Head-mounted Display - Performance Parameters

Performance Parameters

• Ability to show 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 borne in mind that so-called 'Optical Infinity' is generally taken by flight surgeons and display experts as about 9 metres. This is the distance at which, given the average human eye rangefinder "baseline" (distance between the eyes or Inter-Pupillary Distance (IPD)) 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.
• Inter-Pupillary Distance (IPD). This is the distance between the two eyes, measured at the pupils, and is important in designing Head-Mounted Displays.
• Field of view (FOV) – Humans have an 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. To convert the manufacturer's apparent screen size to a desktop monitor position, just divide the screen size by the distance in feet, then multiply by 2. Consumer-level HMDs typically offer a FOV of about 30-40° whereas professional HMDs offer a field of view of 60° to 150°.
• 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. 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/°, 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° (50° to the left of the nose and 50° 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°) or as a percentage 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.
• On-board processing and operating system. Some HMD vendors offer on-board operating systems such as Android, allowing applications to run locally on the HMD and eliminating the need to be tethered to an external device to generate video. These are sometimes referred to as Smart Goggles.