A pulse oximeter is a clever tool used to measure a person’s blood oxygen saturation (SpO2) and pulse rate.
Within your body, inside each red blood cell exists around 270 million hemoglobin proteins that transport oxygen molecules from your lungs to the rest of your body. Oxygen saturation is the percentage of hemoglobin that is carrying oxygen. The typical range is 95–99% saturation.
This meter uses 2 lights, one red and one infrared, that alternate blinking through your fingertip about 30 times per second.
Saturated and desaturated blood each absorb these light frequencies at different rates. So a sensor on the opposite side of your finger measures how much of each light makes it all the way through your finger and calculates the ratio.
You may wonder how this can universally work on everyone’s hands regardless of skin, bone, fingernails, tattoos, nail polish, etc. Since all of those are constants and the pulsing of your blood through your finger is the only change occurring that would affect the light absorption, the computer can isolate the measurement of blood by only tracking changes in the absorption level.