Photoplethysmography — PPG — is the measurement of blood with light. A small LED shines into the tissue of your finger and a photodiode measures how much of that light comes back. With every heartbeat the arteries expand and absorb slightly more light, then relax and absorb slightly less. That rhythm, repeated all day and all night, is the raw pulse signal.
It is the oldest optical measurement in wearables and still the most useful one, because almost everything else on this page is derived from it.
Why the finger
The arteries in a finger sit close to the surface and carry a strong pulsatile signal. A wrist device reads through a thicker, more mobile tissue bed and loses a great deal of that signal to movement. This is the single reason a ring can be smaller than a watch and still read more cleanly.
How Aurora reads it
Green light is absorbed strongly by haemoglobin and is relatively resistant to ambient light, which makes it the working wavelength during the day and during movement. Infrared penetrates deeper and behaves better when you are still, which makes it the overnight wavelength and the one used alongside red light for oxygen saturation.
What comes out of the sensor is not a heart rate. It is a series of intervals between beats. Heart rate is the average of that series. Heart-rate variability is its spread. Much of the sleep staging is built on how both of them behave across the night.
What it cannot tell you
PPG is not an ECG. It measures a pressure wave travelling through your arteries, not the electrical activity of the heart, and it cannot be used to detect, diagnose or rule out arrhythmia or any cardiac condition.
It also degrades in predictable ways: cold hands reduce perfusion, a loose ring lets ambient light in, and heavy motion swamps the signal. If a night looks strange, check the fit before you check yourself.