Week 1 - Visual Efficiency Flashcards
What is ergonomics?
Scientific discipline concerned with human interactions with others or systems in carrying out purposeful activity
- Improves human wellbeing and compatibility
- First developed after ww2
Why do optoms need to be aware of ergonomics?
• Consider environmental and occupational needs of patients
• Screen patients for ocular abnormalities
Posture hints with monitor:-
• 15-30° below eyesight
• No light shining in eyes/screen
• Forearms + thighs 90° from spine
3 focus points for Visual ergonomics:-
• Visual performance
- Illumination
- Glare
•Visual comfort
- Light intensity
- colour rendering
• Visual environment
- Shadiness
- Direction of light
- Light colour
Difference between night day + cells in retina:-
• Difference between night/day is 10^11 fold change in intensity
• Cones: 6-7million (red 64%, green 32%, blue 2%; small receptive fields
• Rods: 120million rods; not sensitive to colour; large receptive fields
Cellular Receptive field facts:-
• Receptive neuron field is a region where presence of stimulus alters firing of neurone
• “On centre”
- Inhibiting surround, excitatory centre
• “Off centre”
- Inhibiting centre, excitatory surround
Pupil and light adaptation:-
• Pupil diameter determines amount of light illuminates retina
• Age reduces pupil diameter (senile miosis)
- Reduces overall level of light, reducing ability to cope with different levels of light
What age factors influence on light adaptation?
• Transmission through lens decreases due to ocular media opacification
• Retinal changes; AMD
• Elderly have problems adapting to night
- Differences in adaptation between young/old differ by factor of 100-1000
define light adaptation, who is it important for and what it can be affected by:-
• Light adaptation: quick adaptation of the eye to the background illumination in order to be able to distinguish objects in this background.
- Changes in contrast
- Can either be dark>light or light>dark
• Important for sailors, military, drivers
- Dark adaptation affected by; Retinitis pigments and Vit A Deficiency
Time taken for a visual response:-
• Space scotoma
- Partial alteration in visual field with degradation of visual acuity that is surrounded by normal acuity. Blind spot is example.
• 0.1 second for brain to see image and project a visual sensation, therefore pilots traveling 150ft in 0.1s will have a space scotoma of 0.1s/150ft
What is flicker:-
• Critical flicker fusion frequency is highest temporal frequency that can be resolved (Hertz)
• Ferry-porter law:- CFF is proportional to log of luminance of flickering stimulus
Flicker examples:-
• Driving with flicker through sunlight through array of trees
• Elderly people who have smaller pupils: less prone to flicker
• Rotation of machinery at more than CFF can blur rotating blades into invisibility
- Stroboscopic effects between lights sources and machinery may occur
- Neutral density filters could reduce the apparent flicker
Position in visual field/visual field size:-
• Must scan visual field to place fovea on object
• Normal human VA: 60 deg nasally, 100 deg temporarily, 60 deg above, 75 deg below
• UK minimal requirement is 50 deg either side vertically, and 20deg above and below
• The macula: Central 13 deg of visual field
• The fovea: Central 3 deg of visual field
Viewing distance is affected by:-
• Depends on accommodation and convergence
• Accommodation amplitude falls with age
• Accommodation response is slow
Viewing distance definition:-
• Weston devised a system for specifying distances, based on demands of accommodation and convergence
- Teloramic (d>2) (D<0.5)
- Mesoramic (2>d>0.25) (0.5<D<4)
- Ancoramic (d<0.25) (D>4)
Visual Classification of task detail:-
Classification:-
• Large : >12’ : 6/72
• Medium : 4’-2’ : 6/12 to 6/24
•Small: 2’-1’: 6/6 to 6/12
•Very small: 1’ : >6/6 (magnification)
Contrast:-
Contrast = Lmax - Lmin/ Lmax
• Object contrast is reduced in foggy weather, snowy conditions and rain or at night
Clarity:-
• Clarity related to image sharpness
• Poor lighting impairs the perception of small details
• Poor weather can affect clarity of drivers
Task surround:-
• Larger light area surrounding a task gives better visual efficiency than a small surround
Colour of task:-
• Colour vision is important for recognition of traffic lights or road signs
• Accurate matching of colour in paints, fabrics, vegetables requires excellent colour discrimination
Motion vision is measured as:-
• Dynamic Visual Acuity:- DVA: Ability to perceive lateral motion and motion in depth
• Dynamic VA is lower than static visual acuity
Stereopsis Task Requirements examples:- (2 total)
• Monocular vision - problems:
pouring coffee
• Binocular vision:
positioning large objects with a crane.
Visual field requirements for driving tips:-
• Good peripheral vision is essential for driving.
• A restriction of the visual field can never be fully overcome.
Improved by:-
• Head and eye movements and adding extra mirrors to the car can help.
Reduced by:
•Artificial reductions eg: thick spectacle frames and car design.
• Pathological: Glaucoma, retinitis pigmentosa, cataracts may cause problems.
