A pulse oximetry device that is mounted on a wrist strap and fixates an area above a distal end of the ulna with a dome shaped structure. This area is used as measuring area. The measurement is carried out by a detector positioned above the fixated area, that detects light emitted by light sources having different wave lengths that are located at a periphery of the fixated area. Hence, the reflections are measured at neither a reflection mode nor a transmission mode, but are at an angle between 20° and 160° from the emitted light. This mode, termed trans-illumination, allows achieving an excellent signal to noise ratio that for the first time enables continuous and reliable measurement of oximetry data on the wrist.