Visual acuity for optmetry students by DR.Ihsan Bazirgan

luctures for optometry students

1. Visual Acuity
Dr. Ihsan Ali BAZIRGAN
URUK University
Baghdad, IRAQ

2. Visual Acuity
ConceptofVisualAcuity.
VisualDiscriminationTasks.
TypesofVisualAcuity
ScalesofVisualAcuity
OptotypesformeasuringVisualAcuity
FactorsthataffectVisualAcuity

3. Concept of visual acuity
Visual Acuity is the ability to detect,
determination or recognize the details of
objects
the visual system’s ability of resolution.
It is quantified by the minimum dimension
that an object must have in order for the
observer to be able to identify, distinguish or
simply detect it.

4. Definition
The resolving power of the eye used to
assess and quantify the eye’s ability to
resolve varying letter sizes
Visual acuity is dependent on:
The refractive error of the eye
The health and the integrity of the eye
The test targets used
The test conditions

5. Why need a visual acuity test?
Diagnostic tool
Baseline data
Measures progression of disease
Evaluates treatment
To measure the patient’s progress
visual acuity must be assessed at
every presentation.

6. Measurement
The measurement of VA is done to monitor
change in vision with disease progression
of disease and/or treatment plan
It is a way to measure “blur” vision
Visual acuity must be measured on every
patient at every visit!

7. Standard Test Distances
Distance visual acuity (DVA)
20ft or 6m is equivalent to optical
infinity
Near visual acuity (NVA)
40cm

8. Classification of Visual Acuity
Measurements
Minimum visible
Ability to perceive the presence of a single target
Minimum detectable(‘distinguishable’)
Ability to identify features or internal arrangements in a
visible target
Minimum separable
Ability to detect spatial separation between adjacent
lines i.e2 lines are separated by an interval
Black-&-white grating tests

9. Different acuity measures
Hyper acuity
Ability to perceive the alignment and
orientation of linear stimuli
Vernier acuity
Minimum recognisable
Ability to determine the smallest
optotypes
Symbols, pictures, letters
Snellen and LogMARcharts

10. Concept of visual acuity
Is normally expressed as the inverse of the
angle in minutes subtended by the minimum
detail of the object. )('
1
u
VA

11. Tasks of Visual Discrimination.
Types of Visual Acuity
•Task of detection or minimum visible
•Task of resolution or minimum separable
•Task of recognition or minimum recognizable
•Task of localization
Detection , resolution, recognition and localization
Tasks of visual discrimination. Types of
visual acuity

12. Tasks of Visual Discrimination.
Types of Visual Acuity
The observer should be able to decide on
the presence or absence of an object in
their field of vision
This is often calculated by the angle that is
subtended by the object that is observed
by the observer.
Task of detection or minimum visible
The capacity of detection is measured through the
inverse of the minimum angle that an object should
subtend in order to be perceived.

13. TasksofVisual Discrimination.
TypesofVisual Acuity
Task of detection of the minimum visible
Types of test:
-Luminous points on black background
-Black point on luminous background

14. Tasks of Visual Discrimination.
Types of Visual Acuity
Task of detection or minimum visible
Test Limit
Luminous Object Points 1”
Dark Object Points15” a 30”
Lines Until 1’
Standard values for minimum visible

15. Tasks of Visual Discrimination.
Types of Visual Acuity
Task of resolution or minimum separable
The observer should be able to decide if
two objects that are very close together
are separated or not.
It is usually quantified by the minimum
angle of separation that should exist
between the two objects so that they can
be perceived as separate by the observer.

16. Tasks of Visual Discrimination.
Types of Visual Acuity
Type of test:
-Points or lines saperated by a variable
distance, which are clearly distinguishable if they
are presented individually.
Task of resolution or minimum separable

17. Tasks of Visual Discrimination.
Types of Visual Acuity
Test Limit
Points 1’ to 3’
Lines 40” to 1’
Foucault gratting Until 1’
Standard values of minimum separable
Task of resolution or minimum separable

19. Tasks of Visual Discrimination.
Types of Visual Acuity
Task of recognition or minimum recognizable
The observer should be able to recognize
the details of the object
Usually quantified by the inverse of the
angle that subtends the object to be
recognized.
The minimum recognizable is the Visual
Acuity normally used by an optometry clinic

20. Tasks of Visual Discrimination.
Types of Visual Acuity
Types of test (Optotypes):
-Letters
-Numbers
-Snellen “E”
-Landolt Rings
Task of recognition or minimum recognizable

21. Tasks of Visual Discrimination.
Types of Visual Acuity
Size of the test (Optotypes):
The size of the optotypes used is often a 5sx5s box
or a 4sx5s rectangle, with s being the minimum
detail of the object to recognize.
Recognition or minimum recognizable

22. Snellen Chart
Snellen= visual
angle subtended at
the nodal point of the
eye by altering the
size of the component
parts of a letter

23. Snellen Chart


24. VA is recorder in the form of
d/D
• d is the testing distance
(numerator)
• D is the line containing the
smallest letter identified
(denominator)
• For example, if a chart
specifies that a client is
positioned six meters from
the chart (6) and can read the
nine line (9), they have a VA
of 6/9.

25. Snellen Letter Construction
Each component of a letter
subtends 1 minute of arc at
the nodal point
1 cone stimulated & 1 cone
gap for appreciation
Whole letter subtends 5
minute of arc at the nodal
point
Many can actually
discriminate 0.5 min of arc

28. 
15mm
7mm
N

29. Tasks of Visual Discrimination.
Types of Visual Acuity
Test Limit VA
Optotypes 1’ 1
Task of recognition or minimum recognizable
Standard values of the minimum
recognizable

30. Tasks of Visual Discrimination.
Types of Visual Acuity
Task of localization.
The observer should be able to distinguish
small displacements within the test
It is usually calculated by the minimum
angle of displacement that the observer
can perceive.
The classic measurement of localization is
called Vernier acuity

31. Tasks of Visual Discrimination.
Types of Visual Acuity
Types of test:
-Lines that shift until the observer sees
them aligned
Task of localization.
u

32. Minimum Discriminable Acuity-angular size of the smallest
changein a feature (e.g., position) that one can identify.
Vernier acuity is termed a hyperacutiy, limit of 3 arcsec(0.0008deg).
This is 10 times smaller
than width of a foveal
cone. Optics of eye spread
out the photons, and the
information to distinguish
A from B is present, but it
must be cortical neurons
that interpolate this
information with high
resolution.

33. Tasks of Visual Discrimination.
Types of Visual Acuity
Test Limit
Alignment
(Vernier)
<1”
Standard values of minimum
displacement
Task of localization.

34. Scales of visual acuity
DECIMAL OR MONOYER SCALE
Visual Acuity values are expressed as the inverse
of the angle in minutes of arc that subtends the
minimum detail of the test)('
1
u
VA
d
The standard value is considered to be AV = 1
The rest of the acuities are expressed as a decimal
Example: 0.8, 1, 1.2

35. Concept of Visual Acuity
Relation of visual acuity with the details
size and distance of observation.)('
1
u
VA s
d
VA
4
109'2


 d
s
radu )( rad
4
10921

.'

36. 1 degree = 60 minute
1 minute of an arc = 1/60 degree
180 degree = pi radian=л
1 degree = pi/180 radian
therefore 1/60 of pi/180 = 1minute
0r 1 minute = pi /( 60 .180) radian
therefore 1 min . = 2.9 1.10^-4 radian

37. Concept of Visual Acuity
Relation of visual acuity and observation
distance.s
d
VA
s
d
VA
2
4
2
1
4
1
109.2
109.2





 1
2
12
d
d
VAVA

38. Concept of Visual Acuity
Relation of visual acuity with size of
object.2
1
12
s
s
AVAV 2
4
2
1
4
1
109.2
109.2
s
d
VA
s
d
VA







39. Scales of Visual Acuity
SNELLEN SCALE
Visual acuity values are expressed as a fraction
between the distance that the observation is made
(d’) and the distance that the detail of the test
subtends an angle of 1 minute (d)d
d
VA
S
'

•Usually, the distance d’ is 6 meters or 20 feet.
•The standard value for VA is considered to be =
6/6 or 20/20.
•Example: 6/9, 6/6, 6/5; 20/30, 20/20, 20/15

40. Recording
Record the VA line for which more
than 2 letters (or less than half the
number) are read incorrectly

41. Examples
Two out of five letters on the 6/6 line are read
incorrectly
6/6 -2 OR 6/6 P
All letters on the 6/6 line are read correctly,
and one letter on the next line down is read
correctly
6/6 +1
One of the letters on the 6/6 line is read
incorrectly, and two letters on the 6/5 line are
read correctly
6/6 -1/+2

42. Scales of Visual Acuity
SNELLEN SCALE AND DECIMAL
SCALE RELATIONSHIP
If we have a visual acuity in the Snellen Scale we
obtain its decimal value by dividing the two
distances of the Snellen ScaledS VA
ud
d
VA 
)('
1'
Example: dS
dS
dS
dS
VAVA
VAVA
VAVA
VAVA




0.1
6
6
2.0
30
6
5.0
12
6
125.0
48
6

43. Scales of Visual Acuity
SNELLEN SCALE AND DECIMAL SCALE
RELATIONSHIP
If the visual acuity is in the decimal scale, we can
obtain the Snellen scale value by deducing the
value of d, once d’ is fixed at a value of either 6 or
20. d
sd
VA
od
dVA
d
d
u
VA
)206(''
)('
1

Example: 30
6
30
2.0
6
6'2.0
5
6
5
2.1
6
6'2.1


sd
sd
VAddVA
VAddVA

44. Minimum Angle Of Resolution (MAR)
50.1
76.0
11
......76.0.......
9
6
5.0
2
11
........2.......
3
6
1
1
11
.......1.......0.1
6
6



d
ds
d
ds
d
ds
VA
MARVAVA
VA
MARVAVA
VA
MARVAVA

45. Scales of Visual Acuity
LogMAR Scale
In the logarithmic scale of the minimum angle of
resolution (MAR), as the name indicates, visual
acuity is expressed by the logarithm of the angle
subtended by the recognizable detail of the test.MARVA
MARlog
The standard value is VA
MAR = log MAR = 0
Example: 0.2, 0, -0.1
It is the only scale where the value for visual acuity can
be negative. In addition, a negative value is considered to
be a good value.

46. Scales of Visual Acuity
DECIMAL SCALE-logMAR SCALE
RELATION
If we have a visual acuity in the decimal scale we
can obtain the logMAR value by calculating the
logarithm of the angle expressed in minutes, which
is the inverse of the decimal visual acuityVA
uMAR
VA
u
u
VA
d
1
log)('loglog
1
)('
)('
1

Example: 1.025.1log
8.0
1
log
1
loglog8.0
08.083.0log
2.1
1
log
1
loglog2.1


VA
MARVA
VA
MARVA
d
d

47. Scales of Visual Acuity
Acuity
Snellen
Acuity
decimal
Mínimum angle of
resolution (MAR)
LogMAR
6/3 2.00 0.50 -0.30
6/4 1.50 0.67 -0.18
6/5 1.20 0.83 -0.08
6/6 1.00 1.00 0.00
6/9 0.67 1.50 0.18
6/12 0.50 2.00 0.30
6/18 0.33 3.00 0.48
6/24 0.25 4.00 0.60
6/36 0.17 6.00 0.78
6/60 0.10 10.00 1.00

48. Optotypes for measuring visual
acuity
In a clinical practice it is common to use
Optotype cards
An optotype card is formed by rows of optotypes
of progressively smaller sizes.
They are designed to be presented at a fixed
distance and on one side they usually show the
visual acuity value belonging to each row.

49. Optotypes for measuring visual
acuity
Optotype cards
Letter Cards
Snellen

50. Optotypes for measuring visual
acuity
Optotype Cards
Letter Cards
Bailey-Lovie

51. Optotypes for measuring visual
acuity
Optotype Cards
Letter Cards
Optotype Projectors

52. Optotypes for measuring visual
acuity
Optotype Cards
Number Cards
Optotype Projectors

53. Optotypes for measuring visual
acuity
Optotype Cards
One Character Optotype Cards
Landolt Rings

54. Optotypes for measuring visual
acuity
Optotype Cards
One Character Optotype Cards
Snellen “E”

55. Optotypes for measuring visual
acuity
Optotype Cards
Infant Visual Acuity
Other Cards