Table 1. Specifications of PPG Sensors.

		Apple S5	Fitbit Versa 2	Polar M600
Green LED	Number	4	1	6
	Wavelength	523 $\pm$ 16 nm	531 $\pm 15$ nm	520 $\pm 15$ nm
	Dimension	0.5 × 0.5 mm2	0.4 × 0.6 mm2	1.25 × 1.25 mm2
	Effective radius	0.28 mm	0.27 mm	0.7 mm
IR LED	Number	2	1	-
	wavelength	945 $\pm$ 25 nm	949 $\pm 14$ nm	-
	Dimension	0.3 × 0.3 mm2	0.3 × 0.3 mm2	-
	Effective radius	0.17 mm	0.17 mm	-
Photodetector	Number	8	3	1
	Dimension	2.5 × 1.5 mm2	3 × 1.5 mm2	2.75 × 2.75 mm2
	Effective radius	1.09 mm	1.19 mm	1.55 mm
Green LED S-D separations	Closest S-D pair	3.3 mm	2.4 mm	5.0 mm
	farthest S-D pair	3.8 mm	5.5 mm	-
IR LED S-D separations	Closest S-D pair	3.0 mm	2.4 mm	-
	farthest S-D pair	5.0 mm	5.5 mm	-
	Symmetric S-D pair	4.0 mm	-	-