A 25-year retrospective review of visual complaints and illusions associated with a monocular helmet-mounted display

Abstract

In the early 1980s the U.S. Army fielded the first integrated helmet-mounted display (HMD) for use in the AH-64 Apache helicopter. To reduce head-supported weight and minimize center-of-mass offsets, a monocular optical design was selected. Although early design concerns of binocular rivalry and the Pulfrich phenomenon never materialized, user surveys have documented persistent reports of visual complaints and illusions in peacetime training flights. However, a recent evaluation conducted under battle conditions in Operation Iraqi Freedom found statistically lower reports of complaints and illusions. While these reported problems are short-term, questions have been raised regarding potential long-term physiological effects resulting from long-term use of this monocular display. A 10-year longitudinal study, currently underway in the U.K., has been implemented to definitively answer the question of whether long-term exposure produces any degradation in binocular visual function.

Introduction

In the 1970s, the U.S. Army decided to build a modern attack helicopter (designated as the AH-64 Apache) (Fig. 1) to replace its aging fleet of AH-1 Cobra helicopters, which had been in service since the early 1960s. The AH-64 Apache was designed to provide enhanced nocturnal capability through the use of a nose-mounted, forward-looking infrared (FLIR) sensor that provides a thermal image of the outside world to the pilot. This imagery is presented to the pilot via a head-up helmet-mounted display (HMD).

Due to engineering technology limitations of the era, the HMD was constructed to present pilotage imagery only to one eye, the right eye. This monocular design was driven by the paramount need to minimize head-supported weight and any shift in center-of-mass (CM). This monocular presentation to the human visual system, which is binocular in nature, raised concerns among vision scientists involved in the early design. While these particular concerns never manifested themselves, a host of visual complaints and illusions have plagued AH-64 pilots ever since the fielding of this aircraft in the early 1980s.

In this paper, the 25-year history of the visual issues associated with the monocular design of the AH-64 HMD are reviewed and discussed. These problems were predicted by early designers, have been documented in performance surveys conducted over nearly 25 years of fielding, and potentially may have long-term physiological effects on binocular vision.