Introduction. Proper fitting and left to right eye ocular alignment is necessary to maintain user performance and comfort with the use of binocular head-worn displays (HWDs). Towards that objective, measuring alignment of binocular HWDs for performance was investigated. Methods. A new, deployable, ruggedized Helmet-Mounted Display Binocular Alignment measurement tool (HMD BAMT) was developed that provides accurate alignment measurements to 15 microns. The tool was developed for measurement of any HWD/HMD in use in the field. Validation of the tool was accomplished using a commercially available binocular HMD. The HMD was provided to the U.S. Army C5ISR Center Research and Technology Integration Directorate at Fort Belvoir, VA for additional validation against the Near-Eye Display Test Station (NDTS), a much larger and more expensive system designed for laboratory measurement of HWDs. Results. Measurements showed the BAMT has very good accuracy and repeatability (i.e. milliradian accuracy). The BAMT capability and comparison to the NDTS measurements are described in this paper. Discussion. Further development of the alignment tool is ongoing and expanding the capability to include other measurements such as but not limited to uniformity, contrast, modulation transfer function (MTF) and distortion. The tool has been ruggedized for field support of binocular displays. The tool design updates have 3-axis capability and will provide fast, automated testing capability for production rate measurements. The new alignment tool will enable measurements to characterize quality of alignment of binocular HWDs in use in the field which will, in turn, support research concerning individual tolerance to representative levels of misalignment.
Binocular head-mounted displays (HMD) utilizing augmented reality (AR) strategies can greatly increase the information that reaches the visual system of the user. For example, binocular presentation allows for elements to appear in stereoscopic depth and with a higher perceived resolution and AR can improve the quality of a low visibility scene. But with two independent optical channels, a binocular HMD can easily become misaligned, which can potentially be detrimental to both performance and comfort (Gavrilescu et al., 2019; SPIE DCS). Here, we quantify the effect that global binocular misalignment in an HMD has on both operational and visual performance during a simulated flying task. Using a platform consisting of 3 85-inch displays providing out-the-window imagery and head-tracked AR overlay (e.g., DAS) within the HMD (Posselt et al., 2021; SPIE DCS), subjects were instructed to adhere to flight commands while periodically discriminating the orientation of a target aircraft. In different blocks the two optics of the HMD were either well-aligned, misaligned vertically by 0.67°, or rolled in opposite directions by 4°. In the well-aligned condition, subjects could discriminate the orientation of the target plane on average nearly 1000 ft farther than in either of the misaligned conditions. Curiously, adherence to the flight commands was affected only by the vertical misalignment, which may represent a strategy of selectively ignoring grossly misaligned imagery in one eye. These results obviate the need to quantify and maintain well-aligned visual channels in binocular HMDs that utilize AR imagery.
A pilot’s Helmet-Mounted Display (HMD) is now a critical part of the aircraft system. Next generation HMDs will be able to display information and imagery binocularly, with stereoscopic depth. Stereo 3D (S3D) can potentially be used to enhance situational awareness and improve performance. The degree to which performance is improved may be linked to individual visual capabilities of the user, in particular, stereo acuity. Stereo acuity varies tremendously in the general population with up to 30% being classed as ‘stereo blind’. For most military aviators there is a minimum stereo acuity standard, however current test methods are crude and fallible. Many previous S3D studies do not accurately characterize individual stereo acuity, and in some cases do not even screen for its presence, making their results difficult to interpret. The Operational Based Vision Assessment (OBVA) laboratory has developed a flight simulation platform using an SA Photonics SA-62 HMD to display stereoscopic symbology and five 85-inch displays to provide the “out the window” view. After completing a battery of vision tests, participants fly various mission profiles while responding to a combination of navigational instructions and warning alerts displayed in the HMD. The warning alerts are displayed in 2D (flashing at 1 Hz), intermittent S3D (flashing on and off at depth), persistent S3D (alternating between 2 depth planes), and dynamic S3D (motion in depth). We present preliminary data examining whether a stereoscopic HMD could be used to improve performance when responding to a critical warning alert, and discuss potential implications for military aviator vision standards as well as HMD requirements
This paper describes an evaluation of the capability of two tests of vision, stereoacuity and fusion recovery range, to predict depth discrimination performance for subjects using a hyperstereoscopic display system. For the hyperstereo performance evaluation, 14 subjects completed a depth discrimination task presented at multiple positions and depths in a remote vision system (RVS) simulation similar to that used by air refueling operators on the KC-46 aircraft. Prior to performing the hyperstereo task, each subject completed automated tests of stereoacuity and binocular fusion recovery range. Evaluation results indicate that both stereoacuity (R2 = 0.64) and recovery range (R2 = 0.45) reliably predict (p ≤ 0.01) hyperstereo depth discrimination performance. The use of a two-factor model improves predictive capability (R2 = 0.73); however, the utility of including recovery range scores depended on viewing conditions. When easy viewing conditions (high contrast stimuli presented near the depth of the display) were used, performance was predicted by stereoacuity and the prediction was not improved using recovery range scores. Under difficult viewing conditions (low contrast stimuli, background clutter, crosstalk, and dipvergence), the prediction of hyperstereo performance was significantly improved by including recovery range scores. These results suggest that a binocular fusion recovery range test should be used in conjunction with a stereoacuity test to predict the performance of operators using hyperstereoscopic displays under the more difficult viewing conditions that can be expected in operational environments. Stereoscopic display design considerations and the importance of computer-based vision testing will be discussed in detail.
Head-mounted displays (HMDs) generally exhibit significant image distortion, which must be reduced/eliminated prior to effective use. Additionally, biocular or binocular near-eye displays must be carefully aligned to enable overlapping two- or three-dimensional image synthesis without causing eye strain, fatigue, or performance loss. Typically, HMDs include distortion correction maps supplied by the manufacturer that are often generated by theoretical calculations that do not precisely match the as-built optical system or account for manufacturing variance. However, HMD users often assert that manufacturer-supplied distortion maps are not accurate enough for some alignment-critical applications. In this work we present the design and validation of a relatively low cost alignment and distortion characterization toolset (hardware and software) for characterization of biocular HMDs. This toolset is able to replicate the ocular alignment of most human observers by emulating a user’s ocular position to examine both on- and off-axis distortion and alignment over a wide range of viewing angles and eye positions. This enables accurate characterization of distortion changes experienced as a user’s eyes move to view different regions of the display (e.g., viewing off-boresight symbols in a well-aligned HMD or viewing a new alignment after an HMD has “slipped” to a slightly different position). The toolset characterizes distortion through image registration of distorted patterns displayed in the HMD to undistorted reference patterns. This work is intended to be of interest to HMD manufacturers, vision scientists, and operators of biocular HMDs for use in precision-critical applications.
Over the past few decades the term “eye-limited resolution” has seen significant use. However, several variations in the definition of the term have been employed and estimates of the display pixel pitch required to achieve it differ significantly. This paper summarizes the results of published evaluations and experiments conducted in our laboratories relating to resolution requirements. The results of several evaluations employing displays with sufficient antialiasing indicate a pixel pitch of 0.5 to 0.93 arcmin will produce 90% of peak performance for observers with 20/20 or better acuity for a variety of visual tasks. If insufficient antialiasing is employed, spurious results can indicate that a finer pixel pitch is required due to the presence of sampling artifacts. The paper reconciles these findings with hyperacuity task performance which a number of authors have suggested may require a much finer pixel pitch. The empirical data provided in this paper show that hyperacuity task performance does not appear to be a driver of eye-limited resolution. Asymptotic visual performance is recommended as the basis of eye-limited resolution because it provides the most stable estimates and is well aligned with the needs of the display design and acquisition communities.
With increased reliance on head-mounted displays (HMDs), such as the Joint Helmet Mounted Cueing System and F-35 Helmet Mounted Display System, research concerning visual performance has also increased in importance. Although monocular HMDs have been used successfully for many years, a number of authors have reported significant problems with their use. Certain problems have been attributed to binocular rivalry when differing imagery is presented to the two eyes. With binocular rivalry, the visibility of the images in the two eyes fluctuates, with one eye’s view becoming dominant, and thus visible, while the other eye’s view is suppressed, which alternates over time. Rivalry is almost certainly created when viewing an occluding monocular HMD. For semi-transparent monocular HMDs, however, much of the scene is binocularly fused, with additional imagery superimposed in one eye. Binocular fusion is thought to prevent rivalry. The present study was designed to investigate differences in visibility between monocularly and binocularly presented symbology at varying levels of contrast and while viewing simulated flight over terrain at various speeds. Visibility was estimated by measuring the presentation time required to identify a test probe (tumbling E) embedded within other static symbology. Results indicated that there were large individual differences, but that performance decreased with decreased test probe contrast under monocular viewing relative to binocular viewing conditions. Rivalry suppression may reduce visibility of semi-transparent monocular HMD imagery. However, factors, such as contrast sensitivity, masking, and conditions such as monofixation, will be important to examine in future research concerning visibility of HMD imagery.
One of our previous studies examining the integration of a
head-mounted visual display with a faceted flight simulator
display showed that a monocular condition was the most uncomfortable and it also resulted in poorer operator
performance. In the present study, we investigated whether this reduction in performance was dependent on eye
dominance and whether it could be reduced or eliminated through training. Our performance measure was the amount of
time it took operators to make correct decisions on a simplified targeting task using a see-through monocular headmounted
display and a large-screen display upon which was presented an
out-the-window view of a desert scene. A
binocular on-screen viewing condition served as baseline. The results revealed that response time significantly decreased
with training but that eye dominance did not exert a significant effect. These results are interpreted within the context of
training regimes for using HMDs with sparse symbology.
When wearing a monocular head-mounted display (HMD), one eye views the HMD symbology while both eyes view
an out-the-window scene. This may create interocular differences in image characteristics that could disrupt binocular
vision by provoking visual suppression, thus reducing visibility of the background scene, monocular symbology, or
both. However, binocular fusion of the background scene may mitigate against the occurrence of visual suppression, a
hypothesis that was investigated in the present study. Observers simultaneously viewed a static background scene and
HMD symbology while performing a target recognition task under several viewing conditions. In a simulated HMD
condition observers binocularly viewed a background scene with monocular symbology superimposed. In another
condition, viewing was dichoptic (i.e. completely different images were presented to the left and right eyes).
Additionally, one control condition was implemented for comparison. The results indicate that for continuously
presented targets binocular rivalry did not have significant effects on target visibility. However, for briefly presented
targets, binocular rivalry was shown to increase thresholds for target recognition time in HMD and dichoptic viewing
conditions relative to the control. Impairment was less in the HMD condition. Thus, binocular fusion of a background
scene can partially mitigate against the occurrence of visual suppression. However, some suppression still exists which
occurs between monocular pathways. Implications for the integration of monocular HMDs into Air Force training
environments will be discussed.
The Joint Helmet Mounted Cueing System (JHMCS),is being considered for integration into the F-15, F-16, and F-18 aircraft. If this integration occurs, similar monocular head-mounted displays (HMDs) will need to be integrated with existing out-the-window simulator systems for training purposes. One such system is the Mobile Modular Display for Advanced Research and Training (M2DART), which is constructed with flat-panel rear-projection screens around a nominal eye-point. Because the panels are flat, the distance from the eye point to the display screen varies depending upon the location on the screen to which the observer is directing fixation. Variation in focal distance may create visibility problems for either the HMD symbology or the out-the-window imagery presented on the simulator rear-projection display screen because observers may not be able to focus both sets of images simultaneously. The extent to which blurring occurs will depend upon the difference between the focal planes of the simulator display and HMD as well as the depth of focus of the observer. In our psychophysical study, we investigated whether significant blurring occurs as a result of such differences in focal distances and established an optimal focal distance for an HMD which would minimize blurring for a range of focal distances representative of the M2DART. Our data suggest that blurring of symbology due to differing focal planes is not a significant issue within the range of distances tested and that the optimal focal distance for an HMD is the optical midpoint between the near and far rear-projection screen distances.
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