The small electronic device a nurse attaches to the end of your index finger with a plastic clip calculates how much oxygen is carried in your blood. It's based on measuring the difference between levels of the red pigment hemoglobin which carries oxygen in your blood. It turns out that oxygenated hemoglobin ("oxyhemoglobin") and nonoxygenated hemoglobin ("deoxyhemoglobin") in your blood are different colors. That thing on your finger has two light-emitting diodes - one that sends out invisible infrared light and one that sends out red light.
The exact colors are chosen to coincide with the maximum differences in colors of light absorbed by each form of hemoglobin. The difference between the two gives a measure of what fraction of the hemoglobin in your blood is oxygenated and thus how much oxygen is being carried around inside you.
Things are not quite so simple, however, as the light must pass through your skin, there's bone in the way, etc. so there's one more trick that's used. In time with your pulse, your finger gets a little bigger and smaller depending on how engorged it is with blood, but the skin and bone remain the same. By comparing the amount of each color of light that gets through with your finger most and least full of blood it's possible to see the effects of the blood itself - and that's finally what gets you a clean oxygen reading without the need to break your skin.
Doctors find this information useful to figure out if your heart/ lungs/ blood are all working together to get oxygen delivered to all the various parts of your body. The test isn't perfect - for example, you could have poor overall blood flow and still a high level of oxygenation.
The test can also be faked out by other things that can influence blood color. For example, exposure to carbon monoxide can give an incorrect reading, but more sophisticated pulse oximeters can use other colors of light to sort out how much hemoglobin has got carbon monoxide stuck to it instead of oxygen.
Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to firstname.lastname@example.org or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.