theme 4: visual acuity

Theme 4: Visual Acuity

• Concept of Visual Acuity.• Visual Discrimination Tasks. Types of

Visual Acuity• Scales of Visual Acuity• Optotypes for measuring Visual

Acuity• Factors that affect Visual Acuity

Concept of visual acuity

Visual Acuity is the capacity to detect, resolve or recognize the details of

objects

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.

Concept of visual acuity

Is normally expressed as the inverse of the angle in minutes subtended by the

minimum detail of the object.

)('

1

uVA

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

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 quantified by the angle that is subtended by the object that is

perceived by the observer.

Task of detection or minimum visible


Task of detection of the minimum visible

Types of test:

- Luminous points on black background

- Black point on luminous background


Task of detection or minimum visible

Test Limit

Luminous Object

Points 1”

Dark Object Points 15” a 30”

Lines Until 1’

Standard values for minimum visible


Task of resolution or minimum separableThe 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.


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


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


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


Types of test (Optotypes):

- Letters

- Numbers

- Snellen “E”

- Landolt Rings



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


Test Limit VA

Optotypes 1’ 1


Standard values of the minimum recognizable


Task of localization.

The observer should be able to discriminate small displacements

within the test

It is usually quantified by the minimum angle of displacement that

the observer can perceive.

The classic measurement of localization is called Vernier acuity


Types of test:

- Lines that shift until the observer sees them aligned


u


Test Limit

Alignment (Vernier) <1”

Standard values of minimum displacement


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

uVAd

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

Relation of visual acuity with the details size and distance of observation.

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1

uVA

Concept of Visual Acuity

s

dVA

4109'2

ds

radu )(

rad410921 .'


Relation of visual acuity and observation distance.

s

dVA

s

dVA

24

2

14

1

109.2

109.2

1

212 d

dVAVA


Relation of visual acuity with size of object.

2

112 ss

AVAV

2

4

2

1

4

1

109.2

109.2

s

dVA

s

dVA

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

dVAS

'

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


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 Scale

dS VAud

dVA

)('

1'

Example:

dS

dS

VAVA

VAVA

66.030

20

2.15

6


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.

dsd VA

oddVA

d

d

uVA

)206(''

)('

1

Example:

25

2025

8.0

2020'8.0

5

65

2.1

66'2.1

sd

sd

VAddVA

VAddVA


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.

MARVAMAR logThe standard value is VAMAR = log MAR = 0Example: 0.2, 0, -0.1It 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.


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 acuity

VAuMAR

VAu

uVAd

1log)('loglog

1)('

)('

1

Example:

1.025.1log1

loglog8.0

08.083.0log1

loglog2.1

VAMARVA

VAMARVA

d

d


DECIMAL SCALE-LogMAR SCALE RELATIONSHIP

With a visual acuity in the logMAR scale we obtain its value in the decimal scale

calculating the antilogarithm of the LogMAR and then calculating the inverse.

MARantVAMARantuuMARVA dMAR log

1log)(')('loglog

Example:

8.026.1

1

log

126.1log1.0log

2.183.0

1

log

183.0log08.0log

MARantVAMARantMARVA

MARantVAMARantMARVA

dMAR

dMAR


Minimum angle of resolution

Acuity decimal

Acuity Snellen

LogMAR

0.50 2.00 6/3 -0.30

0.67 1.50 6/4 -0.18

0.83 1.20 6/5 -0.08

1.00 1.00 6/6 0.00

1.50 0.67 6/9 0.18

2.00 0.50 6/12 0.30

3.00 0.33 6/18 0.48

4.00 0.25 6/24 0.60

6.00 0.17 6/36 0.78

10.00 0.10 6/60 1.00

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.


Optotype cards

Letter Cards

Snellen


Optotype Cards

Letter Cards

Bailey-Lovie

Optotype Cards

Letter Cards

Optotype Projectors


Optotype Cards

Number Cards

Optotype Projectors



Optotype Cards

One Character Optotype Cards

Landolt Rings


Optotype Cards

One Character Optotype Cards

Snellen “E”


Optotype Cards

Other Cards

Infant Visual Acuity

Factors that affect Visual Acuity

PHYSICAL FACTORS (Observation conditions)Type of test used Luminance level of testContrast of testINTERNAL FACTORS (Ocular Optics)DefocusPupil diameter

Factors that affect visual acuity

Type of test used

-Foucault gratting provide a greater acuity than Landolt rings or Snellen E’s

- Easy letters (Z,U,E,F,T,L) or difficult letters (V,R,D,B,G)


Luminance level

It is very important to always measure Visual Acuity with luminance values of

greater than or equal to 100 cd/m2


Contrast

It is very important to always measure Visual Acuity with contrast values

greater than 0.9 (regular situation in a clinical practice)


Defocus

DVA

25.0

Grade of defocus (D)

Visual Acuity

1.25>D2.5 <0.10

0.5>D1.25 <0.20

0>D0.5 <0.5


Pupil Diameter

- Maximum visual acuity for pupil diameters between 2mm and 4mm.

- For larger diameters the aberrations are more important.

- For smaller diameters diffraction is important.


Pupil Diameter

- If the diameter is small the size of the circle of defocus is reduced.

Stenopeic glasses

Small circular diaphragm of 1 to 2 mm on a black background. It improves vision when defocus exists.

theme 4: visual acuity

Documents

visual acuityfactors

visual discrimination

types of visual acuitysize

concept of visual acuityis

minimum dimension

minimum separablethe

minimum visibletypes

minimum angle of separation