Google's Project Glass is one of the more elegant-looking versions of the latter approach.
The glasses, as modeled by Google co-founder Sergey Brin, are clear, half-rim glasses with an L-shaped module attached to the right temple, stretching from the to the lens, curving around near the top of the lens so the projector is out of the user's direct eyeline but is close enough to project its own images on the lens.
Focusing on an image so close is a big problem, because of how close the HUD projector has to be to the eye and the virtual distance needed to make the HUD image big enough to read.
Human eyes aren't good at focusing on details in both extremely short and extremely long detail at the same time.
The Pentagon's Defense Advanced Research Projects Agency (DARPA) just signed a contract under which an optics company called Innovega in Bellvue, Wash. will develop a version of its dual-focus contact lens and accompanying HUD-display eyeglasses to DARPA's specs, according to an interview company CEO Steve Willey did with the BBC.
The contact lens allows human eyes to simultaneously focus on images that are ultra-close and those that are farther away by piping in the image from an accessory such as a HUD display and display it using the multiple-focus optic splitter designed into the contact lenses.
Making wearable HUD displays smaller, lighter, more energy efficient and easier to use is a huge priority for those within the military eager to put tactical information in the hands of troops who need it. Past efforts have provided useful information – according to the evaluations of the troops testing them in the field – but were too heavy, too awkward or displayed data too out of date to be useful to most ground troops.
The reliability and security of wireless networks connecting individual modules carried by one soldier and connecting troops to one another were bigger problems than the heads-up displays in the failure of the wearable hardware that was part of Future Combat System, which was killed in 2009.