Visual acuity is a measure of the eye's ability to see fine detail and discriminate between objects. It is assessed using charts with letters, symbols, or pictures of decreasing size. The Snellen chart is commonly used, with visual acuity recorded as the distance at which a person can see a symbol subtending an angle of 5 minutes of arc. Other methods include the Landolt C chart and LogMAR chart. Visual acuity depends on factors like stimulus characteristics, retinal location stimulated, and optical elements of the eye. It provides information on visual function, refractive status, and outcomes of treatments.

1. VISUAL ACUITY
DR.ANKITA MAHAPATRA
DEPARTMENT OF OPHTHALMOLOGY
VSSIMSAR, BURLA

2. VISUAL ACUITY
Vision or visual perception :
complex integration of
sense
light
form
contrast
colour
VISUAL
ACUITY
Visualacuityisameasureof thespatialresolutionof the eyeor, in other words, an
estimation of its ability to discriminate between two points.

3. THEORIES OF VISUAL ACUITY
1.RECEPTOR THEORY :
• If the image fell on two cones separated by an
unilluminated cone, then the points of light would be
perceived as two distinct sources.
• In terms of length of the retinal image , two points will be seen
clearly when their image size is more than 4.5 μ . ( diameter of
individual cone – 1.5 μ )
• In terms of Visual angle, two adjacent points seen clearly and
discretely when they produce visual angle more than 1 minute.
• VISUAL ANGLE is the angle subtended at NODAL POINT of eye by an
object in visual field.

4. Dimension of the visual angle depends on size of the object and distance from the eye.

5. RAYLEIGH CRITERION
• Two point sources are resolved from each other when separated by at least
the radius of the AIRY DISC.

6. USES :
• 1. Visual function
• 2. Refractive status
• 3. Outcome measure for a treatment
• 4. medico legal purposes
• 5. Criteria for : a. Person’s fitness to drive
b. Eligibility for entrance into a profession

7. COMPONENTS OF VISUAL ACUITY
MINIMUM
VISIBLE
RESOLUTION RECOGNITION
MINIMUM
DISCRIMINABLE
Ability to detect
whether an object
is present in an
otherwise empty
visual field.
Clinically measure
of Visual acuity.
Discrimination of
two spatially
separated targets.
Discrimination of
spatial characteristics
and identifies the
pattern from memory.
Ex: identification of
faces.
Spatial distinction
by an observer
when threshold
lower than
ordinary acuity.

8. 1.MINIMUM VISIBLE
• Minimum visible spatial threshold level depends upon specification of
stimulus--- a.size b.shape c.illumination
• A. black dot against a white background detected if its diameter is of
the order of 30 seconds of arc or more.
• B. Black square against a white background when length of diagonal is
30 seconds.
• C. Visualization of a thin telegraphic wire against a uniform sky with a
thickness of one half second of arc ----- convergence of subthreshold
signals from a number of individual retinal elements , yields a
suprathreshold level of activity.
• D. Detection of an illuminated object against a dark background
depends on its intensity , not size.

9. 2.RESOLUTION (ORDINARY VISUAL ACUITY )
• Discrimination of two spatially separated targets .
• Minimum separation between two points , which can be discriminated ----
MINIMUM RESOLVABLE
• Measurement of the threshold of discrimination is an assessment of function
of fovea centralis.
• Normal angular threshold of discrimination for resolution ------- 30-60
seconds of an arc. (minimum angle of resolution).

10. 3.MINIMUM DISCRIMINABLE (HYPERACUITY)
• Ability to determine whether or not two parallel and straight lines are aligned
in the frontal plane. ( Vernier acuity )
• Threshold value of Vernier acuity : 2-10 seconds of arc .

11. FACTORS AFFECTING VISUAL ACUITY
STIMULUS RELATED
LUMINANCE OF TEST OBJECT
GEOMETRIC CONFIGURATION
OF STIMULUS
CONTRAST OF STIMULUS
WAVELENGTHOF STIMULUS
LIGHT
EXPOSURE DURATION
INTERACTION EFFECTS OF TWO
TARGETS
OBSERVER RELATED
RETINAL LOCUS OF STIMULATION
PUPIL SIZE
ACCOMODATION
EFFECTS OF EYE MOVEMENT
MEREDIONAL VARIATION IN ACUITY
OPTICAL ELEMENTS OF EYE
DEVELOPMENTAL ASPECTS

14. VISUAL ACUITY TESTS
DETECTION A
TEST
1.DOT VISUAL A
TEST
2.CATFORD DRUM
TEST
3. BOEK CANDY
BEAD TEST
4.STYCAR GRADED
BALL TEST
5. SCHWARTING
METRONOME TEST
RECOGNITIO
N A TEST
DIRECTION
IDENTIFICATIO
N TEST
1. SNELLEN’S E
CHART
2.LANDOLT’S C
CHART
3.SJORGEN’S
HAND TEST
4.ARROW TEST
LETTER
IDENTIFICATIO
N TEST
1.SNELLEN’S
LETTER CHART
2.SHERIDAN’S
LETTER
3.FLOOK’S
SYMBOL
4. LIPMAN’S
HOTV
PICTURE
IDENTIFICATIO
N TEST
1. ALLEN’S
PICTURE CARD
TEST
2.BEALE COLLINS
PICTURE CHART
3.DOMINO CARD
4.LIGHTHOUSE
5.MINIATURE TOY
TEST
PICTURE
IDENTIFICATIO
N ON
BEHAVIOURAL
PATTERNS
1. CARDIFF
ACUITY TEST
2. BAILEY HALL
CEREAL
RESOLUTION
A TEST
1.OPTOKINETIC
NYSTAGMUS TEST
2. PREFERENTIAL
LOOKING TEST
3.VISUALLY EVOKED
RESPONSE

15. Commonly used methods for distant visual acuity,
• Snellen’s chart
• Landolt broken ring (or C chart)
• Tumbling E chart
• Bailey – Lovie chart / LogMAR chart

16. SNELLEN’S CHART

17. HermanSnellen:ADutch ophthalmologist who introduced the
Snellen chart to study visual acuity in 1862.

18. PRINCIPLESOFSNELLEN’SACUITY:
• Each letter is designed in a square with sides 5 times the width of letter strokes
• The breadth of black strokes and white
spaces are equal
• The breadth of line and spaces produce
1’ min of arc at nodal point when
viewed from a certain distance
• Each letter subtends an of 5’ of arc at
nodal point when seen
at a certain distance
VA =
DISTANCE OF THE PATIENT FROM THE CHART
SMALLEST LINE READ BY THE PATIENT

19. • On the 6/6 line each letter is constructed to subtend an angle of 5’ of arc at a
distance of 6 meter
• Other lines are constructed in a similar way, so that letters on the 6/18 line or
6/60 line subtend an angle of 5’ of arc if tested at 18m or 60m from the chart.
VA = TESTING DISTANCE
DISTANCE AT WHICH LETTER SUBTENDS 5MIN OF ARC

21. LANDOLT’S C- CHART

22. TUMBLING E CHART

23. MINIMUM ANGLE OF RESOLUTION :
• The denominator in Snellen grading is an indirect measure of the size of the
letters read and the angle they subtend.
• The classic Snellen fraction is the reciprocal of the minimum angle of
resolution (MAR)

24. LOG MAR SCALE :
• A notation of visual acuity that has the same clinically significant
difference between each line and allows easy recording of every
letter read is the log minimum angle of resolution (MAR) scale.
• The MAR is arrived at by dividing the denominator by the distance
at which the letters were read, i.e. the Snellen fraction is inverted
and reduced. A Snellen acuity of 6/12 or 20/40 therefore
corresponds to a MAR of 2 minutes of arc.
• Allows for constant geometric progression over each step.
• This derivation has been used in the construction of charts such as
the Bailey- Lovie chart

25. 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

26. PROCEDURE FOR RECORDING VISUAL ACUITY :
• Patient seated at 6m distant from the chart .So light rays are parallel and pt
exerts minimal accommodation
• Chart should be properly illuminated
• The pt is asked to read the chart with each eye separately and VA is recorded
• Depending upon the smallest line that the pt can read from 6m distance,
his/her VA is as 6/6, 6/9, 6/12, 6/18, 6/24, 6/36 and 6/60
• If one cannot see the top line from 6m, pt is asked to slowly walk towards the
chart till can read the top line
• Depending upon the distance at which one can read the top line, the vision is
recorded as 5/60, 4/60, 3/60, 2/60, 1/60

28. • If the patient is unable to read the top line even from 1m, he/she is asked to
count fingers (CF) of the examiner
• His/her VA is recorded as CF-3’, CF-2’, CF-1’ or CF close to face depending
upon the distance at which the pt is able to count fingers
• When the pt fails to count fingers, the examiner should move his/her hand
close to pt’s face
• If pt can appreciate the hand movements, the VA is recorded as HM+
• If pt cannot distinguish hand movements, the examiner should test whether
the pt can perceive light or not
• If yes, vision is recorded as PL+ and if not it is recorded as PL- .
• PL+ pts must be tested with projection of rays to represent quadrants - nasal,
superior, temporal and inferior
• Then VA is recorded as PL+, PR

29. PIN HOLE VISUAL ACUITY
• If the vision is subnormal, the visual acuity is again determined by asking
the patient to read the letters through a pinhole.
• To determine if a decrease in vision is correctable by lenses

30. BCVA
To determine the function of the macula in the best optical conditions, the
refraction of the eye must be determined and the visual acuity assessed again
in the same way with the correcting glasses in place.

31. DENSE CATARACT
• LASER INTERFEROMETER forms a diffraction pattern of parallel lines on
the retina even through a moderate cataract. The patient is asked to identify
the orientation of progressively finer lines, to establish the visual acuity likely
to be regained after surgery.
• POTENTIAL ACUITY METER
projects a tiny Snellen chart onto the retina around a lens opacity and the
patient is required to read the alphabets

32. VA IN IMPAIRMENTS :

33. YOUNG CHILDREN
• Maturation of infant visual function has been studied by two
techniques, the pattern visual evoked potential and preferential
looking behaviour.
• In children younger than 2 years the VEP test proves more
successful .

34. Preferential Looking (Keeler / Teller Cards)
Given a choice, an infant prefers to look at patterned rather than unpatterned stimuli.
The infant's preference may be quantified by incorporating patterns which vary in
stripe width.

35. CARDIFF ACUITY CARDS LINEAR KAY PICTURES

36. NEAR VISUAL ACUITY
PROCEDURE :
• The pt is comfortably seat in a chair and asked to read the near vision chart
kept in a distance of 33 – 40 cm
• Good illumination thrown over from his/her back preferably over left
shoulder
• Each eye should be tested separately
• The near vision is recorded as the smallest type that can be read comfortably
by the patient as N5, N6, N8, N10, N12, N14, N18, N24, N36 and N48
• Notation is made as NV = N5 at 30cm

37. NEAR VISION CHARTS :
• Roman test types
• Snellen’s near vision test type
• Jager’s chart
• The purpose is to detect people with near vision difficulties (e.g., uncorrected
high hyperopia, accommodative dysfunction)
• In patients over 40 years old, the reduced near visual acuity is one of the
symptoms of presbyopia

38. PHOTOSTRESS TEST
• The extent of involvement can be assessed clinically by recording vision before
and after exposure to a bright light or photostress.
• The photostress test is performed by covering one eye and asking the patient to
read the smallest possible line on the near chart. A bright light is shone into the
eye for 15 seconds, following which the patient is asked to read the same line of
print and the recovery time noted. The test is repeated with the other eye.
• In normal people and those with optic nerve disease there is no significant
difference in the time taken for the two eyes to recover from the photo stress.
• In a subject with macular disease the recovery time is prolonged.
• The test is useful in early macular disease, particularly central serous
retinopathy, where there may be minimal deterioration in visual acuity and yet
an easily detectable decrease in photoreceptor reserve capacity.

39. THANK YOU