There are displays which project flat 2D images onto a thin layer of water vapor. There are also swept-screen volumetric displays, where images are projected upon a physically rotating screen that sweeps through a volume. But then the entire device needs to be enclosed in a transparent cover, for safety. Some volumetric display devices focus two scanning laser beams into a cubic volume filled with a special gas. Where the two beams meet, the gas glows.
But we want a display like the one we all saw in "Forbidden Planet" and "Star Wars": full 3D figures imaged directly into the air, with no need for a projection screen.
We're going to build one, by scanning a laser through the air and lighting up dust particles. It turns out there are plenty of dust particles in the air.
As the laser illuminates more dust particles in the air, the resolution of the object improves. Below are three examples of the same virtual "teddy bear," with approximately 1070, 3400 and 10600 particles illuminated, respectively.
The dust particles we want are relatively large, on the order of 0.5-1.0 millimeter in length (like stray lint from clothing), so they pose no health hazard. The cloud of particles can be relatively sparse, so that the individual particles are not visible to the unaided eye under normal lighting conditions.
So how do we find the dust particles? The trick is to use two lasers, one visible, and one infrared. The two lasers are lined up (so their beams coincide). We keep the infrared laser always lit, but we switch the visible laser rapidly on and off.
The beam is scanned through the display volume, around 50 times per second (we use a set of rotating or vibrating mirrors to deflect the beam through a 2D scanning pattern).
An optical sensor watches out for infrared dust glints. Whenever the infrared beam hits a dust particle, the dust particle will scatter some of that infrared light into the detector. If we place the detector off-axis of the beam, then we can triangulate to figure out how far along the beam the dust particle was when it was hit.
Then we look that point up in a virtual 3D model (like in the teddy bear example above). If the dust particle is "inside" a virtual object, then we flash the visible laser, visibly lighting up the dust particle. What observers will see is a sparkling object floating in the air.
Won't that be cool?