A peripheral head-mounted display (PHMD) is a visual display (monocular or binocular) mounted to the user's head that is in the peripheral of the user's field of view (FOV) / peripheral vision. Whereby the actual position of the mounting (as the display technology) is considered to be irrelevant as long as it does not cover the entire FOV. While a PHMD provide an additional, always-available visual output channel, it does not limit the user performing real world tasks. The term PHMD includes devices such as Google Glass, which are often misclassified as a Head-up display (HUD) if following the original definition by NASA. While NASA defined this term over centuries of space flight research, it actually describes a display that addresses the eyes-free problem, by absolving the user from the need to angle down their head. Furthermore, it provides augmented information in the user's forward field of view (FOV), which is commonly projected on a windshield. In contrast, the Head-Down Display (HDD) is located at the instrument control panel. Also, a HUD is mainly used to augment additional information into reality, which is technically not feasible yet for products such as Google Glass (lens focus on the display causes a blurred environment – see figure below). This taxonomy for head-mounted displays is based on the property of its functionality and the ability of the human eye to perceive peripheral information, instead of being technology-dependent. In this article Human Factors for visual perception are being summarized, which are important to be taken into consideration when designing visual interfaces for PHMDs.
Characteristics
The most important uniqueness is that the user's FOV is not being fully covered, allowing the user to perform real world tasks without limitations, while not having the pretension to raise or create immersion, such as HMDs often aim for. For current display technologies, while projecting image onto the eye, the screen needs to be focused by the pupil to enable a clear reading of the screen, thus the environment becomes blurred and out-of-focus. So a PHMD such as Google Glass is capable of displaying detailed information, when the pupil is focusing the display itself, as it also allows for peripheral information when the eye focuses on the real world. Still, simple information such as notifications are perceivable when focusing on the real world instead of the display.
Physiological Factors: Visual Perception Research shows that designing an optimal visual output for Head-Mounted Displays is a complex issue, since there are several physiological factors that significantly impact users’ perception. The following effects are known in research:
Depth of Focus / Field Switches permanently by refocusing on objects, which are different in distances to the user. A display mounted somehow to user's eye has fixed focal distance. Focusing information such as presented on a screen leads to a change in the depth of focus. This causes blurring of information presented at other layers, which especially degrades the perception of high spatial frequency information such as text.
Eye-Movements Are actually done at a specific angle of 10°. To focus an object out of this angle, head movements are used automatically for support. However, when wearing an HMD with eye-movements that exceed this angle, since head movements do not have any effect on the interface, a drop in comfort might occur due to tired eye muscle.
Field-Of-View Describes the FOV. The User's eye has a viewing angle of 94° from the center and 62° on the nose side. The vertical angle is about 60° upwards and 75° downwards. HMDs often do not cover the whole FOV, which is also a reason for increased cybersickness.
Binocular Rivalry Describes the phenomenon, which occurs when dissimilar images are presented to the human eye. As the two images captured by each eye are incompatible for stereo processing, they fight for visual dominance over the other eye's side view, resulting in alternating views from the two eyes, where the non-dominant view is almost unseen. This effect often occurs when wearing a monocular HMD. In this setup, researchers also observed objects that completely vanish for several seconds from user's attention.
Visual Interference Describes the phenomenon when both eyes perceive different images that are overlapping, but the brain is not able to distinguish between those. This phenomenon is also known as the inability for visual separation.
Phoria Describes a muscle state of the eye, when the eyes are not focusing on a specific point. There are three different states, which can be distinguished: Esophoria, Exophoria, Orthophoria. While one eye is closed or being obstructed by a display, phoria can occur, which has the potential to cause Vertigo and Nausea as well.
Eye-Dominance Although the user has two eyes, one eye is predominantly used. The other eye is used to make corrections and provide additional spatial information. It is recommended to wear a monocular HMD over the dominant eye.
Peripheral Perception
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