Twenty-first century demands on "Top Gun" pilots are so great that the relative narrow field of view of head-up displays (HUDs), approximately 20° in a fixed direction, is far too limiting and confining for all the tactical information that is needed to be displayed for them to make split-second decisions based on sound judgement. This inadequacy has led, in part, to the development of helmet-mounted displays (HMDs), and thence to helmet integrated systems (HISs). This paper gives highlights from the patent description of a HIS that dates back to World War I and that is a forerunner of the concept of those being developed today.
Gerald F. Marshall,
"Back From The Past: The Helmet Integrated System Of Albert Bacon Pratt (1916)," Optical Engineering 28(11), 281247 (1 November 1989). https://doi.org/10.1117/12.7977127