Visual Acuity Flashcards
(32 cards)
visual acuity
the criterion for an adequate refraction
foundation on which the vision exam is built
not necessarily the final endpoint of an exam
Types of visual acuity
Detection acuity
Resolution acuity
Recognition acuity
Contrast sensitivity
Detection acuity
seeing an object on a background that is different from the object itself
Resolution acuity
the ability to see a misalignment (a line is broken)
Recognition acuity
being able to identify two or more objects on a contrasting background
acuity that is most commonly thought of
Low literacy or non verbal tests
Tumbling E & Landolt C
Recognition acuity tests
Snellen is the most common (E on top)
Logmar chart
Contrast sensitivity
minimum contrast that is still perceivable by the patient
measured by test gratings of equal width
can vary contrast and frequency
30 cycles/degree = 20/20
Visual Acuity Notation Numerator
how far away the patient stands from the object
Visual Acuity Notation Denominator
distance at which the letters subtend 5’ of arc at the nodal point of the eye or the letter the patient reads
normal terms: distance at which the letters take up five minutes (5/60ths) of one degree of space of their visual field
What 20/20 means
If a person is standing 20 feet away from a target, the smallest letters identified would subtend 5’ of the visual field
What 20/40 means
A person standing 20 feet away from a target, the smallest letters identified would subtend 5’ of arc if that person stood 40 ft away
Smallest object a human eye can see
1’ of arc detail
limited by cone density and neural connections
Finding 3 month old visual acuity
fix & follow in both eyes
Finding newborn old visual acuity
pupils react to light
Finding 93 year old visual acuity (cateracts)
finger counting from 1 ft away
if it’s really bad may need to ask for which direction your hand moves
Objective acuity measurements
optokinetic (OKN) drum
catford drum
OKN drum
measures visual perception based on the optokinetic response (subconscious response to movement)
Catford drum
tests for visual perception also using optokinetic response, but has bars of varying sizes
no longer used (scary metal thing)
VA sc
visual acuity without correction
VA cc
visual acuity with correction
Snellen letter size
- indicates at what distance a letter subtends 5’ of arc at the nodal point
- based on the tangent of the angle
- height of a 20/20 letter for a chart designed for 20 ft (6 m) is 8.73 mm (tan(5’) = height * 6000 mm)
Relationship between Snellen Letters
Linear
a 20 ft chart: 20/200 = 87.3 mm, 20/40 = 17.46 mm, etc
Hardest Snellen letter
B
