Optical low angle pass filter for high resolution robust photoplethysmography monitor

We report an optical low angle pass filter (OLAPF) to enhance the signal-to-noise ratio (SNR) using the slanted mirror structure which angle selectively block the light in photoplethysmography (PPG). The reflectance type PPG signal is using the reflected light from the capillary as a signal distorted by the scattered light. The OLAPF can block the light, which scattered by the human tissue, losing their directionality, while transmit the light which has directionality. The device can be used for smart watch, smart phone, and NIRS to get the signal from the underneath of the biological tissue.