The head-up display (HUD) was developed to provide the pilot simultaneous viewing of flight information and the outside environment. The HUD optics were collimated so that the symbols appear at optical infinity as does the distant terrain. Operationally, about 30% of pilots experience an increased tendency toward spatial disorientation when using a HUD. Two experiments were conducted at the Naval Air Development Center to determine whether HUD symbols do in fact cause the eyes to focus at optical infinity. And if not, what are the direction and amount of refocusing necessary to see both distant targets and HUD symbols clearly? Results showed that use of the HUD released accommodation to lapse inward for all 10 experimental observers. When simultaneously using the HUD and viewing distant outside targets, only 3 of 10 subjects consistently focused at or beyond optical infinity; two of these had dark foci near infinity, and the third had a dark focus almost three diopters negative. Previous research indicates that when focus shifts occur, the apparent size and distance of objects also change. Spatial judgments critical to ground-referenced flight can be fatally erroneous. Because individual differences in focusing are large, redesigning HUDs to incorporate manual optical adjustment may be desirable to optimize focus demand for each pilot. Volitional accommodation training should also be investigated.
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