Visual Acuity

OPTOM FASLU MUHAMMED

Vision

Vision Vision or visual perception is a

complex integration of light sense, form sense, contrast sense & colour sense.

Visual acuity is considered a measure of Form Sense

Visual Angle In term of visual angle , VA is

defined as the reciprocal of the minimum resolvable VA measured in minutes of arc for a standard test pattern

Angle subtended at the nodal point of the eye by the physical dimensions of an object in the visual field

Contd…….Depends up on

◦Size of the object◦Distance from the eye

Components of visual acuityMinimum VisibleResolution RecognitionMinimum Discriminable

Minimum VisibleThe ability to determine whether

or not object is present.

Resolution Discrimination of two spatially

separated targets is termed resolution

The normal angular threshold of discrimination for resolution measures 30 – 60 sec of an arc . It is usually called Minimum Angle of Resolution ( MAR )

E

RecognitionNot only discriminates the spatial

characteristics of the test pattern but also identifies the pattern with which one has had an experience

Familiar with the set of test figures

The test targets are called optotypesG L

N

Minimum DiscriminableMinimum Discriminable refers

to spatial distinction by an observer when the threshold is much lower than the ordinary acuity

Vernier acuity

Factors affecting VA1. Stimulus related factors

◦ Luminance of test objects◦ Geometrical configuration of the

stimulus◦ Contrast of the stimulus from the

surround◦ Influence of wavelength◦ Exposure duration◦ Interaction effects of the two

targets

Contd……..2. Observer related factorsRetinal locus of stimulationPupil sizeAccommodationEffect of the eye movementMeridonial variation in acuityOptical element of the eyeDevelopmental aspects

Principle measures of VAUnaided VAHabitual VAOptimal VA

Standard testing conditions1. Letter Size

VA involves the ability to detect a gap subtending an angle of 1 minute of arc

Testing Distance6 m distance ,Parallel rays of

light.

Contd……….If testing distance must be less

than 20 feet ,the use of a mirror system should be considered.

ILLUMINATIONIllumination for VA testing can

be consider in terms of either illuminance or luminance.

ILLUMINANCE 12 to 20 foot candlesLUMINANCE10 to 16 foot lamberts.

Contd…….

Visual acuity Notations

Decimal AcuityDividing the numerator of the

Snellen fraction by its denominator.

D.A decrease with increase in gap (or letter size.)

D.A =8.726mm/height of letter.e.g:- 6/6 =1 6/12 =0.5

PERCENTAGE ACUITYD.A is multiplied by 100.

If D.A is 1 P.A acuity is 100%

D.A 0.1 P.A 10%

SNELLEN ACUITYSnellen Fraction =Testing

Distance/Designation of smallest line read

orSnellen Fraction =Testing

Distance/Distance at which the smallest letter read subtends an angle of 5’ of arc

Contd……..Snellen fraction stated either metric or

english units at 6m distance or 20 feetThe numerator of the snellen fraction

is almost always 6 or 20 depending on whether the metric or english system is used

To convert decimal acuity into the snellen fraction ; it is necessary only to multiply the numerator of the reciprocal of the decimal acuity

Contd……..Eg: 0.5

1/ 0.5 = 22(6)= 12 or 2 (20) = 406/ 12 or 20/40

Contd……….Convert English Acuity to Metric

Acuity Eng D.A Metric Acuity

20/20 1 6/620/200 0.1 6/6020/10 2.0 6/3

Contd………Communicating with patientsSnellen fraction = testing

distance / distance at which a normal eye can see the smallest letter read

Thank u……….