through to the opposite side. In the reflective method, the light source and photodiode are the same surface. The signal levels for reflective are much smaller, so the user needs to be mindful of where it’s placed and how the circuit is designed.