1. Visual acuity is a measure of the ability to resolve fine detail and is defined as the smallest object or angle that can be seen by an observer. 2. There are several types of visual acuity charts used to test visual acuity including Snellen charts, logMAR charts, and picture charts suitable for children. Snellen charts use letters of decreasing size while logMAR charts use a logarithmic progression of letters. 3. Factors that can affect visual acuity include stimulus characteristics, observer factors like retinal location and pupil size, and psychological factors. Visual acuity is measured through tasks that assess minimum resolution ability.

1. VISUAL ACUITY
PRESENTED BY:
KARTIK KUMAR GUPTA
G. BHAVANI REDDY
BV(DU)MC SCHOOL OF
OPTOMETRY-2nd YEAR

2. History(ref. HV Nema)
In 1854, Eduard von Jaeger published a set of reading charts
in German, French and English languages to record the V/A.
Donders 1st time, coined the term visual acuity to describe
the sharpness of vision and defined it as a ratio between
patient’s V/A and standard V/A.
Hermann Snellen published his classical chart recording the
V/A in 1862.
Unlike jaeger, he did not used the front from printing house
but designed special letters, based on 5*5 grid, which he
called optotypes.

3. Visual Acuity:
• Visual acuity is the spatial resolving capacity of the visual system
.It express the angular size of details that can just be resolved by
observer.(ref. clinical borish)
• In terms of visual angle, the visual acuity is defined as the
reciprocal of the minimum resolvable visual angle measured in
minarc for a standard test pattern.(ref. AK Khurana optics and
refraction)
• Visual acuity is considered a measure of form sense ,so it refers
the spatial limit of visual discrimination.(ref. AK Khurana)
• The MAR, that allows a human optical system to identify two
point as different stimuli, is defined as the threshold of
resolution(ref. HV Nema).

4. • In normal eye, the limits of visual acuity are imposed by
optical and neural factor and are equal in magnitude.(ref:
clinical borish, optometry ,Duke Elder)
Optical limitation
Neural limitation
Components of visual acuity:(ref :AK Khurana)
i. Minimum visible/detectable
ii. Minimum resolution
iii. Minimum cognizable/recognition
iv. Minimum discriminable/hyperacuity

5. • Factors affecting visual acuity:(ref: AK Khurana)
In general, the factors that influence the spatial resolution can be
classified into physical, physiological and psychological factors)
 Stimulus – related factors:
I. Luminance of test object
II. Geometrical configuration of the stimulus
III. Contrast of the stimulus from the surround
IV. Influence of wavelength of stimulus light
V. Exposure duration of stimulus
VI. Interaction effects of the two targets

6. • Observer –related factors:(AK Khurana)
I. Retinal locus of stimulation(60% reduction in V/A occurs just at one
degree away from the centre-HV Nema)
II. Pupil size
III. Accommodation
IV. Effect of eye movements
V. Meridional variation in acuity
VI. Optical elements of the eye
VII. Developmental aspects
 Mental status(psychological factor)-ref. HV Nema

7. Chart Formats (ref : Clinical Borish)
Visual acuity charts may be prepared
as printed panels or as slides to be
projected onto a screen, or they may
be generated for video display.
1. Printed panel charts:
•Printed on opaque card or plastic, and
these are directly illuminated.
•Others are printed on translucent
material and mounted on a light box
that provides illumination from the
rear(back illumination).

8. • The panel charts are usually Snellen chart and presented at
6m.
• Although 20 feet or 6 m is the most widely used test
distance , 4m has been recommended by Hofstetter and ,
subsequently , by some authoritative bodies.
• Closer test distances are used when the examination room
does not permit chart presentation at the standard distance
or when the patient has low vision and is unable to read the
largest letters on the chart.

9. Projector Chart

10. • Projector Charts : ( ref :clinical procedures in optometry – J.
Boyd Eskridge )
This instrument projects test objects onto a screen .
The test objects are printed on glass slides that are placed
inside the projector.
Standard testing distance - 10 and 20 ft available .
Chart projection lens modification is used to adjust the size
of the letters.
Proportion formula to calculate letter size w.r.t testing
distance;
 letter size = x .
Testing distance current testing distance

11.  Charts On Display Screens :
They provide the means to select different optotypes, to
change letter sequences, and to vary stimulus parameters
such as contrast, spacing arrangements, and presentation
time .
It provides more detailed recording and analysis of responses.
 illuminance level varies from 150cd/m2 -300cd/m2 .
The pixel structure limits the size of the smallest letters ,and
the screen dimensions limit the size of the largest letters that
can be presented in a row or a singly.

12.  To maintain a 5:1 ratio between the height of the optotype and
stroke or gap width, the number of pixels per letter height must be an
integer multiplied by 5.

13. Visual Acuity Chart Design:
1. Snellen chart:
 Principle: The Snellen test types are constructed
on the principle that the two distant point can be
visible as separate only when the angle
subtended by them at nodal points of eye is 1’.
It was discovered by Dutch ophthalmologist
Herman Snellen .(ref. Borish)
It is a type of legible VA task.(ref.Borish)
The distant central visual acuity is usually tested
by Snellen’s test types.(ref .A.K)
It consists of a series of black capital letters on a
white board ,arranged in lines , each progressively
diminishing in size .(ref. A.K)

14. There is only one letter at the
largest size level ,and the number at
each size level increase
progressively.
Each letter of the Snellen’s chart fits
in a square.
Assume 5/5 grid for letters with
detail separation of 1/5th of the
letter size.

15. Snellen’s Fraction:
Snellen’s visual acuity is recorded in terms of Snellen’s
fraction.
Test distance
Distance at which letter subtends 5’ of arc
Calculation of letter size:
tan5’=h/d
Testing distance:6m
Demerits of Snellen’s chart:
i. Visual acuity task is not same in each line.
ii. Snellen chart VAs are much less repeatable than VA and over
3-times less sensitivity to intraocular difference in VA and
thus less sensitivity to amblyopia and other uniocular visual
acuity loss.

16. Letter size variation:
Serif :ornamental cross stroke at the
end of the limb.
Sanserif : bold type faces and
appears less clutter.
Sloan letters: C,D,H,K,N,O,R,S,V,Z.
British letter :
C,D,E,F,H,K,N,P,R,U,V,Z.

17. 2. Bailey-lovie Design Principle
Bailey and Lovie proposed a set of principle for the design of
visual acuity chart ,and these make the task essentially the same
at each size level.
Such standardization of the visual acuity task requires the
followings:
i. A logarithm size progression.
ii. The same number of letters at each size level.
iii. The spacing between letters and between rows that is
proportional to letter size.
iv. Equal average legibility for the optotypes at each size level.

18. Testing distance:3m and 4m
Characteristics of log-MAR
chart:
It is flat and typically ‘v’
shaped.
Every line has 5 letters and
each letter corresponds to
0.02 log unit of each line.
The letters are sans-serif and
Sloan letter.
.

19. The spacing between the two letters is width of the single
letter and the two line is the height of the lower line letter.
With 0.1 log unit progression each successive steps
represents a change in size by the ratio of 1.2589:1(approx.
5:4)
Letters to letters scoring system.
Brightness gradually decreases.

20. Near Visual Acuity Chart: (Ref. J. Boyd)
1. The Point System:
Points are units used to specify the size of typeset print and
are used in the printing industry.
Each point is 0.35mm or 1/72inch.
1.0M units=1.45mm=8 points(lowercase newspaper style)=
typical newsprint.

21. 2.The Jaeger System:
This consists of 20 letters
sizes classified j1 toj20.
It is mainly used by
ophthalmologist.

22. 3.Reduced Snellen :
This utilizes the conventional
Snellen’s visual acuity chart
photographically reduced such
that 20/20 letters subtended
5’ of arc at the retina at the
testing distance of
16inch(40cm)

23. 4. M-units:
They are used to specify the size of the print by
indicating the distance in meters at which the
height of the smaller letter(the lower case x-
height of typeset print) of the printed materials
subtends 5’ of arc.
Print that is 1.0M unit subtends 5’ of arc at 1m;
accordingly it is 1.45mm high.
Regular news print is usually 1.0M in size.
Visual acuity may easily recorded in terms of
Snellen’s fraction.
A patient who can just read 1.0M print at 40cm
would have his or her visual acuity recorded as
0.40/1.0M.

24. 5.N-Notation :
•To standardize the testing of near vision , the
faculty of ophthalmologists of the U.K
adopted the times new roman font as the
standard font for testing near vision, and they
recommended that the print size be
indicated in points.
•The size label N8 indicates that the standard
near test font is being used and that the size
is 8points.
•The near visual acuity performance is
recorded as the smallest print that can be
read, and the distance is specified.
•A print size recorded in N-notation can be
converted to M-units by dividing the number
by 8.

26.  Pediatric V/A Chart:(Ref. AK Khurana)
 The particular V/A test selected depends upon factor such as
test availability, age of the child and responsiveness of the
child.
At the age above 6-7 years Snellen’s acuity chart are used.
For children younger than this Fern and Manny
recommended the following desirable characteristics
for visual acuity test.
1. Use of Landolt ‘C’ format.
2. Use of single, isolated optotypes rather than a full
line or chart.

27. 3. A two alternative force choice paradigm or a matching
response.
4.Avoidance of the need for a verbal response or a
directional response.
5. A short test distance such as 3m, rather than traditional
6m distance.
6. Borders or contour surrounding the test optotypes to
control for contour interaction.

28. • Measurement of visual acuity in 3-5 yrs :
Illiterate E- cutout test :
This test is useful in children between 2 ½ and 3
yrs of age .
The child is given a cut out of an E and asked to
match this E with isolated Es of varying sizes .
The first trail is always not successful.
The mother may be instructed to teach E – game
at home .
When the child starts understanding the
orientation of E , a visual acuity chart consisting
of Es oriented in various directions may be used.
Tumbling E – pad test is similar to E – cut out
test.

29. Isolated Hand Figure Test:
•Sjogren has replaced the E with
the isolated figure of a hand , and in
some children it works better than
E’s.

30. Sheridan-Gardiner HOTV test:
This test is similar to E-cutout test.
This is an initiative test used to take
vision in the age group of 2-5 years.
The child is handed a card with
HOTV and is asked to match the
letters on the chart.
Snellen’s equivalent of 6/6-6/60 can
be estimated using this method.

32. Pictorial Vision Chart:
When the child is able to verbalize, visual acuity chart showing
picture, rather than symbol may be used.
Many such chart have been devised, and one should be
chosen that present picture of object with which the child is
likely to be familiar.
One of the example is Kay picture test.
The Allen preschool test which present picture in isolated
form, is useful for this purpose.

34. Broken Wheel Test:
A pair of cars in progressively
smaller sizes, one of which has a
wheel cut across, like Landolt C is
shown to the child and the child is
asked to identify one with the
broken wheel.
Two cards are held before the child
and he simply has to identify which
one has broken wheel, which
indicate that he can resolve the gap
size of the Landolt C for that
particular car.

35. Boek candy bead test:
The child is asked to match
beads at 40cm.
Snellen visual acuity
equivalent of 20/200 is
estimated by this method.

36. Light Home Picture Card:
A chart containing an apple, a house and a umbrella
arranged in Snellen’s equivalents of 20/200-20/10 is
used and the child is asked to identify the pictures
along the line.
The test is carried out at 10ft.
 Drawbacks:
They are affected by the child familiarity with the
pictured object. cultural and social factor influences
response to such test, increasing variability and
decreasing reliability.
They require a naming response rather than a
nonverbal pointing response.
The result from picture type test are difficult to
corelate, except in an approximate way, with those
obtain from standard optotype test.

37. Vision Test In 2-3 Years:
Dot V/A Test:
Child is shown an illuminated
box with black dots of different
sizes printed on it.
The smallest dot identified
denotes the visual acuity of the
child.

38. Coin Test:
In this test child is
asked to identify the
two faces of coins of
different sizes held at
different distances.

39. Miniature Toy Test:
In this test the child is shown in miniature toy from a distance of 10ft and is
asked to name or pick from the pair from the assortment.
Vision Test In 1-2 Years:
Marble Game Test:
In children of 6-12 month of age, reaching or placing games can be used to
estimate visual function.
In it the child is asked to place marble in the holes of a card or in box.
This test is not intended to measure visual acuity of each eye but rather to
compare the functioning of the child’s eye when one or other is closed.
The vision of the eye is then noted as being useful or less useful.

41. Sheridan’s Ball Test:
Used a series of Styrofoam balls of progressively smaller sizes.
One record the smallest ball that the infant can fixate and follow at a
distance of 10ft.
Rolling the ball on a white or grey background and asking the child to
pick it up, and noting the smallest size to which the child gives a good
response is a rough way of estimating v/a.

42. Measurement Of V/A In Infants:
Optokinetic Nystagmus Test(OKN):
In this test nystagmus is elicited by passing a succession of black and
white strips through the patient field of vision.
The visual angle subtended by the smallest strip width that still elicits
an eye movement is a measure of v/a.
The only co-operation requires in this test are the infant be awake and
should hold both eyes open.
It is reported that OKN acuity is at least 6/120 in the newborns and
improves fairly, rapidly during the 1st few month of life, reaching to a
level of 6/60 at 2 months and 6/30 at 6 months, 6/6 by 20-30 months.
A normal pupillary response, a positive blind response and an
elicitable OKN indicates good v/a.

44. Preferential Looking Test(PLT):
This test is based on the observation that when presented with 2-
adjacents stimulus field, one of which is striped and other is
homogenous, the infant will tend to look at the stripped pattern for a
greater portion of time.
Test procedures have been developed in which the examiner is hidden
behind a screen on which one projects a homogenous surface on one side
and black and white strips on other side.
This two stimuli are alternated randomly.
Observer is able to look at the eye of the infants through a hole on the
screen but is unaware of which target, stripes or homogenous field is
presented on each side of the screen.
The baby faces the screen and the observer records the direction of head
movements in response to the appearance of the striped stimulus.

45. The location of the stripped pattern is varied at random from left to
right and fineness of the strips is gradually reduce until there is no
longer any correlation between the judged direction of the infants gaze
and the location of the stripped patterns.
This method is suitable for infants up to 4 months of age.
Older infants are too easily distracted.
 It is reported that visual acuity in newborns is 6/240, 6/60 at 3 months
and 6/6 at 36 months of age.
It must be well understood that grating acuity testing cannot be
automatically be equated with acuity testing based on recognition task,
such as naming pictures or Snellen’s letter.
In normal children grating acuity is better than recognition acuity.
The different neural processing mechanism in the brain are involved
with special discrimination and recognition task. So, it is not advisable
to equate grating acuity with recognition acuity.

47. Visual Evoked Response(VER):
It refers to electroencephalographic(EEG) recording made from the occipital
lobe in response to the visual stimuli.
VER is the only clinically objective technique available to access the
functional state of the visual system beyond the retinal ganglion cells.
It is quite useful in assessing visual function in infants.
Flash VER just tells about the integrity of the macular and visual pathway.
Pattern reversal VER is recorded using some patterns stimulus as in the
checker board.
In it the pattern of stimulus is changed but the overall illumination remains
the same.
The pattern reversal VER depends on form sense and thus gives a rough
estimation of the v/a.
In VER v/a in infants to be 6/120 at the age of 1 month,6/60 at 2 months,
and 6/6-6/12 at the age of 6-12 months.

49. Catford Drum Test:
It is a detection acuity test, useful in infants and preschool children.
In this test the child is made to observe an oscillating drum with black
dot of varying sizes.
The smallest dot that evokes pendular eye movements denotes the
level of v/a.
This test is unreliable since it over estimate the vision.

50. AGE(month) OKN PLT VER
1 6/120 6/120 6/120
2 6/60 6/60 6/60
6 6/30 6/30 6/6-6/12
AGE(months
at which 6/6
is achieved)
20-30 24-36 6-12

51. Indirect Assessments Of Visual Acuity:
Indirect assessments of v/a can be made in infants by observing the various
milestone in the development of vision as follows:
a) Blink reflex in response to sound is present since birth.
b) Menace reflex, i.e. reflex closer of the eye on the approach of an object is
usually present after the age of 5 month, if vision is normal.
c) Test based on fixation reflex are useful in making a rough estimation of
vision in infants .These include:
 Fixation behaviour test
 Binocular fixation pattern
 Central, steady and maintained(CSM) method of grating monocular fixation.

52. Low Vision V/A:(ref. clinical procedure in optometry: J.
Boyd Eskridge)
Definition:
• Low vision can be said to be present when the ability to perform
visual task of everyday life is notably impaired by vision loss that is
not correctable by standard spectacle or contact lenses.
Some agencies providing services for the visually impaired require
the v/a to be less than 20/60 or 20/70 to be classified as low vision.
However not all low vision involves reduced v/a.
Significant visual field losses or contrast sensitivity losses can occur
while V/A remains unimpaired and such losses can interfere with the
individual ability to perform visually dependent task of everyday life.

53. In measuring the v/a of low vision patients, there is often more need to
explore the effects of illumination conditions, and often the viewing distance
must be shorten.
Consequently, printed panel chart are most widely used.
Uses of taking VA in low vision patient:
Monitor stability or progression of disease and changes in visual abilities as
rehabilitation progresses
 Assess eccentric viewing postures and skills
 Assess scanning ability (for patients with restricted fields)
Assess patient motivation
Teach basic concepts and skills (i.e., to eccentrically view) relevant to the
rehabilitation process

54. LOW VISION DISTANT VISUAL ACUITY CHART:
1. Feinbloom chart or design for vision chart:
Feinbloom designed a chart in the form of a spiral bound
book of 13 pages, each page being 10 * 13 inches.
Numbers are optotype in this chart.
The range of print size extends from a number subtending 5’
of arc at 700 down to
10ft(700,600,400,350,300,225,200,180,160140,120,100-80-
60,40-30-25-20-10.

55. For the larger sizes
there is one number
per page, in the
intermediate sizes
there are 3-
number/page and for
the print of small size,
the numbers are
arranged in rows.
Testing distance:10ft
but sometimes even
closer distances are
used.

56. 2. Sloan Chart:
Used a set of 10 letters that have approximately equal
legibility.
Used a logarithmic progression and size range extended
from 400-10ft letter.
The number of letter per row and the spacing between
within rows varied throughout the chart.

57. 3.Keller’s Chart:
Use a letters that had stroke width that were 1/7th of the
letter height with geometric size progression.
Number of letters at each size levels and the spacing
between letters of the various level dependent upon letter
size.
4.Bailey-Lovie chart and Snellen’s chart:
(ETDR chart uses Sloan letter which is based on principle of
Bailey-Lovie CHART and is found to be most effective
among all the chart in low vision patients.)

58. CHARTS DESIGNS FOR NEAR VISION:
1. Feinbloom Card(digit Card):
Test optotypes are number and at each size level, there are
numbers in rows of single, widely separated numbers, followed
by sets of double digit number, triple digit numbers, and at
some of the smaller print sizes there are also 4-digits numbers.
Developed a message chart that contain words of
encouragement and guidance to the low vision patient when
reading the chart.
Neither of the Feinbloom chart covered a particularly wide
range of print size, and within the chart there is considerable
variation in the print style, boldness of the print and the
difficulty of the material.

59. 2.Sloan’s Chart:
Testing near vision print size should
be specified in M-units.
Sloan design a set of near vision card
that included samples of paragraph
prepared on a typewriter using an ‘elite
font.’
She use photographic enlargement to
obtain a set of charts in which the size
range was 10,7,5,4,3,2.5-1.5 and 1-M
UNITS.

60. 3.Keeler’s Chart:
Used a logarithm size progression for a reading size in
which the print was in times roman font at all size
levels.
The test materials consisted of sentences or
paragraph.
The size range extend from 10M-1M.

61. 4.Bailey-Lovie’s Chart:
Designed a word reading chart in which successive word
are not connected by syntax.
They used only 4-,7- and 10 letter words.
The range of print size includes 17 different size level
extending from 10M- 0.25M.
For the 11 smaller size level(2.5M-0.25M) there are 6
words at each size level, 6 words on each row and fewer
letters at highest size letter and size level are in geometric
progression(5:4)