Philips DL8790/10 - Manual (Page 9)

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Operating principle
The Philips health watch is a wrist-worn device for 24/7 home use (indoor and outdoor).
The device uses a tri-axial accelerometer to measure motion and green Light-Emitting
Diodes (LEDs) and a photodiode to create a photoplethysmogram (PPG).
The accelerometer output and the PPG-signal are analyzed to determine heartbeat,
activity type, steps taken, and are used as input for estimation of energy expenditure
and other parameters.
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