Head-mounted display

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A head-mounted display (HMD).
A 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 each eye.

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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.

  • Stereoscopic imagery. A 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. 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.
  • 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.
  • 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.
  • HMD Manufacturers. Companies that have produced HMDs include 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.
  • 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.
  • 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. 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.

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

  • 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.

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