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Optical Theory I > Visual Acuity > Flashcards

Flashcards in Visual Acuity Deck (32)
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1

visual acuity

the criterion for an adequate refraction
foundation on which the vision exam is built
not necessarily the final endpoint of an exam

2

Types of visual acuity

Detection acuity
Resolution acuity
Recognition acuity
Contrast sensitivity

3

Detection acuity

seeing an object on a background that is different from the object itself

4

Resolution acuity

the ability to see a misalignment (a line is broken)

5

Recognition acuity

being able to identify two or more objects on a contrasting background
acuity that is most commonly thought of

6

Low literacy or non verbal tests

Tumbling E & Landolt C

7

Recognition acuity tests

Snellen is the most common (E on top)
Logmar chart

8

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

9

Visual Acuity Notation Numerator

how far away the patient stands from the object

10

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

11

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

12

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

13

Smallest object a human eye can see

1' of arc detail
limited by cone density and neural connections

14

Finding 3 month old visual acuity

fix & follow in both eyes

15

Finding newborn old visual acuity

pupils react to light

16

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

17

Objective acuity measurements

optokinetic (OKN) drum
catford drum

18

OKN drum

measures visual perception based on the optokinetic response (subconscious response to movement)

19

Catford drum

tests for visual perception also using optokinetic response, but has bars of varying sizes
no longer used (scary metal thing)

20

VA sc

visual acuity without correction

21

VA cc

visual acuity with correction

22

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)

23

Relationship between Snellen Letters

Linear
a 20 ft chart: 20/200 = 87.3 mm, 20/40 = 17.46 mm, etc

24

Hardest Snellen letter

B

25

Easiest Snellen letter

L

26

Myopia

*aka near sighted
*can't see well far away
*can see nearby
*have elongated eye : light focuses in front of eye : need - lens to correct

27

hyperopia

*aka far sighted
*can't see well nearby
*can see far away
*have shortened eye : light focuses behind eye : need + lens to correct

28

Things that affect visual acuity

pupil size
pinhole acuity

29

Pinhole acuity

has a pinhole of 1 mm diameter
tested with a pinhole occluder

30

Alger brush

Used to help mend rough edged corneal abrasions (cleans up lines and helps epithelial cells quickly & cleanly heal)
*Bowman’s stops it

31

Layers of the cornea

Carneal epithelium
Bowman’s membrane
Stroma
Dua’s layer
Descemet’s membrane
Corneal endothelium

32

Dilation drops

Excitate dilater & paralyze sphincter