Apple explains how it thinks the heart rate measurement system in the new 𝖶𝖠𝖳𝖢𝖧 works here.
However there are some technical errors in that, and it is clear that some folk are still puzzled.
The most traditional way of estimating the heart rate is to feel the pulse at the wrist (or another main artery site, such as the femoral in the groin, or carotid in the neck), and count the number of pulses felt over a minute. In a normal healthy person, each heart beat produces one pulse (impulse), so that is the same as the heart rate; in some heart conditions that may not be the case, though.
The method most common in medical settings such as hospitals has been to measure the electrical fluxes across the body using skin electrodes, to produce an electrocardiogram (ECG, or EKG). This is also used in some ‘sports’ measuring systems, which have a plastic band which is placed around the chest.
There are several reasons why the 𝖶𝖠𝖳𝖢𝖧 was unlikely to use an ECG-based technique. The most obvious is that you get the best (most noise-free, highest amplitude) signals from measurement points which are quite distant. So you would have needed another sensor (at least), perhaps on the opposite wrist, or on the chest. Good electrical contact is also needed, which is fine if you keep the sensors moistened with conductive jelly or sweat, but not really socially acceptable.
More recently a family of medical measurement systems has been developed which uses light reflection from the haemoglobin in red cells in the superficial blood vessels of the skin, to estimate variables. You may have encountered these in the ‘sats’ systems commonly used in A&E (ER) and elsewhere in hospitals, where they will report for example that the arterial blood is 98% saturated with oxygen (good), or 70% (very bad).
One variant of this system has been used to estimate blood flow. Traditional methods of measuring blood flow actually do so indirectly, by measuring the change in volume of part of a limb. Measuring volume is performed using a plethysmograph, so the system used to estimate blood flow by light reflected from haemoglobin also been known as plethysmography, although in fact it does not measure volume change at all.
Apple says that there are two systems for measuring heart rate: one based on green light, the other on infra-red; both are existing technologies used to estimate blood flow and oxygen saturation in clinical systems.
Why green light? As any artist will tell you, green is the complementary colour to red. Because red cells and the haemoglobin inside them appears red, it means that it absorbs green light. So if you shine green light into the skin and measure how much is reflected back, that will give you an estimate of how much is absorbed, and thus how many red cells are going past at any time.
Measure reflected green light very frequently, and you will see troughs of reflection, which are moments when many red cells are going past, which is the local pulse. Thus you can estimate the local pulse rate, which should (in a healthy normal person) be the same as the heart rate. Infra-red methods are fairly similar in principle.
So the technology is derived from a device which (among other things) can estimate blood flow, which has (incorrectly) been named photoplethysmography (PPG).
But measuring heart rate, however you do it, is actually measuring a frequency, so you should call it a ‘heart rate meter’, or if you like more opaque terms, a frequency meter or even tachometer, from the Greek for speed.