The PPG (Photoplethysmography) data is provided by the E4 device. This is also known in the scientific literature as BVP (Blood Volume Pulse).
We suggest you to read the following articles before starting to analyze the BVP data:
- Pulse oximetry KK Tremper - CHEST Journal, 1989
- Sinex, James E. "Pulse oximetry: principles and limitations." The American journal of emergency medicine 17.1 (1999): 59-66.
The PPG data is also extremely related to the IBI (interbeat interval) data. If you want to read more of how they relate to each other read our What should I know to use the PPG/IBI data in my experiment? article. The PPG is the input signal to the algorithm that detects the heart beats and that provides the IBI signal as output.
The processing of the PPG in the E4 wristband happens as you can see in (eq 1)
(eq 1.) GREEN, RED -->[ Algorithm 1] --> PPG --> [Algorithm 2] --> IBI
The E4 wristband implements only a part of working principle of classic pulse oximetry and it relies in the reflective mode principles:
- E4 wristband uses green and red light instead of classic Red/IR coupling
- The way the green and red light are combined is optimized for maximizing the detection of the pulse wave, i.e. the estimation of the heart rate, instead of the oxygen saturation level (which is not part of the Empatica outputs).
The first part of the processing performed by the E4 is represented in (Fig 1)
The lights produced by the Green and Red LEDs are oriented toward the skin and are absorbed by the blood in different ways. Portion of the light is then reflected back and is then measured by the Light receiver.
- The measured light during green exposure contains most of the information on the pulse wave (i.e., the heart beats) and it is typically characterized by a sequences of valleys, whose time occurencies are used to estimete the heart beats. Note that the more the blood is oxygenated, the more the light is absorbed. Thus, during a heart beat, there is a high light absorption, which is observed as a valley in the light output signal.
- The measured light during the red exposure contains a reference light level which is used to cancel motion artefacts.