Depth of focus and visual recognition of imagery presented on simultaneously viewed displays: Implications for head-mounted displays

Objective: We sought to determine the optimal focal distance for a semitransparent monocular head-mounted display (HMD) integrated with a flight simulator display and to investigate whether observers experienced visual discomfort or impaired target recognition when using an HMD set at the optimal distance. Background: When an observer wears a monocular HMD and views a simulator display, focal distances of both displays must be within the observers' depth of focus to prevent blurred imagery. Because focal distance can vary by as much as 0.5 in in U.S. Air Force multifaceted simulator displays, we determined whether a monocular HMD could be integrated with a simulator display without blurred imagery or discomfort. Method: Depth of focus and visual recognition were measured with a staircase procedure, and visual discomfort was measured with a questionnaire. Results: Depth of focus was 0.64 diopters in one condition tested, but it was affected by luminance level and display resolution. It was recommended that HMD focal distance equal the optical midpoint of the range of viewing distances encountered in the simulator. Moreover, wearing an HMD produced a decline in recognition performance for targets presented on the simulator display despite both displays being within observers' depth of focus and producing no visual discomfort. Conclusion: Monocular HMDs can be integrated with multifaceted simulator displays without blurred imagery or visual discomfort, provided that the correct focal distance is adopted. Application: For situations involving simultaneously viewed visual displays.