Game Development Reference
11.3.1 Architecture of a head-mounted display
A head-mounted display must have four main features:
A large visual field;
An immersion of the eyes;
A stereoscopic vision and
A high resolution.
There are two solutions to create a large visual field : Either a very large screen or
two screens close to the eyes. The first option is used in immersive rooms.
The second option is used in head-mounted displays; however it is mandatory to
take two additional measures:
Adding an optical device with a large visual field for the accommodation of eyes
on screens that are very close to them;
And providing two mini-screens with very good resolution or, if the resolution is
low, placing two diffusers in front of the LCD mini-screens to avoid seeing pixels
in the form of large blocks separated by black gaps.
With respect to these two requirements, the technical solutions are often opposed
to each other: for example, it is not easy to manufacture very small screens comprising
a large number of pixels. It is difficult to obtain a large visual field with mini-screens.
However, the main difficulty lies in the creation of optics with a large visual field.
It is the main constraint restricting the horizontal visual field to a maximum of 120
To achieve immersion of the eyes , it is necessary to have a location sensor and
fix two mini-screens with optics on the head. The user can make the most of it if the
ergonomics of the interface is adequate enough to make the interface transparent. This
means that the devices have to be very light in weight, comfortable (good ventilation)
and safe. The problem of selecting the right optics is primordial.
High magnification optics are necessary to project images on mini-screens, but
this leads to geometric distortions, especially if we want large visual fields. For 3D
vision, the optics also have to be correctly positioned to facilitate merging of images.
Various solutions are available for the optics: either with multiple standard lenses,
or with Fresnel lenses or with the device called “Pancake window''. The last two
options help to reduce their weight. The “Pancake window'' system is based on a
semi-transparent spherical mirror and polarizers eliminating unwanted light rays. A
compact and lightweight device was designed by Saab Avionics for the head-mounted
display AddVisor 150.
Theoretically speaking, having two screens and displaying good images on them
for each eye is sufficient to obtain stereoscopic vision . Considering the difficulties
in manufacturing mini-screens with very large visual fields, the manufacturers prefer
increasing the total visual field (of two eyes) by creating a single smaller stereoscopic
overlap zone. The head-mounted displays often have the option to adjust this zone
(acceptable values: 100% to 70%). A manufacturer had offered an overlap of 25%;
which reduces the possibilities of 3D vision to a large extent!