In contrast to handheld mobiles, AR devices worn like eyeglasses or futuristic visors, such as Microsoft HoloLens and Metavision Meta, may be referred to as optical see-through eyewear augmented reality devices, or simply, smartglasses. As illustrated in the following image, they do not use video to capture and render the real world. Instead, you look directly through the visor and the computer graphics are optically merged with the scene:
The display technologies used to implement optical see-through AR vary from vendor to vendor, but the principles are similar. The glass that you look through while wearing the device is not a basic lens material that might be prescribed by your optometrist. It uses a combiner lens much like a beam splitter, with an angled surface that redirects a projected image coming from the side toward your eye.
An optical see-through display will mix the light from the real world with the virtual objects. Thus, brighter graphics are more visible and effective; darker areas may get lost. Black pixels are transparent. For similar reasons, these devices do not work great in brightly lit environments. You don't need a very dark room but dim lighting is more effective.
We can refer to these displays as binocular. You look through the visor with both eyes. Like VR headsets, there will be two separate views generated, one for each eye to account for parallax and enhance the perception of 3D. In real life, each eye sees a slightly different view in front, offset by the inter-pupillary distance between your eyes. The augmented computer graphics must also be drawn separately for each eye with similar offset viewpoints.
One such device is Microsoft HoloLens, a standalone mobile unit; Metavision Meta 2 can be tethered to a PC using its processing resources. Wearable AR headsets are packed with hardware, yet they must be in a form factor that is lightweight and ergonomic so they can be comfortably worn as you move around. The headsets typically include the following:
- Lens optics, with a specific field of view
- Forward-facing camera
- Depth sensors for positional tracking and hand recognition
- Accelerometer and gyroscope for linear and rotational motion detection and near-ear audio speakers
- Microphone
Furthermore, as a standalone device, you could say that HoloLens is like wearing a laptop wrapped around your head--hopefully, not for the weight but the processing capacity! It runs Windows 10 and must handle all the spatial and graphics processing itself. To assist, Microsoft developed a custom chip called holographic processing unit (HPU) to complement the CPU and GPU.
Instead of headphones, wearable AR headsets often include near-ear speakers that don't block out environmental sounds. While handheld AR could also emit audio, it would come from the phone's speaker or the headphones you may have inserted into your ears. In either case, the audio would not be registered with the graphics. With wearable near-eye visual augmentation, it's safe to assume that your ears are close to your eyes. This enables the use of spatial audio for more convincing and immersive AR experiences.