A head-mounted display is a display device worn on the head or as part of a helmet, that has a small display optic in front of one (monocular HMD) or each eye (binocular HMD). A typical HMD has either one or two small displays with lenses and semi-transparent mirrors embedded in a helmet, eye-glasses (also known as data glasses), or visor. The display units are miniaturised and may include CRT, LCDs, Liquid crystal on silicon (LCos), or OLED. Some vendors employ multiple micro-displays to increase total resolution and field of view.
HMDs differ in whether they can display just a computer generated image (CGI), show live images from the real world, or a combination of both. Some HMDs allow a CGI to be superimposed on a real-world view. This is referred to as augmented reality or mixed reality. Combining real-world view with CGI can be done by projecting the CGI through a partially reflective mirror and viewing the real world directly. This method is often called ‘Optical See-Through.’ Combining real-world view with CGI can also be done electronically by accepting video from a camera and mixing it electronically with CGI. This method is often called ‘Video See-Through.’
Ruggedized HMDs are increasingly being integrated into the cockpits of modern helicopters and fighter aircraft. These are usually fully integrated with the pilot’s flying helmet and may include protective visors, night vision devices, and displays of other symbology. Military, police and firefighters use HMDs to display tactical information such as maps or thermal imaging data while viewing the real scene. In 2005, the Liteye HMD was introduced for ground combat troops as a rugged, waterproof lightweight display that clips into a standard military helmet mount. The self-contained color monocular OLED display replaces the NVG (night vision device) and connects to a mobile computing device. It has see-through capability and can be used as a standard HMD or for augmented reality applications. The design is optimized to provide high definition data under all lighting conditions, in covered or see-through modes of operation. The unit has a low power consumption, operating on four AA batteries for 35 hours or receiving power via standard USB connection.
Engineers and scientists use HMDs to provide stereoscopic views of CAD schematics. These systems are also used in the maintenance of complex systems, as they can give a technician what is effectively ‘x-ray vision’ by combining computer graphics such as system diagrams and imagery with the technician’s natural vision. There are also applications in surgery, wherein a combination of radiographic data (CAT scans and MRI imaging) is combined with the surgeon’s natural view of the operation, and anesthesia, where the patient vital signs are within the anesthesiologist’s field of view at all times. Research universities often use HMDs to conduct studies related to vision, balance, cognition, and neuroscience.
Low cost HMD devices are available for use with 3D games and entertainment applications. One of the first commercially available HMDs was the Forte VFX-1 which was announced in 1994; it had stereoscopic displays, 3-axis head-tracking, and stereo headphones. Another pioneer in this field was Sony who released the Glasstron in 1997, which had as an optional accessory a positional sensor which permitted the user to view the surroundings, with the perspective moving as the head moved, providing a deep sense of immersion. One novel application of this technology was in the game MechWarrior 2, which permitted users of the Sony Glasstron or Virtual I/O’s iGlasses to adopt a new visual perspective from inside the cockpit of the craft, using their own eyes as visual and seeing the battlefield through their craft’s own cockpit.
A HMD system has been developed for Formula One drivers by Kopin Corp. and the BMW Group. According to BMW, ‘The HMD is part of an advanced telemetry system approved for installation by the Formula One racing committee… to communicate to the driver wirelessly from the heart of the race pit.’ The HMD will display critical race data while allowing the driver to continue focussing on the track. Pit crews control the data and messages sent to their drivers through two-way radio. Recon Instruments released in 2011 two head mounted displays for ski goggles, MOD and MOD Live, the later based on an Android operating system.
Another key application for HMDs is training and simulation, allowing to virtually place a trainee in a situation that is either too expensive or too dangerous to replicate in a real-life. Training with HMDs cover a wide range of applications from driving, welding and spray painting, flight and vehicle simulators, dismounted soldier training, medical procedure training, and more.
A binocular HMD has the potential to display a different image to each eye. This can be used to show stereoscopic images. It should be borne in mind that so-called ‘Optical Infinity’ is generally taken by flight surgeons and display experts as about 9 meters. This is the distance at which, given the average human eye rangefinder ‘baseline’ (distance between the eyes or Inter-Pupillary Distance) of between 2.5 and 3 inches (6 and 8 cm), the angle of an object at that distance becomes essentially the same from each eye. At smaller ranges the perspective from each eye is significantly different and the expense of generating two different visual channels through the Computer-Generated Imagery (CGI) system becomes worthwhile.
Humans have a field-of-view (FOV) of around 180°, but most HMDs offer considerably less than this. Typically, a greater field of view results in a greater sense of immersion and better situational awareness. Most people do not have a good feel for what a particular quoted FOV would look like (e.g. 25°) so often manufacturers will quote an apparent screen size. Most people sit about 60 cm away from their monitors and have quite a good feel about screen sizes at that distance. Consumer-level HMDs typically offer a FOV of about 30-40° whereas professional HMDs offer a field of view of 60° to 150°. HMDs usually mention either the total number of pixels or the number of pixels per degree. Listing the total number of pixels (e.g. 1600×1200 pixels per eye) is borrowed from how the specifications of computer monitors are presented. However, the pixel density, usually specified in pixels per degree or in arcminutes per pixel, is also used to determine visual acuity. 60 pixels/° (1 arcmin/pixel) is usually referred to as eye limiting resolution, above which increased resolution is not noticed by people with normal vision. HMDs typically offer 10 to 20 pixels/°, tho
The most rudimentary HMDs simply project an image or symbology on a wearer’s visor or reticle. The image is not slaved to the real world (i.e., the image does not change based on the wearer’s head position). More sophisticated HMDs incorporate a positioning system that tracks the wearer’s head position and angle, so that the picture or symbology displayed is congruent with the outside world using see-through imagery. Slaving the imagery. Head-mounted displays may also be used with tracking sensors that allow changes of angle and orientation to be recorded. When such data is available in the system computer, it can be used to generate the appropriate computer-generated imagery (CGI) for the angle-of-look at the particular time. This allows the user to ‘look around’ a virtual reality environment simply by moving the head without the need for a separate controller to change the angle of the imagery. Eye trackers measure the point of gaze, allowing a computer to sense where the user is looking. This information is useful in a variety of contexts such as user interface navigation: by sensing the user’s gaze, a computer can change the information displayed on a screen, bring additional details to attention, etc.
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