ABSTRACT
The pilot's helmet has been used as an anchorage for useful aids since the early days of aerial combat. A German airman in WW1 was reputed to have attached a dental mirror close to his brow to give him the crucial edge of all-round awareness; eyes in the back of his head. Although the veracity of the story and the success of the device are both unknown, it provides a nicely allegorical introduction to the display devices that have been devised subsequently. Like the HUD, described in the previous chapter, helmet-mounted displays (HMD) project virtual images, but the images are aligned with the orientation of the head rather than the orientation of the aircraft. A HUD mounted on the helmet not only frees the pilot from the need to glance down and re-focus to see information presented within the cockpit, but he no longer has to glance forward. The intention has been to extend the convenience and conformality of a HUD to provide the imagery wherever the head is pointing. Although the size of the visible image is limited to the display field-of-view, the total visual envelope in which imagery can be presented (the field of regard, FoR), is limited only by the mobility of the head.
