A wearable computing device includes a display device for displaying augmented reality (AR) images and a biometric sensor for obtaining a heartrate of a user wearing the wearable computing device. The biometric sensor includes a photosensor that emits a light into the surface of a body part of the user. Using photoplethysmography, the photosensor measures voltages from the light reflected from or transmitted through the users body part at a sampling rate of at least 100 Hz. The measured voltages are then filtered and normalized. Slopes for the resulting set of voltages are then determined on a sliding window basis of approximately 90 milliseconds (ms). Interpulse intervals are then determined for consecutive local minima within each set of sliding windows. The biometric sensor then computes a real-time heartrate for the user from the interpulse intervals, which may then be displayed on the display device of the wearable computing device.
