# Optical head-mounted display

An optical head-mounted display (OHMD) is a wearable device that has the capability of reflecting projected images as well as allowing the user to see through it. In some cases, this may qualify as augmented reality (AR) technology. OHMD technology has existed since 1997 in various forms, but despite a number of attempts from industry, has yet to have had major commercial success.

## Types

Various techniques have existed for see-through HMDs. Most of these techniques can be summarized into two main families: "Curved Mirror" (or Curved Combiner) based and "Waveguide" or "Light-guide" based. The curved mirror technique has been used by Vuzix in their Star 1200 product, by Olympus, and by Laster Technologies. Various waveguide techniques have existed for some time. These techniques include diffraction optics, holographic optics, polarized optics, and reflective optics:
* Diffractive waveguide – slanted diffraction grating elements (nanometric 10E-9). Nokia technique now licensed to Vuzix.
* Holographic waveguide – 3 holographic optical elements (HOE) sandwiched together (RGB). Used by Sony and Konica Minolta.
* Polarized waveguide – 6 multilayer coated (25–35) polarized reflectors in glass sandwich. Developed by Lumus.
* Reflective waveguide – A thick light guide with single semi-reflective mirror is used by Epson in their Moverio product. A curved light guide with partial-reflective segmented mirror array to out-couple the light is used by tooz technologies GmbH.
* "Clear-Vu" reflective waveguide – thin monolithic molded plastic w/ surface reflectors and conventional coatings developed by Optinvent and used in their ORA product.
* Switchable waveguide – developed by SBG Labs.

## Input devices

Head-mounted displays are not designed to be workstations, and traditional input devices such as keyboards do not support the concept of smart glasses. Input devices that lend themselves to mobility and/or hands-free use are good candidates, for example:
* Touchpad or buttons
* Compatible devices (e.g. smartphones or control unit)
* Speech recognition
* Gesture recognition
* Eye tracking
* Brain–computer interface

## Recent developments
### 2012

* On 17 April 2012, Oakley's CEO Colin Baden stated that the company has been working on a way to project information directly onto lenses since 1997, and has 600 patents related to the technology, many of which apply to optical specifications.
* On 18 June 2012, Canon announced the MR (Mixed Reality) System which simultaneously merges virtual objects with the real world at full scale and in 3D. Unlike the Google Glass, the MR System is aimed for professional use with a price tag for the headset and accompanying system is $125,000, with $25,000 in expected annual maintenance.

### 2013

* At MWC 2013, the Japanese company Brilliant Service introduced the Viking OS, an operating system for HMD's which was written in Objective-C and relies on gesture control as a primary form of input. It includes a facial recognition system and was demonstrated on a revamp version of Vuzix STAR 1200XL glasses ($4,999) which combined a generic RGB camera and a PMD CamBoard nano depth camera.
* At Maker Faire 2013, the startup company Technical Illusions unveiled castAR augmented reality glasses which are well equipped for an AR experience: infrared LEDs on the surface detect the motion of an interactive infrared wand, and a set of coils at its base are used to detect RFID chip loaded objects placed on top of it; it uses dual projectors at a framerate of 120 Hz and a retroreflective screen providing a 3D image that can be seen from all directions by the user; a camera sitting on top of the prototype glasses is incorporated for position detection, thus the virtual image changes accordingly as a user walks around the CastAR surface.

### 2016

* The Latvian-based company NeckTec announced the smart necklace form-factor, transferring the processor and batteries into the necklace, thus making facial frame lightweight and more visually pleasing.

### 2018

* Intel announces Vaunt, a set of smart glasses that are designed to appear like conventional glasses and are display-only, using retinal projection. The project was later shut down.
* Zeiss and Deutsche Telekom partners up to form tooz technologies GmbH to develop optical elements for smart glass displays.

## Market structure

Analytics company IHS has estimated that the shipments of smart glasses may rise from just 50,000 units in 2012 to as high as 6.6 million units in 2016. According to a survey of more than 4,600 U.S. adults conducted by Forrester Research, around 12 percent of respondents are willing to wear Google Glass or other similar device if it offers a service that piques their interest. Business Insider's BI Intelligence expects an annual sales of 21 million Google Glass units by 2018.
According to reliable reports, Samsung and Microsoft are expected to develop their own version of Google Glass within six months with a price range of $200 to $500. Samsung has reportedly bought lenses from Lumus, a company based in Israel. Another source says Microsoft is negotiating with Vuzix.
In 2006, Apple filed patent for its own HMD device.
In July 2013, APX Labs founder and CEO Brian Ballard stated that he knows of 25-30 hardware companies who are working on their own versions of smart glasses, some of which APX is working with.

## Comparison of various OHMDs technologies

| Combiner technology | Size | Eye box | FOV | Limits / Requirements | Example |
| Flat combiner 45 degrees | Thick | Medium | Medium | Traditional design | Vuzix, Google Glass |
| Curved combiner | Thick | Large | Large | Classical bug-eye design | Many products (see through and occlusion) |
| Phase conjugate material | Thick | Medium | Medium | Very bulky | OdaLab |
| Buried Fresnel combiner | Thin | Large | Medium | Parasitic diffraction effects | The Technology Partnership (TTP) |
| Cascaded prism/mirror combiner | Variable | Medium to Large | Medium | Louver effects | Lumus, Optinvent |
| Free form TIR combiner | Medium | Large | Medium | Bulky glass combiner | Canon, Verizon & Kopin (see through and occlusion) |
| Diffractive combiner with EPE | Very thin | Very large | Medium | Haze effects, parasitic effects, difficult to replicate | Nokia / Vuzix |
| Holographic waveguide combiner | Very thin | Medium to Large in H | Medium | Requires volume holographic materials | Sony |
| Holographic light guide combiner | Medium | Small in V | Medium | Requires volume holographic materials | Konica Minolta |
| Combo diffuser/contact lens | Thin (glasses) | Very large | Very large | Requires contact lens + glasses | Innovega & EPFL |
| Tapered opaque light guide | Medium | Small | Small | Image can be relocated | Olympus |