When's a screen not a screen? When, err, it's made of thin air we suppose — like this touchscreen, which is constructed from light and ultrasound vibrations alone.
Called HaptoMime, the screen uses uses reflective surfaces to create a holographic display. Then, the presence of someone's hand at the (non-existent) surface is detected using an infrared sensor, which causes ultrasound transducers to beam out high-frequency sound towards your fingertips. The sound causes the sensation of pressure — effectively making it feel like you're touching a real, solid surface.
The holographic images are high enough resolution to read 6 point Times New Roman font, and the video shows that it easily allows someone to play a toy keyboard. The applications are endless, but it seems like a particularly sensible solution for situations where the lack of physical touch would be beneficial — think kitchen, operating theatres ant the like. [New Scientist]