Visual acuity is a measure of form sense and the ability to resolve fine detail. It is defined as the reciprocal of the minimum visual angle that can be resolved, measured in minutes of arc. Visual acuity depends on factors like the size and distance of an object, as well as characteristics of the observer such as accommodation and the optical elements of the eye. There are several methods used to measure visual acuity clinically, including assessing unaided acuity, habitual acuity, and optimal acuity under standardized testing conditions of illumination, testing distance, and letter size. Visual acuity test results can be recorded using decimal notation, percentage acuity, or the traditional Snellen fraction.
3. 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
4. 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
8. 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 )
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9. Recognition
Not 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 optotypes
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10. Minimum Discriminable
Minimum Discriminablerefers to
spatial distinction by an observer
when the threshold is much lower than
the ordinary acuity
Vernier acuity
11. Factors affecting VA
1. 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
12. Contd……..
2. Observer related factors
Retinal locus of stimulation
Pupil size
Accommodation
Effect of the eye movement
Meridonial variation in acuity
Optical element of the eye
Developmental aspects
16. Contd……….
If testing distance must be less than
20 feet ,the use of a mirror system
should be considered.
17. ILLUMINATION
Illumination for VA testing can be
consider in terms of either illuminance
or luminance.
ILLUMINANCE
12 to 20 foot candles
LUMINANCE
10 to 16 foot lamberts.
20. Decimal Acuity
Dividing 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
21. PERCENTAGE ACUITY
D.A is multiplied by 100.
If D.A is 1 P.A acuity is 100%
D.A 0.1 P.A 10%
22. SNELLEN ACUITY
Snellen Fraction =Testing
Distance/Designation of smallest line
read
or
Snellen Fraction =Testing
Distance/Distance at which the
smallest letter read subtends an angle
of 5’ of arc
23. Contd……..
Snellen fraction stated either metric or
english units at 6m distance or 20 feet
The 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