Robotics boffins in the States say they have developed a brilliant three-dimensional holographic projection system - ideal, for instance, for playing 3D Tetris - which is based on falling droplets of water.

Here's a vid of a prototype system:

It seems that the boffins were actually trying to develop a headlight system for cars which could penetrate rain better, by using cunning technology to track individual raindrops and shine light between them rather than having it reflect back from them.

This turned out to be too difficult.

"What we realized is that it was much easier to shine light on the drops themselves," says Srinivasa Narasimhan, Carnegie Mellon robotics prof.

It occurred to to Narasimhan and his colleagues that if they could control the distribution of droplets, they could cause light to be emitted from a chosen point within a given volume. As droplets fell through the chosen point they would be lit up, forming a "volumetric pixel" or "voxel".

"The beauty of water drops is that they refract most incident light, so they serve as excellent wide-angle lenses that can be among the brightest elements of an environment," says Narasimhan. "By carefully generating several layers of drops so that no two drops occupy the same line-of-sight from the projector, we can use each drop as a voxel that can be illuminated to create a 3-D image."

The frustrated headlight engineers turned instead to development of a cunning array of water-drippers able to release droplets in staggered rows at the top of a 3D display, such that selected falling drops at any point in the tank could be lit up without being obscured by a drop in front of them. The drops are tracked by a camera and lit by an ordinary projector.

It seems that a rate of ten drops per second is enough to fool the human eye into perceiving a continuously illuminated voxel, but the team's drippers can actually deliver up to 60 for improved brightness.

The kit now has a trade name, Aqualux 3D, and the designers hope it will move on to achieve greater resolution and take the world by storm.

"People can touch the water drops and alter the appearance of images, which could lead to interactive experiences we can't begin to predict," says Narasimhan.

"We look forward to the day when creative people can fully explore the potential of this display."

There's a longer vid with proper technical details here. ®