Visual Acuity

1. VISUAL ACUITY MEASUREMENT,
CONTRAST SENSITIVITY
PRESENTER : DR.OM PATEL
MODERATOR : DR.ALKA

2. VISUAL ACUITY PRINCIPLES
• The visual acuity is determined by the smallest
retinal image the form of which can be
appreciated
• For discriminating the form of an object its
parts must be differentiated
• It is necessary that two individual cones must
be stimulated with one between them
remaining unstimulated

3. VISUAL ANGLE
• It is found that the object must subtend a
visual angle of 1 minute at the nodal point of
the eye

4. • VA test types consists of a series of letters of
diminishing size.
• Each letter is shaped such that it can be placed in a
square, the sides of which are five times the breadth
of the constituent lines. Hence the whole letter will
subtend an angle of 5 min. at the nodal point of the
eye at the given distance.

5. COMPONENTS OF VISUAL
ACUITY
• Detection or visibility
• Resolution
• Recognition

6. 1. Detection or Visibility
• Ability to determine whether or not an object is
present in an otherwise empty visual field is
termed visibility.
• This depends upon the specification of stimulus
such as size, shape, & illumination.
• A black dot against a white background can be
detected if its diameter is of the order of 30 sec
or more

7. 2. Resolution (Ordinary VA)
• Discrimination of two spatially separated
targets is termed resolution
• It is essentially an assessment of function of
the fovea centralis
• This component of VA is measured clinically
using Snellen’s or other test types

8. 3. Recognition
• Virtue by which an individual identifies the
test patterns with which he had some
experience
• It involves cognitive component in addition to
spatial resolution
• E.g. Identification of faces

9. MEASUREMENT OF VISUAL ACUITY
• The visual acuity is a highly complex function
• In clinical practice, VA is considered
synonymous with the measurement of
minimum resolvable only
• Hence, examination with various VA charts is
quite satisfactory, although incomplete

10. MEASUREMENT OF VISUAL ACUITY IN ADULTS
• The distant central VA in
adults is tested by :
• Snellen’s test types
• Landolt’s C test types
• E chart

11. SNELLEN’S TEST TYPE
• Basis of the test : Two distant
points are visible as separate
only when they subtend an
angle of 1 min at the nodal
point of eye
• Black capital letters on white
board, arranged in lines, each
progressively diminishing in size

12. • Each letter of the chart
is so designed that it fits
in a square,the sides of
which are 5 times the
breadth of constituent
lines
• The line comprising the
letters have such a
breadth that they will
subtend an angle of 1
min at the nodal point

13. METHOD
• Patient seated at 6 meters from the chart
• Illumination is 20 foot candles
• Each eye is tested separately.
• VA is recorded as a fraction (6/60, 6/36,
6/24,6/12,6/9, 6/6)
Numerator: distance of the pt. from the chart
Denominator: smallest letters accurately read

14. • If pt. cannot see the top line from 6m, he is asked to
slowly move towards the chart till he can read the top
line(5/60, 4/60, 3/60, 2/60, 1/60)
• If the pt. is unable to see even from 1m, he is asked to
count fingers of the examiner
• When the pt. fails to count fingers, the examiner moves
his hands close to the pt’s face & asks whether he could
appreciate the movements or not. (HM +/-)
• When the patient cannot appreciate hand movements,
perception to light is noted.
(PL +/-) with projection of rays in four quadrants.

15. LANDOLT’S TEST TYPES
• Similar to Snellen’s test types
• Instead of letters, broken rings
are used & the patient is asked
to detect the direction of the
break in the circle
• Each broken ring subtends an
angle of 5 min. at nodal point

16. E CHART :-
• Similar to snellen’s and landolt’s the difference
is that in this chart E of different sizes are
arranged
• Pt. is asked to tell the direction towards which
the arms of the E are pointing

17. SNELLEN’S EQUIVALENT
• In U.S., the metric system is not usually
employed & the values are converted to feet
• ( 6m = 20 feet)
VA 6/6 = 20/20
VA 6/60 = 20/200
VA 3/60 = 20/400

18. Decimal acuity:-
• In this system, the Snellen’s fraction is reduced to
a decimal no.
• Higher VA is represented by a numerically larger
number, which is reverse in the Snellen’s grading
• Ex- 6/6 = 1.0
6/9 = 0.67
6/60 = 0.10

19. The Bailey-Lovie logMAR chart
• Principle:-
 Used logarithmic scale.
 Each step indicates increase of 25% in letter size.
 Letter sizes ratio as we move up is a constant value of 1.26(0.1log unit
steps).
 Incorporated 5 letters in every row.
 Spacing b/w 2 adjacent letters = width of 1 letter.
 Spacing b/w 2 hz rows = height of the letter on lower row.

20. • Results of this chart were
obtained in terms of logMAR
score i.e log of minimum angle
of resolution
• As each letter size changes by
0.1logMAR units per row &
there are 5 letters on each row
,therefore each letter can be
assigned value of 0.02
• Thus final logMAR takes
account of every letter that has
been correctly read

21. Snellens Chart
• Irregular progression of letter
size
• Variable number of letters in
each line
• Variable legibility (difficulty) of
test letters
• Distance between each letter is
not uniform
logMAR Chart
• Uniform progression of letter
size
• Same number of letters in each
line
• All letters with similar legibility
• The distance b/w each letter is
equal to the width of the letter

22. Snellen fraction
(20ft)
Snellen fraction
(6m)
Snellen Acuity
(Decimal)
Minimal Angle of
Resolution
(inverse of sn.frac)
Log-MAR
20/200 6/60 0.10 10 1
20/160 0.12 8 0.9
20/125 ~6/36 0.8
20/100 0.20 5 0.7
20/80 6/24 0.25 4 0.6
20/63 ~6/18 0.5
20/50 0.40 0.4
20/40 6/12 0.50 2 0.3
20/32 0.67 0.2
20/25 0.80 0.1
20/20 6/6 1 1 0
20/16 -0.1
20/12.5 -0.2
20/10 -0.3

23. Measurement of VA for Near
• Near vision is tested by asking the patient to
read a near vision chart kept at a distance of
25cm to 35cm
• Each eye should be tested separately
• The near vision is recorded as the smallest
type which the patient can read comfortably

24. 1) Jaeger’s charts :-
• Consist of ordinary
printer’s fonts of
varying sizes
• Prints are marked
from 1 to 7 and
accordingly pt’s
acuity is labeled as J1
to J7 depending
upon the print he
can read

25. (2) Roman test types :-
• Consists of Times Roman fonts with standard
spacing.
• -The near vision is recorded as N5, N6, N8,
N10, N12, N18, N36 and N48.

26. (3) Snellen’s near vision test types :-
• Constructed on the same principles as of the
distant types.
• The graded thickness of the letters is about
1/17 of the distant vision chart letter.
• The letter equivalent to 6/6 line subtend an
angle of 5 min. at he average reading distance.

27. VISUAL ACUITY MEASUREMENTS IN
CHILDREN
• OBJECTIVE TESTS
Preverbal children ( < 2 ½ yrs )
• SUBJECTIVE TESTS
Verbal children ( > 2 ½ yrs )

28. OBJECTIVE TESTS
• Fixation & following behaviour
• Preferential looking test ( PLT)
• Optokinetic Nystagmus (OKN)
• Visual evoked potential (VEP)

29. 1) FIXATION & FOLLOWING
• Bright colored objects with
high contrast edges are used.
• Best target however is the
human face.
• Binocular fixation is assesed
first.
• Monocular fixation – reveals
the defective vision in one
eye.
• Infant may not fix with the
defective eye and objects to
occlusion of the better eye

30. • Quality of fixation behaviour – C S M
C – Central – foveal fixation
S - Steady – no nystagmus
M – Maintained – fixation after a blink

31. QUANTITATIVE METHODS
• Methods to detect the resolution acuity
• More sophisticated method of visual assesment
than mere fixation assesment.
• Include -
PREFERENTIAL LOOKING TEST
OPTOKINETIC NYSTAGMUS
VISUAL EVOKED POTENTIAL

32. (2) Preferential looking test
• Assumes that the child will
prefer to look at an area of
higher visual interest, rather
than a neutral grey field
• Child presented with two
adjacent stimulus fields,one
which is striped and other
homogenous
• Method suitable for infants
upto 4 months of age
• Ex- Lea’s paddles, Teller’s
acuity cards

33. Procedure
• On a screen homogenous
surface is projected on
one side & black and
white strips on the other
• These two stimuli are
alternated randomly
• The eyes of the infant are
observed and the
movements recorded

34. PROCEDURE
• Gradually the fineness of stripes is reduced
unless there is no longer correlation between
direction of gaze & location of the striped
pattern.
• Visual acuity ranges from 6/240 in
newborn,6/60 at 3 months and 6/6 at 36
months

35. (3) Optokinetic Nystagmus Test (OKN)
• Nystagmus is elicited by
passing a succession of black
and white stripes through the
patient’s field of vision
• The visual angle subtended by
the smallest strip which elicits
an eye movement is a
measure of VA
• OKN acuity is 6/120 in
newborns,6/20 at 2
months,6/6 by 20-30 months

36. (4) Visual evoked response (VER)
• Refers to EEG recording made from the
occipital lobe in response to visual stimuli.
• It is useful in assessing visual function in
infants.
• Only clinically objective technique available to
assess the functional state of visual system
beyond the retinal ganglion cells.
• Two types – flash & pattern reversal VER

37. Visual evoked response (VER)
• Flash VER tells about the integrity of macular
and visual pathway
• Pattern reversal VER uses some patterned
stimulus like checkerboard
• The pattern of stimulus is changed and so it
gives an idea of form sense
• VER studies shown VA in infants to be 6/120 at
1 month,6/60 at 2 months & 6/6 at 1 yr of
age.

38. SUBJECTIVE TESTS
• Optotype-
symbol whose identification implies VA
• Eg; Lea’s symbols ,HOTV, Snellen’s chart,
Landolt-C ,E chart.
• These tests are employed in verbal children.

39. Lea’s symbols
• Pt. is asked to match the
picture shown to the
hand held cards
• 4 shapes are included in
this : Circle, Square, Hut
& Apple
• Done at 3m

40. HOVT /SHERIDAN GARDINER TEST
• It is done at a distance of
6 meters from the child
• It includes simple
alphabets such as
H,O,T,V,X,A,U
• The patient matches the
letter being displayed with
the hand held cards

41. Snellen’s,E Chart,Landolt’s C

42. IN SUMMARY
• INFANTS -
Fixation methods
Preferential looking test
Optokinetic nystagmus
Visual evoked potential

43. • TODDLERS - LEA’S SYMBOLS
HOTV TEST
• CHILDREN > 5 YRS & ADULTS-
SNELLEN’S CHART
LANDOLT’S C CHART
E – CHART
ETDRS