Performance Evaluation of Airborne Ultrasound Focus Measurement Using Thermal Imaging on the Surface of a Finger

Mid-air haptics generated by ultrasound requires measurement and correction of the sound field on the surface of the user’s finger for optimal tactile presentation. Thermal images enable the rapid observation of sound pressure patterns without interfering with the acoustic field. In this study, we discuss the performance of ultrasound focus measurement using thermal images. Using a dummy finger with an embedded microphone, we confirmed that the temperature and sound pressure patterns of the ultrasound focus matched. Furthermore, we confirmed that the temperature change on the human finger surface determines the focal point position in 0.2 s with an accuracy of less than 1 mm. The results suggest that the temperature pattern directly reflects the position and shape of the focus and enables rapid and accurate calibration with a fixed finger by repeating the focus measurement and positional correction. In addition to increasing the reproducibility of tactile presentation, thermal measurement enhances the tactile experience by allowing the presentation of precisely controlled stimulation.