Human factors
Ruth Aylett
Topics
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Human senses and their limitations
– Sight
– Hearing
– Touch
– Vestibular system
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Health and safety issues
Why are human factors
important?
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Human capabilities define desired technology
attributes
Different senses have different data rates
Guidelines for best display and organisation
of information for understanding
Health and safety issues
Predict overall impact of a VE
Human senses
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Visual
Auditory
Tactile
Olefactory
Vestibular (balance)
Others:
– Proprioception: sense of position and
movement of body
Visual perception
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Field of view (FOV)
Acuity
– Resolution
– Dark v light
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Stereopsis
– Depth cues
– Binocular rivalry/eye dominance
Vision
Visual process
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Light enters through cornea/pupil
– Focused by lens on retina
– Lens has to adjust focus:accommodation
• Uses ciliary muscles
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Responds to a narrow band of
electromagnetic radiation
– 400nm to 700nm, overall peak at 559nm
– Well-matched to spectral emission of sunlight
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Response from single photon to fluxes one
trilliontimes stronger
Visual process cont.
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Colour
– Via RGB cones (rods do night vision)
• Three kinds of cones, sensitive to different
wavelengths
– “long”: 575nm, about 64%
– “middle”: 535nm, about 32%
– “short”: 445nm,
• Peripheral cells very sensitive to movement
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Blind spot
– Where optic nerve emerges
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Eye in permanent motion
– Saccades at about 4Hz
– Stabilized images disappear
– Saccadic suppression leaves these motions largely
unnoticed
FOV
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Horizontally
– -59˚ to +110˚
– 118º overlap where stereopsis occurs
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Vertically
– -70˚ to +56˚
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Visual acuity
– Can separate 2 bright light points 1.5mm apart at 10m
– 40’’of arc; 2µm on retina
– To match this, requires “typical” desktop display of
4800x3840 (18.4 million pixels)
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Light to dark adaptation: 25000:1
Visual persistence
– Critical fusion frequency: c 20 Hz
Visual
processing
Stereopsis
The geometry
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Can detect depth
distances of 0.05m
at .5m or 4mm at
5m
Represented as:
qD=IPD
r 
— ——
r r +r
IPD= Inter-Pupil
Distance
QuickTime™ and a
TIFF (LZW ) decompressor
are needed to see this picture.
Stereo cues
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Motion parallex cues
– Image speed across the retina to judge depth
– Only needs one eye
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Convergence
– How much eyes turn in
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Perspective depth cues
– Where we ‘know’ the size of an object
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Textural cues
– Light and shadow
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Very important for greater distances
Oculomotor cues
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Accommodation
–
–
–
–
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Physical stretching and relaxing of the lens
Parallel rays entering the relaxed eye will focus on the retina
Relaxed eye has a depth of field of 6m to infinity
To focus objects within 6m it is necessary to alter the optical
system of the eye
Vergence
– Rotation of the eyes (convergence: inward rotation
corresponding to viewing closer object)
– Muscular feedback in converging and focusing the eyes is
cue to the depth of viewed object
– Relatively weak, but coupled depth cues!
Hearing
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Air vibrations (rapid changes in air pressure)
converted to mechanical vibrations in middle ear
Sound has:
– Amplitude: Magnitude of the pressure variation
– Frequency: Pressure variation rate
– Phase
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Acoustic reflex: Adaptation to high-intensity sounds;
temporarily reduced auditory sensitivity
Acoustic stimuli necessarily have temporal
component
Constant sounds drop out of conscious awareness
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Sounds are perceived from sources in all directions
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Localisation
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Different factors influence ability to perceive
location of sound sources
– Interaural level difference: Difference in volume of
sound reaching each ear
– Interaural time difference:Time difference of sound
reaching each ear (sound is slow)
– Motion cues
• Doppler effect: Frequency shift resulting from relative
motion between sound source and observer
• Changing volume:Sound perceived as approaching
when volume gradually increases (and vice versa)
External ear - pinnae
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Why do stereo speakers reproduce the
original sound stage?
– Effect of head + pinnae
– Hence in-ear speakers produce stage
‘inside head’
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Head-related transfer functions (HRTF)
– Modify signal
– Simulate effect of head and pinnae
Haptic perception
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Touch is a mechanical or thermal contact with the
skin.
Mechanical stimuli produce sensation of touch:
– Displacement of skin over an extended period
– Transitory (milliseconds) displacement of skin
– Transitory displacement repeated at constant or variable
frequency
– Different below-skin sensors for skin with/without hair
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Also thermal sensing
Difficult to characterize in quantitative way
Sensations of skin adapt with exposure to stimuli
Skin
Force sensing
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Receptors in muscles and joints
– Inside the body
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Perception of movement, position, and
torque of limbs and other body parts
Varying joint angles and muscular
length
Vestibular system
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Movement and
orientation in space
– See
http://en.wikipedia.org/wiki/V
estibular_system
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Inner ear: vestibulum
– Orthogonal canals for
rotation
– Saccule/utricle for
linear acceleration
Vestibulo-occular reflex
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To be able to
see while
moving
– Push-pull
between RH
and LH canals
– 3 neurons link
to eye
– <10 ms lag in
eye for head
movement
Health and safety issues
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Flicker
– Can trigger epilepsy
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Eyestrain
– Dissociation of accommodation and
convergence
– Lack of visual calibration, especially HMDs
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Use of laser light
Movement injuries
Cybersickness
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Reverse form of motion sickness
– Conflicting signals from visual and vestibular
systems
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Caused by:
– Vection: illusion one is moving in a VE
– Lag: delay in visual feedback
• Especially bad in HMDs
– FOV: wide and narrow. Impacts vection
– Lack of interactive control
Symptoms
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Vestibular disturbances
– Head spinning
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Reduced hand-eye coordination
Nausea
Effects take time to reduce after
exposure
Nervous system does adapt