Real-time heart activity monitoring with optical illusion using a smartphone

In this study, we obtained a pulsatile photoplethysmogram (PPG) signal from a fingertip using the built-in camera of an iPhone 6 s and displayed a real-time heart activity images with holographic projection on a smartphone screen. Our proposed heart activity hologram is simple and can be easily realized with only a smartphone and overhead projector (OHP) film. A square pyramid-shaped OHP film was positioned on a smartphone screen. The actual cardiac cycle on the smartphone screen was projected onto the film while measuring the pulsatile signal from a fingertip placed on the smartphone’s camera. The heart’s beat-to-beat time interval was then recorded. This approach enables observation of one’s own virtual heart activity in real time, rather than seeing a pulsatile signal graphically on a smartphone. To investigate the feasibility of this heart activity monitoring based on holographic projection, we tested it under different conditions in terms of environmental light, smartphone screen light intensity, and film color, and quantified the contrast-to-noise ratio for comparison.