Visual acuity

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OPTOM PRATIKSHA PATIL

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• TOTAL POWER OF THE EYE?
• CORNEA – 2/3rd eye power (+44.00D)
• LENS- 1/3rd eye power (+16.00D)
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VISUAL ACUITY
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• HOW WILL YOU QUANTIFY VISION?
• Visual function assessment / vision
components?
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HISTORY
• 1679- Hooke
• Early 19th century- Purkinje & Young
• 1863 – Professor Hermann Snellen
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CONCEPT OF VA
• Visual acuity(VA) is the spacial resolving
capacity of visual system.
• It express the angular size of detail that can
just be resolved by observer.
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VISUAL ANGLE
• VISUAL ANGLE IS THE ANGLE SUBTENDED AT THE
NODAL POINT OF THE EYE BY THE PHYSICAL
DIAMENSION OF THE OBJECT IN THE VISUAL FIELD
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• Visual angle ?
• At nodal point?
• Minimum angle required to see at retina?
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COMPONENTS OF VA
• Minimum visible or detectable acuity
• Minimum resolvable acuity
• Minimum recognizable acuity
• Minimum discriminable acuity or hyperacuity
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Minimum Visible/Detectable
• Refers to the smallest object that one can
detect.
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Minimum resolvable
• Refers to the smallest angular separation
between neighboring objects that one can
resolve.
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Minimum recognizable
• Refers to the angular size of the smallest
feature that one can recognize or identify.
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Minimum discriminable or hyperacuity
• Refers to the angular size of the smallest
change in a feature (e.g. a change in size,
position or orientation) that one can
discriminate.
Fig.: Two “hyperacuity” configurations: Vernier
acuity (A) and bisection acuity (B).
In both cases, the subject’s task is to judge
the relative position of the features.
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(A)
(B)
(C)
(D)
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FACTORS AFFECTING VISUAL ACUITY
Stimulus/ object Related
1. Luminance/ illumination
2. Contrast
3. Geometry of letters
4. Time of exposure of the target
5. Influence of wavelength
6. Interaction effect
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Observer Related
1. Area of the retina stimulated
2. Pupil size
3. Accommodation
4. Effect of eye movement
5. Refractive error
6. Optical element
7. Developmental aspect
FACTORS AFFECTING VISUAL ACUITY
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OPTOTYPES
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REFERENCE: Borish’s clinical refraction(William J. Benjamin) 2nd ed.
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Specification of visual acuity
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TERMINOLOGY
1. Snellen’s Fraction
2. Decimal notation
3. Minimum Angle of resolution(MAR)
4. LogMAR
5. Visual Acuity Rating(VAR)
6. Visual efficiencies
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Snellen’s Fraction
• Angular size of optotypes
• VA=testing distance/distance at which
Optotype subtend 5min of arc
Eg. 20/200 feet or 6/60 m
E5min of arc
6 meter testing
distance
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Snellen’s Fraction
• 20/200 means: test distance - 20 feet and the
smallest letters that could be read would subtend 5
minarc when at a distance of 200 feet.
• The angular size of such letters at 20 feet is 50
minarc.
• 20/200, 40/400, 10/100, 5/50, and 6/60 are all
visual acuity scores that represent the same angle
(letters subtend 50 minarc);
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Decimal Notation
• Gives single number to quantify an angle.
• Does not indicate test distance.
• Eg. 20/200 = 0.1 ; 20/20 = 1.
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Minimum angle of resolution(MAR)
• Expressed in min of arc.
• Indicates angular size of critical detail within
just resolvable optotype.
• Critical detail= ⅕ of letter height.
• MAR= 1/ decimal acuity value
• Eg.20/40 = 2 min of arc
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LogMAR :
• It is common Logarithm of MAR (logMAR)10
• Eg.
VA= 20/20
MAR= 1 min of arc, so logMAR = 0
• When VA is better than 20/20, logMAR is
negative.
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Visual acuity rating
• Derived from the logMAR values.
• VAR = 100-50(logMAR)
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Visual efficiency
• Visual efficiency scale used to quantify visual
loss for legal & compensation purpose.
• AMA later used VE scale for quantifying loss in
visual field & ocular motility.
• VE= 0.2(MAR-1)/9
• VE for visual acuity 6/6 is 1.0, for visual acuity
6/60 is 0.2
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• VA is tested by different ways depending
upon age group:
– Infant:- less than 1 year
– Toddlers:- 1-2 year
– Children:- 2-12 year
– Adolescents:- 12-18 year
– Adult:- more than 18 year
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Visual Acuity test in adolescent
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Snellen’s chart
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Reference: Textbook of visual science and clinical optometry - Bhattacharya , 2009
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CLINICAL TESTING OF VISUAL ACUITY
CHART
FORMAT
CHART
LUMINANCE
REFRACTIVE
ERROR
TESTING
DISTANCE
TESTING
PROCEDURE
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• Testing distance is 6 m.
• Patient is asked to read the chart with each eye
separately.
• When patient is able to read upto to 6m line the
VA is recorded as 6/6 it is normal. Similarly
depending upon the line that the patient can
read from distance of 6m patient vision is
recorded as 6/9, 6/12, 6/18, 6/24, 6/36 and
6/60.
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• If patient cannot see the top line from 6m patient is
asked to slowly walk towards chart till he can read
the top line.
• Depending upon distance at which patient can read
the top line patients VA is recorded as 5/60, 4/60,
3/60, 2/60 and 1/60.
• If patient is unable to read the top line from 1m,
Patient is asked to count fingers of examiner his or
her vision is recorded as CF-3’, CF-2’, CF-1’ or CF
close to the face depending on the distance at which
he counts fingers.
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• When patient fails to count finger, the examiner
moves his or her hand close to the patient’s face. If
patient appreciates the hand movement (HM), VA is
recorded as HM positive.
• When the patient cannot distinguish the hand
movements, the examiner notes whether the patient
can perceive light (PL) or not. If yes, vision is recorded
as PL positive and if not it is recorded as PL negative.
• If PL positive then ask the patient from which
quadrant light is coming.
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UNAIDED
PINHOLE
AIDED
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• Testing distance of near
• Chart is 35-40 cm
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• Jaeger’s chart : In this chart, prints are
marked from 1 to 7 and accordingly patient’s
acuity is labeled as J1 – J7, depending upon
the print one can read.
• Roman test chart : consists of “Times
Roman” type font with standard spacing and
the near vision is recorded as N5, N6, N8,
N10, N12, N18, N36 and N48.
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• Reduced Snellen’s near vision chart: the graded
thickness of the letters of different lines is about is
1/17th of the distance vision chart letters.
• M notation Chart:In this chart, prints are marked
from 0.4 to 4 accordingly the patients visual acuity
is labelled 0.4M to 4M, depending upon the print
one can read.
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Roman test chart Jaeger’s chart
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• Borish’s Clinical Refraction- William J Benjamin
• Adler’s physiology of the eye, 11th ed. -
William M Hart, jr
• Theory & practice of optics & refraction- A K
Khurana
• Clinical visual optics - Bennett & Rabbetts’
• Primary care optometry - Theodore Grosvenor
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