                                                                                            !"     I. INTRODUCTION idair displays which project floating images in free space have been seen in SF movies for several decades [1]. Recently, they are attracting a lot of attention as promising technologies in the field of digital signage and home TV. And then, novel technologies are developed to render images hovering in air without special glasses. FogScreen [2] and Heliodisplay [3] use a thin layer of fog as a projection screen. Holo [4] provides floating images from an LCD by utilizing a concave mirror. SeeReal Technologies is working on a realtime, computergenerated, and 3D holography [5] through the use of an eyetracking technology for reduction of calculation amount. With them, you can see a virtual object as if it is really hovering in front of you. Furthermore, by applying the visionbased and markerless hand tracking techniques demonstrated in Holovizio [6] or GrImage [7], you can handle the projected images with your hands as if it really exists. Then, tactile feedback will be the next demand. If tactile feedback is added, the usability of the interaction systems will be highly improved. There are three types of conventional strategies for tactile feedback in free space. The first is attaching tactile devices on user's fingers and/or palms. Employed devices are, for example, vibrotactile stimulators (CyberTouch [8]), motordriven belts (GhostGlove [9]), and pinarray units (SaLT [10]). In this strategy, the skin and the device are always in contact and that leads to undesired touch feelings. The second is controlling the position of tactile devices so that they contact with the skin only when tactile feedback is required. In the masterslave system shown in [11], the encountertype force feedback is realized by the exoskeleton master hand. The detailed tactile feedback for each finger is Takayuki Hoshi and Hiroyuki Shinoda are with the Department of Information Physics and Computing, the Graduate School of Information Science and Technology, the University of Tokyo, Tokyo, Japan (phone and fax: +81358416927; email: {star, shino}@alab.t. utokyo.ac.jp). Daisu Abe is with the Department of Mathematical Engineering and Information Physics, the Faculty of Engineering, the University of Tokyo, Tokyo, Japan. Fig. 1 Developed interaction system. An aerial imaging system, a Wiimotebased handtracking system, and a noncontact tactile display are combined. In this figure, the ultrasound is radiated from below. When the user hits the floating virtual ball, he feels an impact on his palm. provided by the electrotactile display attached on the finger part of the master hand. The drawback of this strategy is that it requires bulky robot arms. The last is providing tactile feedback from a distance without any direct contact. For example, airjets are utilized in [12] to realize noncontact force feedback. Although airjet is effective for rough “force” feedback, its spatial and temporal properties are quite limited and it cannot provide detailed “tactile” feedback. We have proposed a method for producing tactile sensation with airborne ultrasound [13]. The method renders desired pressure pattern in free space by using wave field synthesis with high spatial and temporal resolution. Users can feel the pressure with their bare hands. In [13], the prototype consisting of 91 ultrasound transducers was introduced and the feasibility of the proposed method was discussed. It can move a focal point along Z axis and the force generated at the focal point is 0.8 gf. In this paper, an interaction system is shown which tracks user's hand and provides tactile feedback when collision occurs between the hand and a virtual object (Fig. 1). After that, we try to increase the output force in order to represent the feeling of impact which is one of the primitive touch feelings. As a result, the force is sextuplicated (i.e. 4.8 gf) for a shorttime output. We also try to reduce the air flow generated around the focal point to make it clear. The methods and experiments are described.   #  $ Takayuki Hoshi, Daisu Abe, and Hiroyuki Shinoda M The 18th IEEE International Symposium on Robot and Human Interactive Communication Toyama, Japan, Sept. 27-Oct. 2, 2009 TuIAH.2 978-1-4244-5081-7/09/$26.00 ©2009 IEEE 7