An attempt to evaluate finger load and surface texture by biometric finger sensor using ultrasonic method

Abstract

The potential of an ultrasonic biometric finger sensor to evaluate the finger load and surface texture in contact opposition was investigated under the range of quasi-static load and low sliding speed. The ultrasonic probe, which has a respective vibration element for reception and transmission, attached to the nail plate has a transducer 5 mm in diameter with 5 MHz center frequency. The reflection echo ratio H₁ from the nail bed is reduced with increment of nail bed stiffness (acoustic impedance) due to increasing finger load W. In contrast, the reflection echo ratio H₂ from the finger-pad is increased with increment of finger load since the amplitude of interference wave reflected from the finger-pad is increased due to the phase decrement of the reflected wave from there by flattening of the finger pad. Therefore, the potential for the estimation of finger load W using the echo ratio H₁ or H₂ was cleared. Additionally, it becomes possible to evaluate the surface texture in contact opposition since the finger-pad echo ratio H₂ is strongly affected by local displacement on finger-pad. Since the nail bed echo ratio H₁ during catalyzed migration of the finger depends on only finger load without being influenced by the texture of the opposition surface, the potential for the substitution of tactile impression of the person by the simultaneous observation of H₁ and H₂ became clear.