Head-mounted Displays

Of course, making the observer stand behind a stationary combiner is not very pragmatic -- no augmented-reality system would be of much practical use if the user had to stand in a fixed location. That's why most systems require that the user carry the computer on his or her person, either in a backpack or clipped on the hip. It's also why most systems take advantage of head-mounted displays, or HMDs, which assemble the combiner and optics in a wearable device.

There are two types of HMDs: optical see-through displays and video see-through displays. Optical see-through displays look like high-tech goggles, sort of like the goggles Cyclops wears in the X-Men comic books and movies. These goggles provide a display and optics for each eye, so the user sees the augmented reality in stereo. Video see-through displays, on the other hand, use video-mixing technology to combine the image from a head-worn camera with computer-generated graphics.

Photo courtesy Columbia University Computer Graphics and User Interfaces Lab Video see-through display

In this arrangement, video of the real world is mixed with synthesized graphics and then presented on a liquid-crystal display. The great advantage of video see-through displays is that virtual objects can fully obscure real-world objects and vice versa.

The scientists who have developed optical-camouflage technology are currently perfecting a variation of a video see-through display that brings together all of the components necessary to make the invisibility cloak work.

Photo courtesy ©Tachi Laboratory, the University of Tokyo Prototype head-mounted projector

They call their apparatus a head-mounted projector (HMP) because the projection unit is an integral part of the helmet. Two projectors -- one for each eye -- are required to produce a stereoscopic effect.