All sorts of 3D-imaging technologies tend to get lumped under the label "hologram." But there's actually a variety of distinct technologies that can create the appearance of depth. Now, we can add another to the list: the photophoretic-trap volumetric display. The device uses one set of optical hardware to control the motion of a tiny sphere and a second set to illuminate the sphere as it travels. Provided the sphere can be kept moving fast enough, the result is a true-color image that has real depth since it's built from light reflected from different locations.
The downside is that a single sphere can't cover all that much ground in the amount of time our brain needs to construct an image. As a result, photophoretic-trap volumetric display is currently limited to either small images or showing only part of an image at a time.
The recent work, from a team at Brigham Young University, is a variation on volumetric displays. These involve projecting a changing image onto a moving reflective surface. If the change in the image is properly matched to the changing location of where it's projected, the result will be the appearance of depth, since the light you see will actually be reflected at different locations. On the plus side, this doesn't require the viewer to wear any hardware, and multiple people can view the image at the same time, each seeing it from the appropriate perspective.