Head-mounted display

A binocular head-mounted display (HMD).

A head-mounted display or helmet-mounted display, both abbreviated 'HMD', 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).

Overview

A typical HMD has either one or two small displays with lenses and semi-transparent mirrors embedded in a helmet, eye-glasses or visor. The display units are miniaturised and may include CRT, LCDs, Liquid Crystal on Silicon (LCos), or OLED.

Performance Parameters

• Stereoscopic imagery. 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 heeded that so-called 'Optical Infinity' is generally taken by flight surgeons and display experts as about 9 metres. This is the distance at which, with the average human eye rangefinder "baseline" (distance between the eyes or Inter-Pupillary Distance (IPD)) of between 2.5 and 3 inches, 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.

• Field Of View (FOV) - Humans have around 180 degree FOV, but most HMDs offer considerably less than this. Typically, greater field of view results in 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 degrees) so often manufacturers will quote an "apparent screen size". For example, "Our new HMD presents an image that looks like a 72 inch screen at 10 feet." However, this tends to be quite misleading since people focus on the screen size rather than the distance and manufacturers will often increase the distance until the screen size sounds good. Most people sit about 2 feet away from their monitors and have quite a good feel about screen sizes at that distance. To convert the manufacturers "apparent screen size" to your desktop monitor position, just divide the screen size by the distance in feet, then multiply by 2. So 72 inches at 10 feet becomes 14.4 inches. A 14 inch monitor on your desk sounds a lot less impressive than "72 inches at 10 feet" which is why they do not explain it that way.

• Resolution - HMDs usually mention either the total number of pixels or the number of pixels per degree. Listing the total number of pixels (e.g. 1600x1200 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/degree (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/degree, though advances in micro-displays help increase this number.

• Binocular overlap - measures the area that is common to both eyes. Binocular overlap is the basis for the sense of depth and stereo, allowing humans to sense which objects are near and which objects are far. Humans have a binocular overlap of about 100 degrees (50 degrees to the left of the nose and 50 degrees to the right). The larger the binocular overlap offered by an HMD, the greater the sense of stereo. Overlap is sometimes specified in degrees (e.g. 74 degrees) or in percent indicating how much of the visual field of each eye is common to the other eye.

• Distant Focus ('Collimation'). Optical techniques may be used to present the images at a distant focus, which seems to improve the realism of images that in the real world would be at a distance.

• Inter-Pupillary Distance (IPD). This is the distance between the two eyes, measured at the pupils, and is important in designing Head-Mounted Displays.

Peripherals

• Head Tracking - 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. In radio-based systems (compared to wires), the wearer may move about within the tracking limits of the system.

• Eye Tracking - 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 into attention, etc.

HMD Manufacturers

Companies that have produced HMDs include VUZIX Corporation (formerly ICUITI),Fifth Dimension Technologies (5DT) in South Africa; Canon, Olympus and Sony in Japan; Cybermind and SEOS in the UK. In the USA, CAE Tampa, Kaiser Electro-Optics (KEO), the Link simulation division of L-3 Communications, Myvu (formerly MicroOptical), NVIS (formerly n-Vision), Sensics, Emagin (EMAN) and Virtual Research (VSRI), TDVision, and Headplay.Forte Technologies (U.S.A)well worthy of mention also.

Applications

• See-through imagery. Some HMDs have the facility to view a computer-generated image (CGI) superimposed upon a real-world view. This is sometimes referred to as augmented reality. This is done by projecting the CGI through a partially reflective mirror. The real world view is seen directly. Such systems have been used for gaming, where 'virtual' opponents may peek from real windows as a player moves about. This type of system has applications in the maintenance of complex systems, as it 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, as it allows the combination of radiographic data (CAT scans and MRI imaging) with the surgeon's natural view of the operation.

• Fast jet cockpits. Ruggedized HMDs are increasingly being integrated into the cockpits of modern fast jet fighter aircraft such as the French Rafale, the European Eurofighter Typhoon and particularly the American F-35. These are usually fully integrated with the pilot's flying helmet and may include protective visors, night vision devices and displays of other symbology.

• Other applications. Military, police and firefighters can use HMDs to display tactical information such as maps or thermal imaging data while viewing the real scene. Recent investigations into novel military applications have included the use of HMD for paratroopers . The i-Vue company make a Night Vision HMD unit with a night vision camera (image intensifier) that feeds imagery to the HMD optics. Engineers and scientists use HMDs to provide stereoscopic views of CAD schematics. Finally, low cost HMD devices are available for use with 3D games and entertainment applications.

Human Measurements

The 95 percentile adult male in the USA has an IPD of 70 mm (2.8 in) and the 5 percentile, 58 mm (2.3 in). For adult females in the USA the figures are 65 and 53 mm (2.6 and 2.1 in). For Europeans the figures are 1 mm smaller.

• Anthropometry. The IPD is one of the many measurements used in anthropometry, measurements of the human body. The statistical spread of these measurements, usually expressed as percentiles, is used for many purposes such as designing the size of airline seating and clothing sizes. In the case of IPD, it is used in specifying the size range not only for Head-Mounted Display systems but also for eyeglasses (spectacles), binoculars and other optics.

See also

• Augmented reality (AR)
• Computer-mediated reality
• Mixed Reality (MR)
• EyeTap
• Stereoscopy
• Virtual Reality (VR)
• Virtual retinal display
• VRML
• Head-up display (HUD)

Commercial Products

• http://www.i-glasses.com, a line of high resolution HMD's for both video and PC based applications
• Nintendo Virtual Boy, a video game platform with an HMD
• Headplay, a 800x600 resolution commercial HMD, capable of stereoscopy http://www.headplay.com
• Sensics a commercial HMD with panoramic field of view http://www.sensics.com
• Shimadzu Data Glass 3/A, a 800x600 resolution commercial HMD [1]
• TDVision 720p personal viewer http://www.tdvision.com
• Oculon OLED 800x600 resolution HMD http://www.oculon.com.tw/
• Glasstron The Sony Glasstron was a portable head-mounted display. http://en.wikipedia.org/wiki/Glasstron

External links

• Sortable comparison charts for a wide range of HMDs
• HMD/headset/VR-helmet Comparison Chart
• User survey of HMD requirements
• Open Directory Project Listings for Wearable Displays
• Resolution analysis for HMD helmets / Comparison Chart
• OpenInterface Platform, OpenInterface Multimodal Interaction Designer Framework

HMD manufacturer pages

• Trivisio
  • Video-see through and Opaque HMDs for Virtual Reality and Augmented Reality applications
• i-glasses: Hi-Tech LCD Display Glasses for Video and PC
• i-Vue
• 720p TDVisor
• Sensics: Professional HMD with panoramic field of view and high resolution

Other HMD-related pages

• Cakmakci and Rolland. Head-Worn Displays: A Review. Journal of Display Technology. Vol. 2, Issue 3, 2006.
• A recent Head-Worn Display Prototype with a 20 degree field of view, 8mm exit pupil, 15mm eye clearance, 1.5 arcminute resolution (based on a 640x480 VGA panel), operating at 490-635 nanometer range, using only two plastic elements
• Forte VFX-1 at the Museum of Interesting Tech
• News item about recent, very portable "video glasses" (zdnet.com, May 5, 2006)
• Helmet Mounted Sights and Displays -Air Power International