Measurement of Airborne Ultrasound Focus on Skin Surface Using Thermal Imaging.

In recent years, tactile presentation technology using airborne ultrasound has attracted attention. To achieve an ideal tactile presentation using ultrasound, the acoustic field on the user's skin surface must be determined, particularly the location of the focal point. Previous studies have suggested that thermal images can be used to immediately visualize sound pressure patterns on finger surfaces. In this study, we comprehensively investigated the performance of thermal imaging for measuring the ultrasound focus on the skin. First, we confirmed that the sound pressure peak at the focus and the temperature change peak were matched using silicone that mimicked the skin. In addition, we confirmed that when human skin was irradiated, a temperature increase was observed at above 4.0 kPa in 9 out of 10 participants. Moreover, a 5.5 kPa focus could be employed to track the focal position if the moving velocity was less than 100 mm/s and to detect the orbit if the velocity was less than 2000 mm/s. These results clarify the situation in which the focus can be measured by using thermal images and provide guidelines for practical use.