Visual Acuity Flashcards
What are the four types of visual acuity?
Detection/discrimination
Vernier (hyperacuity)
Resolution
Recognition
What is detection/discrimination acuity?
What’s its minimum size?
The ability to perceive an object
0.5-0.15 secsarc
What is resolution acuity?
What’s its minimum size?
The ability to perceive two objects as separate
0.5 minarc
What is recognition acuity?
What’s its minimum size?
What other types of acuity are required for this to be possible?
The ability to identify what the object is
1 minarc
Detection and resolution
What is vernier acuity?
What’s its minimum size?
The ability to detect misalignment
2-6 secsarc
What factors can affect VA?
Age
Retinal eccentricity
Ocular disease (esp macular)
Retinal illuminance
Uncorrected refractive error
Test used
Letter spacing (crowding)
What is aliasing?
When a stimulus is detected, but the orientation of the stimulus cannot be perceived due to undersampling (signal doesn’t reach threshold)
What are the tests that can be used to measure VA?
Bailey-Lovie (logMAR)
Snellen
Tumbling E’s
Landolt C’s
Preferential looking
Vanishing optotypes
Letter matching
Picture matching (Snellen Kay picture test)
Visual evoked potentials (VEPs)
Optokinetic nystagmus
Cardiff acuity cards
What are the advantages of a Snellen chart?
Cheap
Portable
Quick and easy to measure VA
Easy to understand
What are the advantages of a Bailey-Lovie chart?
More accurate scoring than Snellen
Partial lines easier to score
Even and equal letter and line spacing
What are the disadvantages of a Snellen chart?
Unequal spacing between lines and letters (can induce crowding)
Different number of letters on each line
Hard to score partial lines
Possibility of patient memorising letters (becomes unreliable)
What are the disadvantages of a Bailey-Lovie chart?
Large - hard to transport
Expensive
Time-consuming
Hard for patient to understand score
