The lecture by Dr. Sania Aslam discusses visual acuity measurement and contrast sensitivity, explaining the principles and clinical tests used to assess visual acuity, including Snellen's test, Landolt's test, and near vision tests. It details the components of visual acuity such as detection, resolution, and recognition, along with methods for testing both adults and children. Additionally, it highlights how visual acuity is expressed in various grading systems like Decimal Acuity and LogMAR.

1. VISUAL ACUITY
MEASUREMENT, CONTRAST
SENSITIVITY
Lecture By
Dr. Sania Aslam OD

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 0.1 logMAR 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
• 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
logMAR Chart

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
• Optotypesymbol 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