Effects of Wavelength and Optical Path Length in Design of Fingernail Touch Force Sensing

This paper experimentally identifies the optimal optical parameters for the transmittance of light through the human fingernail bed to measure the change in blood volume beneath the fingernail, which is directly related to touch forces exerted on the fingertip. When forces are applied to the fingertip, various red and white coloration regions occur along the fingernail area, hence the transmittance of light through the fingernail bed is a function of applied force. The fingernail sensor uses optical reflectance photoplethysmography to measure the change in blood perfusion in the fingernail bed where an array of LEDs and photodetectors is mounted on the fingernail surface, and the fingertip force is estimated on the basis of the photodetector outputs. In this paper, we conduct thorough experimental characterization of the sensitivity and efficiency of light transmittance through the fingernail bed to wavelength and optical path length. Results show that the optical parameters for the transmittance of light through the human fingernail bed are optimized when using green light (525 nm) and when the surface mount LED and photodiode are placed as close together as possible.