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……….
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