Photoplethysmography - How the Apple Watch measures your heart rate for the Non-Technical

Photoplethysmography - How the Apple Watch measures your heart rate for the Non-Technical

By shooting a blinking green light  (hundreds of times per second) into your wrist a powerful SIP in the Apple Watch processes the data that measures your heart rate

The heart rate sensor in the Apple Watch uses what is known as photoplethysmography.

Terms for the Non-technical 

:)🙋your welcome:)

Photoplethysmography - 

(how to say it - (click here) 

The change in volume [of blood in your wrist] caused by the pressure pulse is detected by illuminating the skin with the GREEN light from a light-emitting diode (LED) and then measuring the amount of light either transmitted or reflected to a photodiode. Each cardiac cycle appears as a peak, as seen in the figure. Because blood flow to the skin can be modulated by multiple other physiological systems, the PPG can also be used to monitor breathing, hypovolemia, and other circulatory conditions.^[2] Additionally, the shape of the PPG waveform differs from subject to subject, and varies with the location and manner in which the pulse oximeter is attached.
https://en.wikipedia.org/wiki/Light-emitting_diode

 https://www.youtube.com/watch?v=cZPwTdC3ShU)  

Terms for the Non-technical 

:)🙋your welcome:)

SIP  (WHAT IS SIP) - System in Package

 The SiP in Apple Watch Series 1 contains a dual-core CPU with the graphics processor (GPU) 
its processes all the data and samples rates coming from your apple watch sensors)

Terms for the Non-technical 

:)🙋your welcome:)

The Apple Watch Infrared LED (red light) 

the purpose of the IR LED is to basically detects blood through your the thickness of your skin  

ALSO DID YOU KNOW - IR  can also detect through paint

(a completely unrelated article that proves IR detection of blood through thick material - here)

How the Apple Watch measures your heart rate for the Non-Technical

(The quick one sentence answer: 

Blood is red because it reflects red light and absorbs green light.

• a blinking GREEN Light shoots into your wrist and a sensor measures the light coming back! 

Fig. 1.1 Green LED light reflection (~510nm)

The green light absorption (~510 nm) is greater when your heart beats during blood flow throughout your your wrist and 

HOWEVER

Between beats the green light absorption is alot less - (there is less green light being absorbed by the body)

do not confuse "the ABSORPTION of green light" with the wavelength of green light (~510nm) and also don't forget your trig equations (the location of green light bulb click here) 

So by shooting a blinking green light  (hundreds of times per second) into your wrist a powerful SIP in the Apple Watch processes the data that measures your heart rate

👰Whoa - way too technical to for the non-techs
SIP (WHAT?) WHAT IS SIP -
System in Package
 The SiP in Apple Watch Series 1 contains a dual-core CPU with the graphics processor (GPU) It processes all the data and samples rates coming from your apple watch sensors)

Why Green?  Why not Purple (I like purple light) 

Who doesn't  like purple light or what is also known as "indigo" or a "black light". 
source NASA.GOV 
Indigo appears between blue and violet in a rainbow. Purple grapes and blueberries are indigo

source NASA

😎Purple Lights - "They are great at parties" 

However a green light shooting out an LED is more useful. 

Fig 1.2 Sensor Example  and Datasheet Source: 
http://www.mouser.com/ds/2/321/28380-ColorPAL-Documentation-370265.pdf

Here is why... Tell me more...   

 The Apple Watch uses green LED lights paired with light‑sensitive photodiodes to detect the amount of blood flowing through your wrist at any given moment. When your heart beats, the blood flow in your wrist — and the green light absorption — is greater. Between beats, it’s less. By flashing its LED lights hundreds of times per second, Apple Watch can calculate the number of times the heart beats each minute — your heart rate. In addition, the heart rate sensor is designed to compensate for low signal levels by increasing both LED brightness and sampling rate

The heart rate sensor in Apple Watch uses what is known as photoplethysmography.
Blood is red because it reflects red light and absorbs green light.

The heart rate sensor can also use infrared light. This mode is what Apple Watch uses when it measures your heart rate in the background.

For best results, start with a good fit

Even under ideal conditions, Apple Watch may not be able to get a reliable heart rate reading every time for everybody. And for a small percentage of users, various factors may make it impossible to get any heart rate reading at all. But there are things you can do to help Apple Watch get the most consistent and best heart rate readings possible.

Too loose

If your Apple Watch doesn’t stay in place, or the sensors aren’t reading your heart rate, tighten the band a bit.

Just right

Your Apple Watch should be snug but comfortable.

What else affects your reading

Many factors can affect the performance of the Apple Watch heart rate sensor. Skin perfusion is one. A fancy way of describing how much blood flows through your skin, skin perfusion varies significantly from person to person and can also be impacted by the environment. If you’re exercising in the cold, for example, the skin perfusion in your wrist may be too low for the heart rate sensor to get a reading.
Motion is another factor that can affect the heart rate sensor. Rhythmic movements, such as running or cycling, give better results compared to irregular movements, like tennis or boxing.
Permanent or temporary changes to your skin, such as some tattoos, can also impact heart rate sensor performance. The ink, pattern, and saturation of some tattoos can block light from the sensor, making it difficult to get reliable readings.
If you’re not able to get a consistent reading because of any of these factors, you can connect your Apple Watch wirelessly to external heart rate monitors such as Bluetooth chest straps.
When doing an Open Water Swim* in the Workout app, water may prevent a heart rate measurement, but Apple Watch will still track your calories using the built-in accelerometer. When doing a Pool Swim* in the Workout app, water may prevent a heart rate measurement, but Apple Watch will still track your calories, laps, and distance using the built-in accelerometer.

Heart rate is just one of many factors that Apple Watch uses to measure your activity and exercise. Depending on your workout, it selects the most appropriate inputs for that activity. For example, when you’re running indoors, it also uses the accelerometer. When you’re cycling outdoors, it uses the GPS in your Apple Watch Series 2 or in your iPhone for your Apple Watch (1st generation). And even when you’re not in a dedicated workout, it tracks how much you move each day. So Apple Watch can give you the information — and the motivation — to improve your fitness and your health.
* Available on Apple Watch Series 2 only. Apple Watch Series 1 and Apple Watch (1st generation) aren't suitable for swimming.