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Human factors

Ruth Aylett

Topics

Human senses and their limitations– Sight– Hearing– Touch– Vestibular system

Health and safety issues

Why are human factorsimportant?

Human capabilities define desired technologyattributes

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

Visual Auditory Tactile Olefactory Vestibular (balance) Others:

– Proprioception: sense of position andmovement of body

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Visual perception

Field of view (FOV) Acuity

– Resolution– Dark v light

Stereopsis– Depth cues– Binocular rivalry/eye dominance

Vision

Visual process

Light enters through cornea/pupil– Focused by lens on retina– Lens has to adjust focus:accommodation

• Uses ciliary muscles

Responds to a narrow band ofelectromagnetic radiation– 400nm to 700nm, overall peak at 559nm– Well-matched to spectral emission of sunlight

Response from single photon to fluxes onetrilliontimes stronger

Visual process cont. 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

Blind spot– Where optic nerve emerges

Eye in permanent motion– Saccades at about 4Hz– Stabilized images disappear– Saccadic suppression leaves these motions largely

unnoticed

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FOV Horizontally

– -59˚ to +110˚– 118º overlap where stereopsis occurs

Vertically– -70˚ to +56˚

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) Light to dark adaptation: 25000:1 Visual persistence

– Critical fusion frequency: c 20 Hz

Visualprocessing

StereopsisThe geometry

Can detect depthdistances of 0.05mat .5m or 4mm at5m

Represented as:θD=IPD Δr — —— r r +Δr

IPD= Inter-PupilDistance

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Stereo cues

Motion parallex cues– Image speed across the retina to judge depth– Only needs one eye

Convergence– How much eyes turn in

Perspective depth cues– Where we ‘know’ the size of an object

Textural cues– Light and shadow

Very important for greater distances

Oculomotor cues Accommodation

– 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 rotationcorresponding to viewing closer object)

– Muscular feedback in converging and focusing the eyes iscue to the depth of viewed object

– Relatively weak, but coupled depth cues!

Hearing 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

Acoustic reflex: Adaptation to high-intensity sounds;temporarily reduced auditory sensitivity

Acoustic stimuli necessarily have temporalcomponent

Constant sounds drop out of conscious awareness Sounds are perceived from sources in all directions

Localisation

Different factors influence ability to perceivelocation 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 relativemotion between sound source and observer

• Changing volume:Sound perceived as approachingwhen volume gradually increases (and vice versa)

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External ear - pinnae

Why do stereo speakers reproduce theoriginal sound stage?– Effect of head + pinnae– Hence in-ear speakers produce stage

‘inside head’

Head-related transfer functions (HRTF)– Modify signal– Simulate effect of head and pinnae

Haptic perception

Touch is a mechanical or thermal contact with theskin.

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

Also thermal sensing Difficult to characterize in quantitative way Sensations of skin adapt with exposure to stimuli

Skin Force sensing

Receptors in muscles and joints– Inside the body

Perception of movement, position, andtorque of limbs and other body parts

Varying joint angles and muscularlength

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Vestibular system

Movement andorientation in space– See

http://en.wikipedia.org/wiki/Vestibular_system

Inner ear: vestibulum– Orthogonal canals for

rotation– Saccule/utricle for

linear acceleration

Vestibulo-occular reflex

To be able tosee whilemoving– Push-pull

between RHand LH canals

– 3 neurons linkto eye

– <10 ms lag ineye for headmovement

Health and safety issues

Flicker– Can trigger epilepsy

Eyestrain– Dissociation of accommodation and

convergence– Lack of visual calibration, especially HMDs

Use of laser light Movement injuries

Cybersickness

Reverse form of motion sickness– Conflicting signals from visual and vestibular

systems

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

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Symptoms

Vestibular disturbances– Head spinning

Reduced hand-eye coordination Nausea Effects take time to reduce after

exposure Nervous system does adapt