Cones for detail
Rods for less light
Iris, Lens, retina
Eye needed for long duration
Red wavelengths larger so used for long distance warning.
We need of light
Finer the task the more illumination is required.
Human eye lens can change its shape and flattens when there is dim light.
Quality and Quantity of light is important.
Environment is affected.
Artificial illumination is affected by ;
Type of source used
Quality of light depends
Quantity of light flux from lamp
sped of retinal impression
Discrimination of brightness
Nervous Muscular tension
Fatigue of ocular muscles can reduce ability to see in detail.
normalcy of heart rate
normal rate of reading
max rate of reading
how precise task is
performance in demonstration in visual task
experience in daylight
1000 increase not make a difference
contrast sensitivity 100 ft candles
ability to distinguish increases with llumination
nervous muscular tension reduces for 10ft candles
to 500 ft candles
rate if blinking needed
after reading for an hour
Blinking reduces as 1 - 100 foot candles.
So blinking rate reduces with increased illumination
Cone nerve cells used for micro vision
decrease in convergence much lower for 100 ft candles.
So nervous muscular tension increases with low illumination
Recommend illumination levels
Black thread on black cloth 800 foot candles
newspaper stock equation need 100 ft candle
typing on dark blue paper 80
Telephone directory 60
newspaper text 40
Great printing 8
what is acuity
distinguish detail depending on brightness of object
characteristics of light entering eye
1 foot 1000 ft lamberts 95% accuracy
Requirements to get good visual acuity depends ion good illumination but beyond a certain point there is saturation.
visual acuity is better when there is brighter surround.
Speed v brightness
80% white background not much increase in speed with increase in brightness
Grey background benefited for increase in brightness
decrease in pupil size
visual acuity reduces
elasticity of pupil
flexibility of optic lens
Illumination needed to increase with age.
Monochromatic light is best for distinguish images.
red green and blue can create all colours.
blue can be sensed fastest green slowest
for good visibility brightness of of surround should be greater than 10,0 ft
Acuity is ability to distinguishes details depends on on brightness of object, characters iof light entering eye contrast maintained
combination of colours reduces acuity known as chromatic aberration