Eli Peli, M.Sc., O.D., FAAO
Optometric Issues with Head-Mounted Displays
This lecture introduces head-mounted displays (HMD) and the optometric consequences
of their wide use. A thorough review is provided to enable the optometrist to respond to
inquiries from patients and parents regarding the use of these devices. With the new
emergence of HMDs as game devices and as personal communication devices (Google
Glass) public attention to the impact of HMD is increasing. Much of the material is
overlapping with the issues of 3D television and movies, another area of current interest.
Side effects of HMD suggested or reported in Literature
(Mon-Wiliams et al ,1993; Wilson, 1995; Howarth, 1995; Howarth 2011)
Effect on visual system performance:
Reduce acuity or stereo vision,
Changes in phoria posture or fixation disparity.
Strabismus
Subjective reports of visual discomfort - asthenopia
Physical discomfort: head, neck, back
Disorientation, nausea, motion sickness, simulation sickness
Lowered cognitive and psychomotor performance
Long term effects including hallucinations, flashbacks and addiction
Effects that were certainly found
Effects presumed to result from known optical aspects of HMD
Effects frequently reported in the media with no known supportive evidence
Classification of HMDs
Monocular vs. Binocular
Bi-ocular
Stereo
See-through vs. Opaque
Synthetic imagery (ie computer games, molecules, models)
Photographic imagery (tele-presence, robotics, endoscopy, LVES)
Augmented reality – use in industry, medicine, and low vision
Optometric issues (Peli, 1990; Peli 1995; Peli 1998; Fortuin, 2010)
Instrument myopia
Convergence and accommodation
Real world situation
Convergence demand and accommodation demand
The demand line as function of IPD
Single clear binocular vision - meeting both demand
Misalignment of convergence and accommodation in HMD
Static misalignment - hardware and optical considerations (Mon-Wiliams et al 1993)
Various designs and their effects
Dynamic misalignment - software - game considerations
Unavoidable
Software design guidelines
Proposed solutions for the misalignment of convergence and accommodation
Accommodative effect of stereo displays
Monocular occlusion in children
The critical period.
Effects of intermittent occlusion on children's vision
Monocular occlusion in adults
Visual Perceptual issues
Head motion, vestibular effects, image motion, and motion sickness
Vestibular mechanism
Adaptation- vestibular ocular reflex (VOR)
Effects of VOR in HMD
Boom (static) system: Nintendo (Virtual boy) - no problem
Adaptation
With HMD
Unequal demand for both eyes
Monocular
Conflicting demands
See-through (Sony, Virtual I-O )
Eye movements and image motion
Saccadic eye movements
Intermittent display (flickering pulsating - relations to asthenopia)
LED display (digital alarm clock, Private Eye (Peli, 1990)
CRT - short persistence phosphors (Neary &Wilkins 1989)
Fluorescent lighting
Normal continuously illuminated scenes
Saccadic suppression (Matin, 1974)
Image motion and eye movement changes
Intermittent binocular displays
Critical Flicker Frequency binocular in and out of phase
In-phase 8% higher CFF than out of phase (Perrin 1954)
Out of phase intermittent display
reduces or eliminates jump (Chen 1993)
Effects of low update rates
Smooth pursuit eye movements
Refresh rate (static imagery)
Update rate (dynamic imagery)
User response tracking
Low update rate schemes (refresh rate double update rate)
Present each updated frame twice - Bright Eye- low vision
Double, low contrast images (Chen, 1993; Lindholm, 1992)
Lower refresh rate to match update rate
Noticeable flicker
Binocular display, hybrid method.
Binocular Rivalry effects in monocular HMD
Two non similar images presented to both eyes.
Unstable perception
Alternating periods of monocular dominance
Factors affecting rivalrous display
Eye dominance in rivalry
Sighting dominance
Users with abnormal binocular vision
Position of monocular HMD
Bioptic (above line of sight)
Bifocular (Below line of sight) (Katsuyama et al 1989)
Design considerations
Accommodation
Fixed (Virtual I-O 4 meters, Optics 1 6 meters)
with or without correcting lenses (effect of un-corrected hyperopia)
Adjustable (Nintendo's Virtual-Boy, Sony's Visortron)
with or without correcting lenses
Convergence
Fixed (Virtual I-O, Optics 1)
Adjustable
(Sony's Visortron)
Convergence in relation to focus setting
IPD
Fixed (Virtual I-O)
Adjustable
(Sony's Visortron, Nintendo, Optics 1, PT- 01)
Prismatic effect
Field curvature
No negative focus.
Magnifiers up to 2 diopters (Katz, & Zikos, 1994)
Does not represent problem for young accommodating users.
Field curvature disparity between eyes.
Require change of accommodation as the gaze is moved across the screen.
If user's IPD does not match that of the system
Tolerances for Quality Control
No Standard exist yet. Relation ANSI Z80.1-1972 standard for spectacles
Vertical alignment
Shift in vertical alignment as a result of direction of gaze
Horizontal Alignment
Magnification, Aniseikonia
Aniso-accommodation
Cyclotorsion.
Product Liability
Age of maturity
Testing of various devices (Peli, 1990; Peli, 1998)
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