Visual acuity (2)

Visual acuity
Farhana Adnin
B.optom,4th batch
ICO,CU

VISUAL ACUITY…?
Resolving power of the eye or
the ability to see two separate
objects as separate.

Visual Angle…
 The angle subtended at the nodal point
of the eye by the physical dimensions of
an object in the visual field.

Theories of visual acuity…
 Receptor theory:
If the images fell on two cones
separated by an unilluminated cone, then
the points of light would be perceived as
two distinct sources.
Means that two distinct points only can
be recognized as separate when they
subtend an angle of 1’ of arc.

 3 types of Cone:
1. Green- Chlorolabe
2. Red- Erythrolabe
3. Blue- Cyanolabe
 Diameter of each cone : 0.002μm

Rayleigh Criterion for resolution
Two “point” (unresolved) sources are resolved
from each other when separated by at least
the radius of the airy disk.
The central disk is surrounded by less intense
concentric rings. (airy disk)

Types of Visual Acuity
 Minimum Detectable ( visible)
 Minimum Separable (resolution)
 Minimum cognizable (recognition)
 Minimum Discriminable ( hyperacuity)

ACUITY TASKS
MINIMUM DETECTABLE:
Ability to determine whether or not an object is
present in a visual field.
The limit for this kind of acuity is ~ 1 arc second.

Minimum Separable (resolution)
 Discrimination of 2 spatially separated targets.
 Normal angular threshold of discrimination for
resolution 30-60 seconds of an arc. (minimum
angle of resolution).

 MINIMUM COGNIZIABLE: not only discrimination
of the spatial characteristics of the test pattern but
also the pattern with which he has previous
experience
(Identification of faces, letters, symbols, pictures etc.)

Minimum Discriminable
 Ability to determine whether or not two
parallel and straight lines are aligned in the
frontal plane.

CLINICAL MEASUREMENT OF
VISUAL ACUITY
 Criteria:
 Normal testing distance (distance VA): 6m
 For near : 33cm/ 40cm.
 Illuminated visual acuity chart.
 Proper room illumination.
 Steps:
 Unaided VA
 Aided VA
 Pinhole VA

Factors affecting visual acuity
Overall
 Illumination of test object.
 Contrast of letters.
 Pupil size
 Cognitive ability
 Ocular health
 Refractive Status

Physical factor
 Those which influence the light characteristics
of the distribution & hence influence the
nature of retinal image.

Physiological factor
Stimulus related
 Luminance of test object
 Geometrical configuration
 Contrast difference of stimulus from background
 Influence of wavelength
 Exposure duration of stimulus
 Interaction effects of 2 targets

Observer related
 Retinal locus of stimulation
 Pupil size
 Accommodation
 Effect of eye movement
 Optical elements of eye
 Development of aspects

Distance Acuity chart
 Snellen’s distance acuity chart
 Bailey-Lovie charts
 Landolt ring (or C) chart
 Feinbloom Distance Test Chart
 Chronister Pocket Acuity Chart

NEAR ACUITY CHARTS
Single letter chart
Reduced Snellen’s chart:
lighthouse near visual acuity test
Designs for Vision Number chart
Reduced Ferris-Bailey ETDRS Chart
Word & Continuous Text charts
Jaeger’s chart
Roman test types
Lighthouse game card
Lighthouse continuous text card for
adults
MNREAD card

Assessment in Children
 Forced choice preferential looking
 Optokinetic nystagmus test
 Stycar ball test
 Static test
 Catford drum test
 Marble game test
 Tumbling E test
 Sheridan gardiner test
 Pictorial test
 Cardiff acuity test (0-1year)
 Dot acuity test
 Coin test (2-3year)
 Miniature toy test

Optotypes
 Landolt Rings (Landolt ‘C’)
 Numbers
 Figures
 Tumbling E
 Letters (snellen)

LETTER STYLE VARIATION
 SERIF: ornamental cross stroke at the end of
the limb
 SANS SERIF;
 bold type faces and appear less cluttered
Easier to read than serif letters

Basics of snellen acuity chart…
Snellen Fraction is the most common notation of acuity.
 The distant acuity is usually tested by snellen chart.
 Consist of a series of black capital letters on a white
board,arranged in lines,each progressively diminishing
in size.
 Each letters fits in a square.
 Assume 5x5 grid for letters with detail separation of
1/5 of letter size.

Formula
Testing distance
VA=
distance at which letter subtends 5min of
arc

…
 The number used to indicate the “size” of the
letter is the distance at which the letter height
subtends 5 min of arc.
 A “20 foot letter” is one that subtends 5 min
of arc at 20 feet.
 A “40 foot letter” is one that subtends 5 min
of arc at 40 feet.
 A “100 foot letter’ is one that subtends 5
min of arc at 100 feet.

Snellen chart…6/60, 6/36, 6/24, 6/18, 6/12, 6/9,
6/6
θ
tan θ = h / d

Testing & Notation
 One eye at a time, OD then OS
 Line by line scoring system
 Count a line as “seen” if over 50% of the Optotypes of line
are correctly identified.
 Using (-) and (+) signs in VA recording
 T Z V E C L (20/20) – read as “T Z V E C K”
 20/20 -1
 read as “T W W E C M” but all of the 20/25 line was
correctly identified
 20/25+3

TYPES OF NOTATION
 Decimal Notation
 Snellen Fraction notated as a decimal
 20/20 = 1.00
 20/40 = 0.50
 20/100 = 0.20

Percentage Acuity
Represent in %
20/20 = 100%
20/200 = 10%
20/2000 = 1%

Minimum Angle of Resolution (MAR)
 The MAR is expressed in minutes of arc
 The angular size of the just-resolvable optotype.
 Normally 1/5 of the letter height
 Reciprocal of the Snellen Fraction or decimal
acuity
 20/20: MAR = 1.0 min of arc
 20/40: MAR = 2.0 min of arc
 20/200: MAR = 10 min of arc

Log MAR(LOGarithm of the minimal
angle of resoluation. )
 Log of the MAR used to notate the acuity
 Used in research and low vision
 Bailey-Lovie Chart
 Their chart and its derivatives follow the
principles:
 Geometric progression of size & spacing by
0.1 Log unit or 4/5
 Found that the relative spacing between letters
and the number of letters per row can cause
substantial variation in acuity scores

 Acuity becomes worse as the log MAR value
increases
…
 Decrease brightness line by line

Characteristics of LogMAR
 It is flat and typical “V” shape.
 Testing distance 4m.
 The letter is sanserif and sollan letters. ex:
s,o,c,v,z
 Every line has 5 letters .
 Point one (0.1) logarithmic progression. (5:4)

 Between two letter space is the width of
a single letter.
 Between two line space is the height of
the lower line letter.

..
..
 Every letter read counts as 0.02 of each line (every line
has 5 letters)
 Letter to Letter scoring system
 Each line = 0.1 log unit The log MAR chart is available
in Landolt C, Tumbling E as well as in numeric and
alphabet optotypes

Log MAR Snellen Equivalents
 20/10 = -0.30
 20/15 = -0.13
 20/20 = 0.0020/80 = 0.60
 20/100 = 0.70
 20/125 = 0.80
 20/160 = 0.90
 20/200 = 1.00
 20/317 = 1.20
 20/502 = 1.40
 20/25 = 0.10
 20/32 = 0.20
 20/40 = 0.30
 20/50 = 0.40
 20/63 = 0.50

Exam…
 Letter to letter Acuity assessment
 Every letter that is correctly read deducts 0.02 from
that line
 Example:
 If the patient reads all the letters of the logMAR 0.40 line
(20/50) and two letters from the 0.30 line (20/40), it is scored
as 0.36
 Snellen Equivalent to this would be 20/50+2

Position/LIMITATION OF V/A
 Standard visual acuity depends on two limitation .
1.Anatomical
 two cones stimulate but one cone
separated by one minute of arc angle
 two separate object as separate
2.Optical
 Airy disc form by diffraction
 Diffraction form in smaller in size of pupil
 Aberration

References…..
 THEORY AND PRACTICE OF OPTICS &
REFRACTION…A.K.KHURANA
 CLINICAL PROCEDURE OF OPTOMETRY
 INTERNET

Visual acuity (2)

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