Visual acuity can be measured in 3 sentences or less as follows: Visual acuity measurement techniques include objective tests for preverbal children like preferential looking tests and subjective letter charts for verbal children and adults. Techniques assess the smallest object or pattern that can be detected, resolved, or recognized at given distances through detection of motion in optokinetic nystagmus tests or identification of shapes, letters, or broken circles in visual acuity charts. Proper measurement requires standardized tests, distances, lighting conditions, and recording of results in accepted notation systems.

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