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According to an article in MIT’s Technology Review publication, the touchable hologram’s visual component is generated by projecting an image from an LCD projector onto a concave mirror. A white paper abstract from the Tokyo University team behind the project explains its tactile (“haptic”) component as follows: “The Airborne Ultrasound Tactile Display is designed to provide tactile feedback in 3D free space. The display radiates airborne ultrasound, and produces high-fidelity pressure fields onto the user’s hands, without the use of gloves or mechanical attachments. The method is based on a nonlinear phenomenon of ultrasound: acoustic radiation pressure. When an object interrupts the propagation of ultrasound, a pressure field is exerted on the surface of the object. This pressure is called acoustic radiation pressure…The acoustic radiation pressure is proportional to the energy density of the ultrasound.
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The spatial distribution of the energy density of the ultrasound can be controlled by using the wave field synthesis techniques. With an ultrasound transducer array, various patterns of pressure field are produced in 3D free space. Unlike air-jets, the spatial and temporal resolutions are quite fine. The spatial resolution is comparable to the wavelength of the ultrasound. The frequency characteristics are sufficiently fine up to 1 kHz. The airborne ultrasound can be applied directly onto the skin without the risk of penetration. When the airborne ultrasound is applied on the surface of the skin, due to the large difference between the characteristic acoustic impedance of the air and that of the skin, about 99.9% of the incident acoustic energy is reflected on the surface of the skin. Hence, this tactile feedback system does not require the users to wear any clumsy gloves or mechanical attachments.” The Airborne Ultrasound Tactile Display is guided by a “vision-based hand tracking system,” the team explains, adding that “the tactile display exerts the radiation pressure on the user’s hands when they ‘touch’ 3D virtual objects.” A demo video of the new technology and the touchable holograms it enables is embedded above. More information on the project is available at: http://www.alab.t.u-tokyo.ac.jp/~siggraph/08/Tactile/SIGGRAPH08_abst.pdf
