Device 10 includes one or more light emitters 2, 3 (e.g. operating at monochromatic red and infrared wavelengths of 600nm/900nm) and a light detector 13 for detecting the light after it has passed through human subject tissue. The device and method involve the conversion of measured characteristics of the light, e.g. light intensity, into corresponding SpO2 oxygenation measurements. The conversion is based upon calibration data obtained using a different, reference device e.g. certified medical pulse-oximeter (100, figure 4). The calibration data may define a linear relationship and there may be a correction offset. The device may allow biometric data collection enabling medical diagnosis and/or during extended time windows, e.g. many hours or days. Also disclosed is a wearable pulse oximeter with a housing having a projection 25 from the case underside 11 defining a contact surface, with an elongated flexible element e.g. a bracelet worn around the wrist.
