Photoplethysmographic (PPG) sensors are suitable for wearable devices, and they can provide a wide range of information such as stress level (calculated from the heart rate interval), respiration rate, heart rate, and blood vessel stiffness. Of particular importance is that reflective PPG sensors can be easily attached anywhere on the body with low wearer constraint. However, PPG sensors are susceptible to body motion artifacts. The output signal of PPG sensors is composed of alternating current (AC), originating from the heart cycle, and direct current (DC), originating from veins and stationary tissue. Motion artifacts affect DC signals, making it difficult to detect AC signals. Thus, it is important to reduce DC signals and increase the AC/DC ratio. In this study, we investigated the effect of a light-blocking structure on the AC/DC ratio. In addition, the AC/DC ratio was estimated when the gap between the light source (LED) and the photodetector was small (3.2 mm) and large (8.0 mm). In this experiment, the measurement part was a fingertip, and the AC/DC ratio was estimated when AC had the highest output with the force from step-by-step contact. As a result, the AC/DC ratio of the light-blocking structure was 2.4%, and the AC/DC of the non-light-blocking type was 0.9%. Also, the AC/DC of the small-gap PPG sensor was 2.4%, and the AC/DC of the large-gap sensor was 7.5%. Thus, the light-blocking structure was effective in increasing the AC/DC ratio, and a larger distance between the LED and photodetector was useful.
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