Visual Ergonomics

Question 1

What are the 14 key points for a Visual Task Analysis?

Answer 1

1. Adaptation effects – light/dark/glare
2. Time taken to respond to particular stimulus
3. Flicker (strobe effect) – depends on (nature of) task
4. Position in VF
5. Viewing distance – how much of a) and b) is required:
   a) Accommodation
   b) Convergence
6. Visual subtense of task detail (size/acuity)
7. Motion of task
8. Contrast of task detail
   o How much dark or light there is to clarify task detail – to make more visible
9. Colour of task
   o E.g. if painter/decorator -> need to know shades/hues so need colour vision
   o Therefore, colour of task in front of person being analysed for visual efficiency needs taken into account
10. Clarity of task detail
11. Stereopsis requirements
12. VF requirements  big VF (pilot) or small VF (watchmaker)
13. Hazards
    o Welding – FBs – light may burn hole in retina  need appropriate protection that ↓ hazard but allows for a good VF
14. Training Requirements

Question 2

Describe Dark Adaptation?

Answer 2

• Eye operates over large range of light
• Difference between a starry night & sunny night = ~10” fold change in light intensity – we can see clearly in this range
• Dark adaptation curve reflects activity of cone and rod pathways
• Cones – 6-7million, small Receptive Field detail
  o ‘Red’ – 64%
  o ‘Green’ – 32%
  o ‘Blue’ – 2%
• Rods – 120million, not sensitive to colour, large RF
• RF of neuron is retinal region where presence of stimulus alters firing of that neuron
• Pupil diameter determines amount of light that illuminates retina
• Age ↓ pupil diameter under all conditions – senile miosis
• ↓ overall level of light for elderly people and ↓ their ability to cope w/ different levels of light
  o E.g. driving at night is a struggle
• As age, transmission of light through ocular media ↓ and lens yellows
• Retinal changes e.g. AMD occur w/ age
• Dark adaptation curves for young & old adults differ by factor of 100- 1000

Question 3

Who is Dark Adaptation important for?

Answer 3

Process of dark adaptation important for sailors, military, drivers -> can be adversely affected by factors such as retinitis pigmentosa (can only see through centre of retina) or Vitamin A deficiencies (malnutrition)

Question 4

Describe Light Adaptation?

Answer 4

• Quick adaptation of eye to background illumination in order to distinguish objects in background
• When first enter extremely bright environment – apparent contrast of object is low
  o In <1min contrast appears normal

Question 5

Describe the time taken for visual response (space scotoma)?

Answer 5

• Space scotoma: area of partial alteration in VF where there’s degradation of VA but surrounded by normal acuity e.g. blind spot
• Eyes take ~0.1 seconds to ‘see’ image and project to brain to initiate visual sensation

Question 6

Describe flicker?

Answer 6

• Critical Flicker Fusion Frequency (CFF) is highest temporal frequency (at high contrast modulation) that can be resolved
• Ferry-Porter law: CFF is proportional to logarithm of luminance of flickering stimulus
• Produced while driving by sunlight incident through a regular array of trees
• Elderly people, w/ smaller pupils, less prone to suffer from flicker (getting less light on retina)
• Rotation of machinery at >CFF can blur rotating blades into invisibility
• Stroboscopic effects between light and machinery may occur
• Neutral density filters may ↓ apparent flicker

Question 7

Desccribe Position in VF?

Answer 7

• Normal human VF:
  o 60° nasally to 100° temporally from vertical meridian
  o 60° above and 75° below horizontal meridian
• Minimum driving requirement: 50° either side of vertical, & 20° either side of horizontal
• Macula: central 13° of VF
• Fovea: central 3°
• Scan VF to put fovea on object – require excellent oculomotor co-ordination
  o Like join the dots -> sent to visual cortex -> image produced

Question 8

Describe Viewing Distance, accommodation and Weston 1962 experiment?

Answer 8

• Demands on observer’s accommodation and convergence
• Accommodation amplitude ↓ with age
• Accommodation response is slow and gets slower with age
• Weston 1962 experiment:
  o Teloramic: distance of task >2m —> accommodation <0.5D
  o Mesoramic: 2>distance>0.25 —> accommodation 0.5<D<4
  o Ancoramic: d<0.25 —> accommodation >4

Question 9

Describe Visual Subtense of Task Detail?

Answer 9

• VA depends on image luminance
• Large - >12’ -> 6/72
• Medium small - 4’-2’ -> 6/12 to 6/24
• Small – 2’-1’ -> 6/6 to 6/12
• V small – 1’ -> 6/6 magnification

Question 10

Describe Contrast (Weber contrast)?

Answer 10

• Weber contrast:
  o (Lmax-Lmin)/Lmax
  o Lmax & Lmin = luminances of detail or surround, depending on which is brighter
  o Object contrast is ↓ in foggy weather, snowy conditions and rain or at night
• Sinusoidal luminance gratings: L to R = low to high
• Spatial frequency

Question 11

Describe Clarity?

Answer 11

• Related to image sharpness
• Poor lighting impairs perception of small details
• Inclement weather e.g. fog or rain can severely restrict visual abilities of drivers

Question 12

Describe Task Surround?

Answer 12

• Layer light area surrounding task gives better visual efficiency than a small surround

Question 13

Describe Colour of Task?

Answer 13

• Colour vision important for recognition of traffic lights and road signs
• Accurate matching of colours in paints/fabrics/vegetables – good colour discrimination required
• Observer should have good colour vision and appropriate lighting (>1000 lux) & colour rendering properties (artificial daylight tubes) should be provided
  o Metamerism: abnormal/adequate lighting can cause colours to appear v differently
• Rod monochromat has no cones – lacks acuity (& colour vision)
• Trichromat – normal viewer – sees lots of colours
• Protanope – cannot see red

Question 14

Describe Motion?

Answer 14

• Dynamic VA:
  o Ability to perceive lateral motion & motion in depth
  o As critical detail (mins of arc) ↑ & motion ↑, lose acuity
• Dynamic VA is lower than static VA
• High demand in cricket or baseball – small ball travelling v fast
  o Low demand in golf, medium demand in football

Question 15

Describe Steropsis Requirements?

Answer 15

• Stereoacuity required to discriminate 2 objects in depth at distance w/ separation of depth
• Monocular vision – problems: pouring coffee – likely to spill it
• Binocular vision – positioning large objects w/ crane

Question 16

Describe VF Requirements?

Answer 16

• Good peripheral vision essential for driving
• Restriction of VF can never fully be overcome
• Head & eye movements & adding extra mirrors to car help
• VFs can be artificially ↓, e.g. thick glasses frames & car design
• Glaucoma, retinitis pigmentosa, cataracts may cause problems