A proposed highly-sensitive finger-photoplethysmograph using a sphere-shaped diffuse reflector for non-invasive measurement of blood constituents

We aim for accurate non-invasive optical measurement of blood constituents, especially alcohol (BAL) and glucose (BGL), by means of a newly designed integrating sphere finger photoplethysmographic sensor (isFPPG). The sensitivity of the new device was compared with that of a conventional transmittance-type sensor (tmFPPG). We used near-infrared light at 1160-nm, a window in the water absorption spectrum and strong BAL absorption peak, and 1600-nm, a strong absorption peak of BGL. In 8 volunteers, simultaneous measurements of both isFPPG and tmFPPG from the index fingers with both wavelengths were made using a laser-based experimental setup. The difference of signal-to-noise ratios (SNRs) between isFPPG and tmFPPG were evaluated. We found statistically significant higher SNRs with the isFPPG, by factors of 1.58 and 4.80 for the FPPG measurements at 1160-nm and 1600-nm, respectively. Thus, the proposed method at these wavelengths could be particularly useful to reduce influences of the strong water absorption bands.