By Dr. Aayush Tandon M.S Ophthalmology

1. Visual Acuity Testing
DR. AAYUSH TANDON

2. • Definition : Visual Acuity is a measure of the spatial resolution of the
eye or, in other words, an estimation of its ability to discriminate
between two points.
• It is quantified by the minimum dimension that an object must have in
order for the observer to be able to identify, distinguish or simply
detect it.

3. In terms of visual angle visual acuity is defined as the reciprocal of the minimum
resolvable visual angle measured in minutes of arc for a standard test pattern
VISUAL ANGLE
Visual angle in the angle subtended at the nodal point of the eye by the physical
dimensions of an object in the visual field.

4. Nodal point is a point on the principle axis of the lens just
anterior to the posterior pole of the lens the rays passing from
where do not undergo deviation.
Two adjacent points (A,B) can be
clearly and discretely seen only
when these 2 points produce
a visual angle not less than
1 minute

5. Minimum visible
• To determine whether an object is
present in the visual field or not
• Depends upon the specification of
stimulus such as size , illumination
• Eg. To detect black dot against white background ,
Visualisation of a telegraph wire against an uniform sky
Components of Visual Acuity

6. Minimum seperable or Minimum resolvable
• Discrimination of two spatially separated
targets
• Quantified by the minimum angle of
separation between two objects so that they
can be perceived as separate by the observer
• Measure of threshold of discrimination is
essentially an assessment of the fovea centralis
and is termed as ‘ Ordinary Visual Acuity ‘
• Eg Snellens chart, Landolts chart, Gratings etc
Gratings

7. Task of recognition or minimum recognizable
• Discrimination of details of the object ( test pattern &
spatial resolution )
• Usually quantified by the inverse of the angle that
subtends the object to be recognized
• Identification of faces is the most common example of
recognition .

8. • The best example of minimum discriminable is ‘Vernier acuity’
which refers to the ability to determine whether or not two
parallel and straight lines are aligned in the frontal plane .
• Quantified by the minimum angle of displacement that the observer
can perceive.
• The threshold values of Vernier acuity are in the range of only a few seconds
( 2-10 sec) of arc
Minimum discirminable or hyperacuity

9. FACTORS AFFECTING VISUAL ACUITY
STIMULUS RELATED FACTORS
1.Luminance of test object
2. Geometrical configuration of the object
3. Contrast of the stimulus from surrounding
4. Wavelength of stimulus light
5. Exposure duration of stimulus
6. Interaction effects of the two targets

10. Observer related factors
1.Pupil size
2.Accommodation
3.Eye movement
4.Meridional variation
5. Optical elements
6. Developmental aspects
7. Retinal locus of stimulation

11. Measurement of Visual Acuity
Detection Acuity Tests
• Dot Visual Acuity Test
• Catford Drum Test
• Boek Candy Bead Test
Recognition acuity test
Direction Identification tests
• Snellens E-Chart Test
• Landolt’s C-chart test
Letter-Identification tests
• Snellen’s Letter chart test
• Sheridan’s Letter test
• Lipman’s HOTV test

12. Picture Identification Charts
• Allen’s picture cards test
• Beale Collins picture charts test
• Domino card test
• Miniature toy test of Sheridian
Tests based on picture identification on
behavioural pattern
• Cardiff acuity cards tests
• Bailey Hall cereal test
 Indirect Assessment
• Blink Reflex
• Menace Reflex
• Fixation Reflex
Resolution acuity tests
• Optokinetic nystagmus (OKN) test
• Preferential looking test (PLT)
• Teller acuity cards test
• Visually evoked response (VER)

13. Vision test in School children (>5 years) & Adults
SNELLEN’S CHART
.Named after the Dutch ophthalmologist Hermann Snellen who developed the chart in 1862.
•Basis of snellens test types in that two distant points can be visible as separate only when they
subtend an angle of 1 minute at the nodal point
Characterstics
•The traditional Snellen chart is printed with eleven lines of block letters.
•Ten Sloan letters C, D, E, F, L, N, O, P, T, Z are used in the traditional Snellen chart.[3]
•The first line consists of one very large letter, which may be one of several letters, for example E,
H, or N.
•Subsequent rows have increasing numbers of letters that decrease in size.

14. • The symbols on an acuity chart are formally known as "optotypes"
• Each letter is perfectly placed in a square which is divided into 25 small squares
• Each letter subtends an angle of 5 minute the nodal point of the eye
• Each component part of the letter subtends an angle of 1 minute at the nodal
point from a given distance in metres
• End point consist of recognition of letter
SNELLENS FRACTION
• Visual acuity = Distance at which test is made / distance at which the letter
subtends an angle of 5 min of arc (Letter size)
• The denominator of the Snellen fraction is termed the foot-letter size

16. Disadvantages of Snellen Test Type
• Letters not of equally legible eg; O and E
• Non-uniform progression of letter sizes
• Unequal number of letters on each line
• Irregular spacing between letters and lines
• Ability to recognize target ( letters) is
influenced by literacy and past experience
• Inaccurate results specially in those with low
vision

17. Landolt test types
• Similar to snellen’s except that instead of letters the
broken circles are used
• Each broken ring subtends an angle of 5 minute at the
nodal point
• Ability to recognize target ( letters) is influenced by literacy and
past experience and hence landolt’s rings were designed
to eliminate these factors and present a more objective test
• End point is detection of the orientation of break in the circle

18. • Patient is kept at 6 metre distance because divergence of rays entering the pupil is so slight that it can be considered
as parallel & thus accomodation is eliminated
• Each eye has to be tested separately
• Patient is asked to close the eye not being tested with the cup of the palm
• Illumination should be adequate ( 100 foot candles)
• Patient is asked to read from the top letter
Vision are taken as 6/60, 6/36, 6/24….
-Numerator indicates distance at which person is.
- Denominator indicates distance at which person with normal vision able to read that letter
• If one cannot see the top line from 6 metre patient is slowly asked to move towards the chart till one can read the top
line . Vision is recorded as 5/60, 4/60, 3/60, 2/60 & 1/60
• If patient is unable to read even from 1 m he/she is asked to count fingers of examiner .Vision is recorded as CF-3m,
CF-2m, CF- 1m & CF close to face
Procedure of testing

19. • If patient cannot count fingers close to face then examiner moves his hand close to the patients face
• If patient can appreciate the hand movements (HM) c VA is recorded as HM close to face.
• If patient cannot appreciate HM he is then taken to a dark room and asked to close one eye firmly with palm and look
straight
• Light is thrown on the open eye from all directions i.e up,down,nasal &temporal. If patient can recognise the light and
indicate its direction then visual acuity is recorded as PL + & PR + is all 4 quadrants .
• if patient is not able to perceive light from a particular quadrant then negative sign is put against that quadrant and is
said to have faulty PR
• If patient can see the glow of light but cannot indicate the side of projected rays then vision is recorded as only PL with
no PR.
• If patient cannot perceive any glow vision is recorded as No PL.

20. Pin Hole Testing
• A test performed on a person who has diminished visual acuity to distinguish a refractive error from organic diseas
e.
• The patient looks through it with one eye at a time, without wearing corrective lenses.
• Light passes only through the centre of the eye's lens, & errors of refraction have no effect while the occluder is
used.
• Pinhole blocks the peripheral rays, only letting those rays which pass through the central portion of the pupil.
• If visual acuity is improved, the defect is refractive; if not, it is organic.

21. Other Scales
1.Decimal system
2. LogMAR - logarithm of the minimum angle of resolution
 Used at a distance of 4 metres
 Ten sloan letters (S,D,K,H,N,O,C.V.R &Z) are used
arranged on the charts in equal lines of 5
 The lines progress in 0.1 logMAR steps
 Each of 5 letters in each line count for a score of as 0.02
(0.1/5)
 It measures visual acuity loss; positive values indicate
vision loss, while negative values denote normal or better
visual acuity.

22. The chart was designed by Ian Bailey
and Jan Lovie in 1980
Eg . If patient reads the 0.4 line entirely he
will have a score of 0.4
• If he reads 0.4 line plus 3 letters of 0.3 line
His score will be 0.34 {0.40 – (0.02x3)}
Bailey & Lovie Chart
(logMar Chart)

23. Advantages of LogMAR over other charts
• Equal number of letters per line ( 5 letters)
• Regular spacing between lines and letters
• Uniform progression in letter size
• Final score based precisely on the total of all
letters read
• Finer grading scale allows for greater accuracy
and improved test/retest reliability

25. Near vision
 Roman test
• Most commonly used chart is N notation.
• Each point is 1/72nd of an inch.
• Test Card starts at 6 point.
• Usually in Times New Roman font.
• Vision is recorded as N5,N6,N8,N12,N18,N3
 Snellen’s near vision test type
• Conventional Snellen distance VA chart reduced (1/17th) .
• Letter equivalents to 6/6 line subtend an angle of 5
minutes at an average reading distance ( 35cm/14 inches)
 Jaeger’s Chart
• Prints are marked from 1-7 and accordindly patients
acuity is measured as J1-J7

26. Wall charts
• Testing VA in literate adults
• Printed on cardboard and mounted on a wall
• Well-suited for vision screenings and doctors’
offices
• Variations: Tumbling E chart, Landolt C chart,
Bailey-Lovie chart
• Hand-held visual acuity cards
• Primarily for children in vision screenings
• Simplest form is the letter E in different sizes
printed onto hand-held cards
• Free space testing
Chart projectors
• Projects test objects onto a screen
• Standard chart projector can be calibrated for
varying testing distances between 10ft and 20ft
• Optically “fold” the testing distance using mirrors
to create appropriate testing distance (20ft is the
standard)

27. Procedure for near vision testing
• Patient is asked to sit in a brightly illuminated
place(light thrown over his left shoulder)
• Near vision chart is held at a distance of 25-35 cm
depending on patients nature of near work
• The Patient is asked to read the chart from bigger print
size to smaller print
• The line which he reads upto is recorded eg N12, N6
etc

28. Vision tests in 3-5 years
• Illiterate E-cutout test
• Tumbling E-test
• Isolated hand figure test (Sjogren)
• Sheridian-Gardiner HOTV test
• Pictorial Vision Charts
o Allen Preschool test
• Broken Wheel test
• Boek Candy bead test
• Light Home picture cards

29. Tumbling E test
Sheridian Gardiner HOTV test
• Four letters (H, O, T and V) are used in the chart. The test is
performed at 10 feet (3m), and is administered similarly to the
Snellen acuity. It comes with a near card so patients can match
the letters at a distance by pointing to the corresponding letter
on the near card.
• Advantage : This test does not have a directional component
• The Tumbling E Test is similar to Snellen in
that it is performed at 20 feet (6m) . The child must tell the
orientation of the legs of the letter ‘E’ (up, down, left, right).
Very useful for non verbal children .
• Disadvantage -right-left disorientation is common in this
age range.

30. Broken wheel test
• The test is performed by placing two pictures side byside.
One picture has complete wheels while the other picture has
sections missing. The child is asked to point to the car with
broken wheels. Pair of cars are kept is progressively smaller
sizes .
Allen picture cards
• Contains line drawings of familiar objects (birthday cake,hand, bird,
house,rotary telephone, jeep).The test distance is 20 feet (6m)
• Disadvantages : -Pictures not construted according to snellens formula
-Children may not be familiar to all images
- Minimum threshold acuity level on the picture chart is 6/9

31. Vision test in 2-3 years
• Dot visual acuity test
• Coin test
• Miniature toy test
Vision test in 1-2 years
• Marble game test
• Sheridans ball test
Other tests:
• Boek’s candy test
• Worth’s ivory ball test

32. Visual acuity testing in infants
• OKN (optokinetic nystagmus ) test
• PLT (preferential looking technique)
• Catford drum test
• Cardiff acuity cards test
Special tests
• VER (Visual evoked response)
• Teller acuity cards
Indirect assessment of visual acuity
• Blink reflex
• Menace reflex
 Test based on fixation reflex
Monocular
• CSM (central study &maintained ) method of
rating monocular vision
Binocular
• Standard Fixation Preference Testing
• Vertical Prism Test (Induced Tropia
Test)

33. Optokinetic Nystagmus (OKN)
OKN testing can be used to verify if thepatient possess a
cortical visual response.
The OKN drum contains black stripes that should be
oriented vertically in front of the patient. The drum is
spun slowly and the examiner observes the patient’s
eye movements as they follow the rotating drum. The
patient should exhibit a nystagmus movement because
their eyes should jump back to look at a new pattern,
as the one they originally followed rotates out of sight.
The visual angle subtended by the smallest strip width
that still elicits eye movement is a measure of visual
acuity.

34. Preferential looking test Teller Acuity Cards

35. • With Visual Evoked Response
(VER), a scalp electrode is used to record
electrical signals from the visual cortex
while the patient views a grating or
checkerboard Stimulus
• Objective technique available to
asses visual system beyond retinal
ganglion

36. Thank you