2. Theoretical Background

2.1. Sensors

• Empatica E4 (Real-time physiological signal sensoring)

• Polar H10 (Real-time heart rate signal sensoring)

• Azure Kinect Developer Kit (3D spatial information sensor device to create computer vision and speech model applications)

2.1.1. Specifications

Device	Sensors	Data Stream	Frequency (set to)	Output
Empatica E4	Photoplethysmography sensor (PPG)	Blood Volume Pressure (BVP)	64Hz	.csv
		Heart Rate (HR)	event-based (derived from BVP)	.csv
		Interbeat Interval (IBI)	event-based (derived from BVP)	.csv
	Electrodermal activity sensor (EDA)	Galvanic skin response (GSR)	4Hz	.csv
	3-axis accelerometer	XYZ-axes per timestep (ACC)	32Hz	.csv
	Optical thermometer (probably infrared)	Skin temperature in celsius per timestep (TEMP)	4Hz	.csv

[Empatica, 2024]

Device	Sensors	Data Stream	Frequency (set to)	Output
Polar H10	Electrocardiography sensor (ECG)	ECG data (ECG) electric potential in microvolts (μV)	130Hz	.csv
		Interbeat Interval (IBI)	event-based (derived from ECG)	.csv
		Heart Rate (HR)	event-based (derived from ECG)	.csv
	3-axis accelerometer (XYZ)	XYZ-axes per timestep (ACC)	200Hz	.csv

[Polar, 2024], [Määttä, 2024]

Device	Sensors	Data Stream	Frequency (set to)	Output
Azure Kinect DK	Depth camera: 1MP Time-of-flight	NFOV unbinned: 640x576 NFOV 2x2 binned: 320x288 WFOV 2x2 binned: 512x512 WFOV unbinned: 1024x1024 Passive IR: 1024x1024	0, 5, 15, or 30 FPS	-
	RGB camera: 12MP CMOS sensor rolling shutter	3840x2160 (UHD) 2560x1440 (QHD) 1920x1080 (FHD) 1280x720 (HD)	0, 5, 15, or 30 FPS	.mkv
	IMU (Inertial Measurement Unit): 3D accelerometer and gyroscope	XYZ-axes linear acceleration in (m/s²); XYZ-axes rotational speed (rad/s)	Sampling rate: 1.6 kHz Reporting rate to host: 208 Hz	-
	Microphone: 7-microphone circular array	Channel Count: 7 channels Sample Format: 16-bit integer (Int16)	Sample Rate: 16000 Hz (16 kHz)	.wav

[orbbec, 2024]

• Important is the distinction between PPG (optical) and ECG (electrodes) where ECG serves as the gold standard for accuracy.

2.2. Theoretical Background

ECG:

• Polar H10 potentially uses two electrodes that make direct contact with the skin.

• These electrodes are made of conductive materials potentially silver or silver chloride (Ag/AgCl) that can detect the small electrical signals generated by the heart.

This information can’t be proven because there is no in detail public information available about this sensor.

INFO
The electrodes in contact with the skin detect the small electrical signals generated by the heart's contractions, measuring the voltage difference between them to produce an electrocardiogram signal.

width=”500”> [Nagasato et al., 2017]

PPG:

• E4’s PPG sensor contains light-emitting diodes (LEDs) with a total 14 mm² sensitive area, that emit light into the skin.

INFO
As the heart pumps blood through the body, the volume of blood in the capillaries under the skin changes. This affects how much light is absorbed or reflected back to the photodiodes.

[Pribadi et al., 2021]