Welcome to Tech Talk, a weekly column about the things we use and how they work. We try to keep it simple here so everyone can understand how and why the gadget in your hand does what it does.
Things may become a little technical at times, as that’s the nature of technology — it can be complex and intricate. Together we can break it all down and make it accessible, though!
Tech Talk
How it works, explained in a way that everyone can understand. Your weekly look into what makes your gadgets tick.
You might not care how any of this stuff happens, and that’s OK, too. Your tech gadgets are personal and should be fun. You never know though, you might just learn something …
What are “Health Sensors”?
Almost every wearable device, as well as plenty of phones, has what are called “health sensors.” That’s a very broad term for components that are also used for a lot of other things, but it kind of fits here.
Most health sensors either emit light or detect light (or both if they’re the expensive kind). That’s because of all the things you can do with light, and two of the most interesting things are included on many wearable devices: blood oxygen level (SpO2) monitoring and heart rate detection.
How can light tell me when my heart beats and how rich my blood is with oxygen? Remember that light doesn’t only mean the light a human can see.
Types of light
We’re not going to go hard and deep with the technical details here; that’s not what this article is about. Light is made of electromagnetic particles (photons) that are emitted at a certain frequency, causing them to fill many parts of the electromagnetic spectrum. It’s fascinating, and I encourage anyone interested to search out more details.
Our eyes can see light between about 400nm (blue) and 700nm (red) in that spectrum, and the lower wavelengths are commonly called ultraviolet, and the upper end is called infrared.
No two people see light the same way. For example, my blue shirt is blue to me, but that color is not the same as what you see. It’s “blue” because of the wavelengths of visible light it reflects, and we learned what “blue” means to our eyes when we were young. “Blue” will change as we age, but so will our concept of what blue means.
Machines aren’t like that. A box of properly calibrated sensors “sees” exactly the same color of blue. That’s how any of this is possible — the sensors know what to look for, and it will be the same from product to product.
The two important bits here are light outside of the spectrum people can see, and that sensors will “see” things the same way.
The process
I’ll use SpO2 reading as an example to illustrate how this can work, but methods such as heart rate detection and glucose monitoring can also be implemented using the same principles.
Inside your wearable, there are tiny LEDs that shine on the thin skin of your wrist. You’ve probably seen them blinking green, but the two used for SpO2 measurement are red and infrared (light you can’t see that penetrates solids like your skin differently). Those LEDs blink at a specific interval, and some of the light they emit goes through your skin and into the tiny blood vessels.
There is a pair of dedicated photodiodes that gather the light reflected through the skin from those blood vessels while trying to block the light that doesn’t go into your skin.
Oxygenated and deoxygenated hemoglobin absorb light differently. Hemoglobin that has been oxygenated absorbs more of the infrared light, so less infrared light will be reflected back to the photodiode. Hemoglobin that has been stripped of its oxygen does the opposite, absorbing more red light and reflecting less of it to the photodiodes.
The computer inside your wearable knows how much of each type of light was sent, based on its intensity. It also knows how much was reflected thanks to the photodiodes that measure it. It can then calculate what percentage of the hemoglobin in your blood is oxygenated based on the difference.
It’s a tough idea to wrap your head around at first, but it’s really just simple math at this point. A certain amount goes in, and a certain amount comes back. Calculate the difference and have the software make the number meaningful.
It’s also the same way those little gadgets that doctors or nurses put on the tip of your finger to get SpO2 readings are used, but those are checked regularly and discarded if they lose calibration.
LEDs and photodiodes are also used for heart rhythm, heartbeat, glucose monitoring, and most anything that involves measuring your blood without actually sampling it. That’s because specific things in your blood react to different light wavelengths differently.
You can combine this with what’s called a Photoplethysmography (PPG for short, which measures volume based on size changes or your finger or wrist) sensor and a component that measures electricity to measure blood volume and electrical signals, and the time it takes them to travel to and from your heart. This can provide an ECG reading or determine your blood pressure.
Yes, if your wearable can do one of these things, then it is capable of doing the other, even if it’s not programmed to do it.
These ideas aren’t new, and they aren’t only used to monitor your health from your ring or watch. However, making the components small enough and using less power was what it took to bring them to your wearable. Remember, you could also measure your heartbeat using many older Galaxy S phones because they had more room for the parts.
In the end, everything turns into math and numbers when you’re a computer. Writing the software to make those numbers mean something is the hard part!
