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VISUAL ACUITY MEASUREMENT,
CONTRAST SENSITIVITY
PRESENTER : DR.OM PATEL
MODERATOR : DR.ALKA
VISUAL ACUITY PRINCIPLES
• The visual acuity is determined by the smallest
retinal image the form of which can be
apprecia...
VISUAL ANGLE
• It is found that the object must subtend a
visual angle of 1 minute at the nodal point of
the eye
• VA test types consists of a series of letters of
diminishing size.
• Each letter is shaped such that it can be placed in...
COMPONENTS OF VISUAL
ACUITY
• Detection or visibility
• Resolution
• Recognition
1. Detection or Visibility
• Ability to determine whether or not an object is
present in an otherwise empty visual field i...
2. Resolution (Ordinary VA)
• Discrimination of two spatially separated
targets is termed resolution
• It is essentially a...
3. Recognition
• Virtue by which an individual identifies the
test patterns with which he had some
experience
• It involve...
MEASUREMENT OF VISUAL ACUITY
• The visual acuity is a highly complex function
• In clinical practice, VA is considered
syn...
MEASUREMENT OF VISUAL ACUITY IN ADULTS
• The distant central VA in
adults is tested by :
• Snellen’s test types
• Landolt’...
SNELLEN’S TEST TYPE
• Basis of the test : Two distant
points are visible as separate
only when they subtend an
angle of 1 ...
• Each letter of the chart
is so designed that it fits
in a square,the sides of
which are 5 times the
breadth of constitue...
METHOD
• Patient seated at 6 meters from the chart
• Illumination is 20 foot candles
• Each eye is tested separately.
• VA...
• 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,...
LANDOLT’S TEST TYPES
• Similar to Snellen’s test types
• Instead of letters, broken rings
are used & the patient is asked
...
E CHART :-
• Similar to snellen’s and landolt’s the difference
is that in this chart E of different sizes are
arranged
• P...
SNELLEN’S EQUIVALENT
• In U.S., the metric system is not usually
employed & the values are converted to feet
• ( 6m = 20 f...
Decimal acuity:-
• In this system, the Snellen’s fraction is reduced to
a decimal no.
• Higher VA is represented by a nume...
The Bailey-Lovie logMAR chart
• Principle:-
 Used logarithmic scale.
 Each step indicates increase of 25% in letter size...
• Results of this chart were
obtained in terms of logMAR
score i.e log of minimum angle
of resolution
• As each letter siz...
Snellens Chart
• Irregular progression of letter
size
• Variable number of letters in
each line
• Variable legibility (dif...
Snellen fraction
(20ft)
Snellen fraction
(6m)
Snellen Acuity
(Decimal)
Minimal Angle of
Resolution
(inverse of sn.frac)
Lo...
Measurement of VA for Near
• Near vision is tested by asking the patient to
read a near vision chart kept at a distance of...
1) Jaeger’s charts :-
• Consist of ordinary
printer’s fonts of
varying sizes
• Prints are marked
from 1 to 7 and
according...
(2) Roman test types :-
• Consists of Times Roman fonts with standard
spacing.
• -The near vision is recorded as N5, N6, N...
(3) Snellen’s near vision test types :-
• Constructed on the same principles as of the
distant types.
• The graded thickne...
VISUAL ACUITY MEASUREMENTS IN
CHILDREN
• OBJECTIVE TESTS
Preverbal children ( < 2 ½ yrs )
• SUBJECTIVE TESTS
Verbal childr...
OBJECTIVE TESTS
• Fixation & following behaviour
• Preferential looking test ( PLT)
• Optokinetic Nystagmus (OKN)
• Visual...
1) FIXATION & FOLLOWING
• Bright colored objects with
high contrast edges are used.
• Best target however is the
human fac...
• Quality of fixation behaviour – C S M
C – Central – foveal fixation
S - Steady – no nystagmus
M – Maintained – fixation ...
QUANTITATIVE METHODS
• Methods to detect the resolution acuity
• More sophisticated method of visual assesment
than mere f...
(2) Preferential looking test
• Assumes that the child will
prefer to look at an area of
higher visual interest, rather
th...
Procedure
• On a screen homogenous
surface is projected on
one side & black and
white strips on the other
• These two stim...
PROCEDURE
• Gradually the fineness of stripes is reduced
unless there is no longer correlation between
direction of gaze &...
(3) Optokinetic Nystagmus Test (OKN)
• Nystagmus is elicited by
passing a succession of black
and white stripes through th...
(4) Visual evoked response (VER)
• Refers to EEG recording made from the
occipital lobe in response to visual stimuli.
• I...
Visual evoked response (VER)
• Flash VER tells about the integrity of macular
and visual pathway
• Pattern reversal VER us...
SUBJECTIVE TESTS
• Optotype-
symbol whose identification implies VA
• Eg; Lea’s symbols ,HOTV, Snellen’s chart,
Landolt-C ...
Lea’s symbols
• Pt. is asked to match the
picture shown to the
hand held cards
• 4 shapes are included in
this : Circle, S...
HOVT /SHERIDAN GARDINER TEST
• It is done at a distance of
6 meters from the child
• It includes simple
alphabets such as
...
Snellen’s,E Chart,Landolt’s C
IN SUMMARY
• INFANTS -
Fixation methods
Preferential looking test
Optokinetic nystagmus
Visual evoked potential
• TODDLERS - LEA’S SYMBOLS
HOTV TEST
• CHILDREN > 5 YRS & ADULTS-
SNELLEN’S CHART
LANDOLT’S C CHART
E – CHART
ETDRS
Visual Acuity Measurement, Contrast Sensitivity
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Visual Acuity

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Visual Acuity Measurement, Contrast Sensitivity

  1. 1. VISUAL ACUITY MEASUREMENT, CONTRAST SENSITIVITY PRESENTER : DR.OM PATEL MODERATOR : DR.ALKA
  2. 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. 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. 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. 5. COMPONENTS OF VISUAL ACUITY • Detection or visibility • Resolution • Recognition
  6. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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 0.1logMAR 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. 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 logMAR Chart • 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
  22. 22. Snellen fraction (20ft) Snellen fraction (6m) Snellen Acuity (Decimal) Minimal Angle of Resolution (inverse of sn.frac) Log-MAR 20/200 6/60 0.10 10 1 20/160 0.12 8 0.9 20/125 ~6/36 0.8 20/100 0.20 5 0.7 20/80 6/24 0.25 4 0.6 20/63 ~6/18 0.5 20/50 0.40 0.4 20/40 6/12 0.50 2 0.3 20/32 0.67 0.2 20/25 0.80 0.1 20/20 6/6 1 1 0 20/16 -0.1 20/12.5 -0.2 20/10 -0.3
  23. 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. 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. 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. 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. 27. VISUAL ACUITY MEASUREMENTS IN CHILDREN • OBJECTIVE TESTS Preverbal children ( < 2 ½ yrs ) • SUBJECTIVE TESTS Verbal children ( > 2 ½ yrs )
  28. 28. OBJECTIVE TESTS • Fixation & following behaviour • Preferential looking test ( PLT) • Optokinetic Nystagmus (OKN) • Visual evoked potential (VEP)
  29. 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. 30. • Quality of fixation behaviour – C S M C – Central – foveal fixation S - Steady – no nystagmus M – Maintained – fixation after a blink
  31. 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. 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. 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. 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. 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. 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. 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. 38. SUBJECTIVE TESTS • Optotype- symbol whose identification implies VA • Eg; Lea’s symbols ,HOTV, Snellen’s chart, Landolt-C ,E chart. • These tests are employed in verbal children.
  39. 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. 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. 41. Snellen’s,E Chart,Landolt’s C
  42. 42. IN SUMMARY • INFANTS - Fixation methods Preferential looking test Optokinetic nystagmus Visual evoked potential
  43. 43. • TODDLERS - LEA’S SYMBOLS HOTV TEST • CHILDREN > 5 YRS & ADULTS- SNELLEN’S CHART LANDOLT’S C CHART E – CHART ETDRS

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