As the the title suggests, how is it that a little light they put on our finger when at the doctors/hospital/etc.. is able to tell us our blood oxygen levels?
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In blood, the oxygen is carried by some molecules.
It just so happens when the oxygen binds to those molecules they change color.
So they shine a light, read the color and that figures out what proportion of those molecules have changed to that color.
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My moment to shine no pun intended! I’m an RN and we use these lights a lot. The principle is pretty simple. See at end for edit:
If you shine a light through your finger, some of the light is absorbed by a protein called hemoglobin. hemoglobin carries oxygen. There is a tiny camera on the opposite side of the light that measures how much of the light was “removed”. A really simple version of this device would just say “yes, you have hemoglobin in there” by seeing that drop off in the light, the “signature” of hemoglobin. So , the really cool part is this: Hemoglobin changes color depending on how much oxygen is stuck to it. If your hemoglobin is 100% packed with oxygen it’s a nice cherry red color and the sensor can see this. As the oxygen in your blood drops, the color of the hemoglobin changes to a darker red, almost purply red when it’s really low. This color tells the sensor that your oxygen level is dropping. In fact the sensors can be thrown off by certain chemicals that alter the color of your blood.
Edit: figured i should clarify there are two light sources in a pulse oximeter. The color change does depend on how fat your finger is! just like if you look down the length of a a long tube of colored water it looks darker than when you look through the side. a computer tries to correct for this by comparing how much light was lost from both light sources.
Interesting side note – some SpO2 sensors can’t tell the difference between oxygen bound to hemoglobin and carbon monoxide that’s bound to hemoglobin. So, it’s possible to have CO poisoning, but still have a 90’s-100% SpO2 reading.
Since this has already been answered. I wanted to point out, some smartphones phones can do this now too. I’m using Galaxy S9 and it has this ability. Just found out when I got Covid a few months ago. Compared it to a hospital “finger light thing” and it was accurate.
A great video to explain it: [https://www.youtube.com/watch?v=4pZZ5AEEmek&t=1s](https://www.youtube.com/watch?v=4pZZ5AEEmek&t=1s)
TLDW: There are things in the world where we know how they will behaving in specific circumstances. One of these things is how much light will be absorbed by blood at any given level of oxygen. By shining a light through it, and then measuring what’s on the other side, we can know.
Youtuber Technology Connections has a very in-depth video on the background, benefits and limitations of such devices available [here](https://www.youtube.com/watch?v=4pZZ5AEEmek).
if anyone wants to get into the specific principles on how it works, this video explains it (to the level of postgraduate critical care specialist exams)
Additionally, there are co-oximeters that test more wavelengths of light, which can help distinguish methaemoglobin etc
My old Samsung phone had one of these on it and i thought it was the coolest thing ever. New phone doesnt have it and im so upset. I guess they want you to buy a smartwatch for that now
I haven’t seen an answer for a 5 year old yet, so ill try.
Oxygen binds to red blood cells. Red blood cells with oxygen absorb light differently than red blood cells without oxygen. You can measure light that passes through your finger and figure out how much oxygen is on your red blood cells.
Extra info. The reason its red light is because the light used is in the infrared spectrum, specifically, 940 and 660nm wave length. Well the 940nm is visible red light.
The reason for two wavelengths is because red cell with oxygen absorbs one wavelength and red cells without oxygen absorbs the other. This is how you get a percentage because its a ratio of oxygenated vs deoxygenated red cells.
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