Effect of Grouping on Lightness
Week 2: Anatomy of the Eye and Visual Acuity
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Brief survey of the anatomy of the eye
Visual size and visual angles
Visual acuity – its measurement and the impact of acuity reduction.
Structures of the Eye
Visual Angle
Angular Size in radians Physical Size / Viewing Distance
Angular Size in degrees 57.3 × Angular Size in radians
= 57.3 × Physical Size / Viewing Distance
where the physical size and viewing distance must be measured in the same units (typically, mm, cm, or inches). These approximations hold when the physical size is significantly smaller than the viewing distance.
How large does your fist look at arm’s length?
Measure the size of your clenched fist, H.
With your arm extended, measure the eye-to-fist distance, d.
Use the formula to compute the visual angle A of your fist in radians.
Convert to degrees.
Three Dimensions of Vision Measurement and Loss
Low vision is usually discussed in terms of:
Reduced acuity
Reduced contrast sensitivity
Visual-field loss.
Demo of Lighthouse Distance Acuity Chart [Position student at 4 m to demonstrate designation of viewing distance, and point out the various lines on the chart as I discuss them.]
What is Visual Acuity?
Visual acuity: determines the smallest visual size (visual angles) of objects that can be recognize.
An object’s visual size is determined by its size/distance ratio.
At a fixed distance, acuity determines the smallest objects that can be recognized, such as letters on an eye chart.
For objects of a given size, acuity determines the greatest distance at which the objects can be recognized, such as the maximum distance for reading road signs.
Lighthouse Letter Acuity Chart
Acuity Ranges
Traditional standard for Normal vision – 20/20
“Normal” Vision – 20/8 – 20/40
Mild Low Vision – 20/40-20/80
Moderate Low Vision – 20/80 – 20/200
Legal Blindness (U.S.) – 20/200
Severe Low Vision 20/200 – 20/1600
“Blindness” (WHO) – < 20/400
Ultra Low Vision - < 20/1600
Light Perception (LP)
Total Blindness
Simulating 5 Levels of Acuity
MNREAD Reading Acuity Chart
Sample MNREAD data
Original 20/40
20/80 20/160
20/320 20/640
Blank Page
20/320 20/640
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