1. Field of the Invention
This invention relates to helmet mounted displays. In this specification the term "helmet" means any article worn by an observer on his/her head and includes protective helmets such as those worn by aircraft pilots or vehicle drivers and simple instrument supporting head bands such as those worn by surgeons and other medical workers. The term "helmet mounted display" shall mean apparatus attached to, located on, or forming part of such a helmet and for presenting images or information to the wearer of the helmet.
2. Description of the Related Art
For combat aircraft use, flight testing and medical examination have confirmed that binocular display to the pilot both aids performance and also reduces brain fatigue.
At present, binocular presentation is provided by either having:
(i) two sources of display or PA1 (ii) one source with optical train containing a split prism.
The adoption of solution (i) doubles the number of devices on the helmet providing the display and hence increases the helmet mass.
The adoption of solution (ii) halves the level of brightness of the display as inherently the split prism has a 50% transmission characteristic.
Consequently both the existing techniques have operational disadvantages. Firstly, excess mass and the adverse physiological effect under g forces; and secondly, low brightness and hence loss of display under high ambient light conditions.
There is alternatively often a requirement to provide the pilot with information f rom two similar or dissimilar data sources simultaneously. Such sources may be for example cathode ray tube, dot matrix, or conventional optical displays. Currently the pilot may view one source only at a time.