Comparative effects of vergence/accommodation conflicts with different interocular separation and viewing distances

As pointed out by Kotulak, vergence/accommodation mismatch in Night Vision Systems, usually due to misadjustments of eyepiece focus, is sometimes a source of visual performance decrement. The increased separation between sensors existing in some modem binocular Helmet Mounted Display systems, creating "hyperstereopsis", was also identified to be potentially responsible for decreased performance at distances less than 5 meters. Based upon basic knowledge pertaining to vergence and accommodation mechanisms, a study was performed using a sensory approach, with the goal of better understanding the problem of dissociation between accommodation and convergence. In this study, different conditions of interocular separation (nominal IPD, X3, X4) and viewing distances (6m, 4m, 2m) were used. Six subjects participated in the experiment and were asked to view Landolt C charts using NVGs and specially developed optical tools allowing changes to sensor separation. The results show that, with a fixed eyepiece focus at 10m, the decrease in resolution performance is roughly proportional to the interocular separation when looking at short distances. A fixed focus at a distance of 4m considerably reduces the conflict and results in improved resolution for increased separation conditions. An additional experiment was conducted to investigate the setting of objectives lenses focus at infinity (nominal landing condition). With this setting, for visual acuity test, the decrease in resolution at short distance was such that effects of the mismatch between accommodation and convergence are no longer apparent regardless of interocular separation.