Effects of fingertip orientation and flash location in smartphone Photoplethysmography

Smartphone based reflective Photoplethysmography (PPG) measures reflected light from blood capillaries, typically from the fingertip of a user. It has gained popularity as a means of unobtrusive affordable physiological sensing. The orientation and relative distance between smartphone flash and camera; fingertip placement direction etc. highly influences the captured PPG signal quality, and hence affects the estimation quality of physiological parameters. In this paper, the frames captured by smartphone camera are divided into smaller blocks and all the blocks are compared for heart rate estimation. Results indicate that the best fingertip video is captured when the finger is pointing towards the flash. It is also demonstrated that the blocks having similar distance to the flash yield similar quality PPG. The observation is validated against two popular off-the-shelf smartphones, having different flash position with respect to the camera.