This document discusses tests used to measure visual acuity and contrast sensitivity. It describes how visual acuity is a measure of spatial resolution and the minimum angle that can be resolved. Common tests to measure visual acuity include Snellen charts for distant and near vision and Landolt C charts. Contrast sensitivity measures the ability to detect differences in luminance and is tested using gratings of different spatial frequencies. Potential vision tests like interferometry and potential acuity meters are used to estimate vision through cataracts by projecting patterns directly on the retina.
2. VISUAL ACUITY
• Visual perception/ Vision – complex
integration of light sense + form
sense + contrast sense + colour sense
• Visual acuity – measure of form
sense
• Defined as reciprocal of the
minimum resolvable visual angle in
minutes of arc
3. Nodal point / axial point
• One of the two points in a compound
optical system, located so that a light
ray directed through the first point
will leave the system through the
second point, parallel to its original
direction
4.
5. • Visual angle – angle subtended at nodal point
of eye by physical dimensions of an object in
the visual field
• 2 adjacent points can be seen discretely and
clearly
if visual angle is >= 1 minute , but depends
on size and distance
If size of retinal image is >4.5 microns
(1stimulated cone 1.5microns +
1unstimulated cone 1.5microns +
1stimulated cone 1.5microns)
6.
7. Components
• V. A - measures threshold of
discrimination of 2 spatially separated
targets – function of fovea centralis
1. Minimum visible / detectable
2. Resolution
3. Recognition
4. Minimum discriminable
8. Minimum visible / detectable
• Ability to detect whether or not an object is
present
• Depends on size, shape, illumination of stimulus
i. Black dot – white background – if diam. >=
30sec of arc
ii. Black square – white background – if
l(diagonal) >= 30 sec of arc
iii. Extended line 0.5sec thick – subthreshold
signals converge to give suprathreshold of V. A.
iv. Illuminated object – dark background –
depends only on intensity
9. Resolution / Ordinary V. A.
• Discrimination of 2 spatially separated targets
• Minimum separation that can be
discriminated = minimum resolvable
• Function of fovea centralis
• Angular threshold at nodal point = 30-60 sec
or arc = Minimum Angle of Resolution (MAR)
• Snellen’s charts
• Landolt’s rings
10. Recoginition
• Identify patterns from past
experience
• Spatial resolution + cognitive
components
• E.g. Identification of faces
13. Factors affecting V. A.
• Stimulus-related factors :
i. Luminance of test object
ii. Geometrical configuration
iii. Contrast from surrounding
iv. Wavelength
v. Exposure duration
vi. Interaction effects of two targets
14. • Observer-related factors :
i. Retinal locus of stimulation
ii. Pupil size
iii. Accomodation
iv. Effect of eye movements
v. Meridional variation
vi. Optical elements
vii. Developmental aspects
15. Measurement
Clinical tests measure minimum resolvable
1. Detection acuity tests : detect
smallest stimulus
i. Dot visual acuity test
ii. Catford drum test
iii. Boek candy bead test
iv. STYCAR graded ball’s test
v. Schwarting metronome test
16. Dot visual acuity test
• Black dots on white background
• Smallest dot child touches is
approximately the visual acuity
• Test distance – 25m
19. 2. Recognition acuity tests : recognize
and distinguish stimulus
A. Direction identification tests-
i. Snellen’s E-chart test
ii. Landolt’s C-chart test
iii. Sjogren’s hand test
iv. Arrows test
20. B. Letter-identification tests-
i. Snellen’s letter chart test
ii. Sheridan’s letter test
iii. Flook’s symbol test
iv. Lipman’s HOTV test
21. C. Picture-identification tests-
i. Allen’s picture card tests
ii. Beale Collins picture chart tests
iii. Domino cards test
iv. Lighthouse test
v. Miniature toy test of Sheridan
D. Tests based on picture identification on
behavioural pattern-
i. Cardiff acuity card tests
ii. Bailey Hall cereal test
22. 3. Resolution acuity tests :
i. Optokinectic Nystagmus (OKN) test
ii. Preferential looking test
a. two-alternate forced choice test
b. Operant variation looking test
c. Teller acuity card tests
iii. Visually evoked response
23. Measurement in school children
(>5yrs) and adults
• Snellen’s test types :
– Distant central visual acuity
– Series of black capital letters on white board, in
lines, progressively decreasing in size
– Breadth of each line will subtend an angle of 1min
at nodal point
– Each letter fits in square whose sides are 5X the
breadth of the constituent line
– So each letter subtends an angle of 5min
– Starting from top, letters should be read clearly at
60,36,24,18,12,9,6,5,4 metres
26. Landolt’s test types
• Each broken ring subtends an angle of 5min
• Detection of orientation of the breakpoint in
the circle
27. Procedure
• Patient at 6mtrs – light rays practically parallel &
minimal accomodation
• Illumination >= 20 footcandle
• Read with each eye separately
• Numerator = distance = 6m
• Denominator = smallest letters read accurately
• If unable to read top line at 6m, patient is asked
to walk towards chart & distance at which patient
reads is noted – 5/60, 4/60 so on
• Unable at 1m – counting fingers
• Hand movements
• Perception of light (PL)
28. Measurement of visual acuity
in 3-5years
• E- cutout test – Child given an E cutout &
asked to match orientation with various Es on
the chart
29. • Tumbling E-pad test – large E 20/200 on
one side and series of five 20/20
tumbling Es on the other – caliberated to
20ft.
33. • Broken wheel test –
– Pair of cars in progressively smaller sizes one of
which has a broken wheel
34. • Boek’s candy bead test – child asked to match
beads at 40cm. Snellen ‘s equivalent 20/200 can
be estimated
• Light home picture cards – at 10ft – 20/200 to
20/10
35. Measurement of visual acuity
in 2-3years
• Dot visual acuity test
• Coin test – identify two faces of coins at
different distances
• Miniature toy test – identify and match
toy at 10ft
36. Measurement of visual acuity
in 1-2years
• Marble game test – place marbles in
holes on cards – to find is ‘useful’ or ‘less
useful’
• Sheridans ball test – balls of progressively
smaller size rolled at 10ft against white
background – smallest size the infant can
fixate
37. Measurement of visual acuity
in infants
1. Optokinetic Nystagmus test (OKN) –
succession of black&white stripes elicit
nystagmus – visual angle subtended by
smallest stripe which elicits nystagmus is
measure of acuity
6/120 – in newborns
6/60 – at 2months
6/36 – at 6months
6/6 – at 20-30months
38.
39. 2. Preferential Looking Test (PLT) – 2adjacent
stimulus fields – on homogenous and one
striped
- examiner notes head movements from
behind the screen through a hole
- infant tends to look at striped pattern for
greater portion of time
-upto 4months
- 6/240 – newborns
6/60 – 3months
6/6 – 36months
40.
41. 3. Visual Evoked Response (VER) – EEG recording
from occipital lobe in response to visual
stimuli
- clinically objective
- functional state beyond retinal ganglion cells
• Flash VER – integrity of macular and visual
pathway
• Pattern reversal VER – checkerboard stimulus
reversed with same illumination
• 6/120 at 1month, 6/60 at 2months, 6/6-6/12
at 6-12months
42.
43. 4. Catford drum test
5. Cardiff acuity cards
6. Indirect assessment –
i. Blink reflex in response to light since birth
ii. Menace reflex – reflex closure of eyes on
approach of object since 5 months
iii. Fixation reflex – fixation behaviour test,
binocular fixation pattern, central steady
mantained (CSM) monocular fixation
44. Measurement of visual acuity for near
• Jaeger’s charts – prints marked 1-7 – acuity J1-J7
• Roman test types – Times Roman font with
standard spacing –
N5,N6,N8,N10,N12,N18,N36,N48
• Snellen’s near vision test types – graded thickness
of letters is 1/17th of the distant-vision chart
letters by photographic reduction – so letters
equivalent to 6/6 line subtend 5min at avg.
reading dist. 35cm/14inches
45.
46. CONTRAST SENSITIVITY
• Ability to perceive slight changes in luminance
between regions that are not separated by
definite borders
• First measured by Schade
• Types –
– Spatial
– Temporal
47. • Spatial – detection of striped patterns at various
levels of contrast and spatial frequencies – Arden
gratings – sine wave gratings of light and dark
bands & minimum contrast at which bars can be
seen at each frequency is measured
• Spatial frequency – number of pairs of light and
dark bands subtending angle of 1degree
• High = narrow bars, Low = wide bars
• Temporal – time-related processing by
presentating a uniform target field modulated
sinusoidal in time
48. Measurement
• Presented with grating frequencies
• Resolution below which contrast is not
possible is threshold level
• Reciprocal of threshold is contrast sensitivity
• L – luminance recorded by photocells
• Contrast sensitivity = (Lmax–
Lmin)/(Lmax+Lmin)
49. Methods
• Arden gratings – 1 screening & 6 diagnostic
plates
– Contrast changes from top to bottom covering
1.76 log units
– Studied at 57cm
– Spatial freq. Increases from 0.2cycles/deg
to6.4cycles/deg
– Score – 1-20/plate, sum of 6plates = upper limit
82 in normal, interocular diff. <12
50. • Cambridge low-contrast gratings – 10 plates at
6m in order of descending contrast, each
paired with a blank page of same reflectance 7
patient has to identify page with gratings
• Conversion table with scores – plate 10 =
score 11
51. • Pelli – Robson contrast sensitivity chart –
letters subtend angle of 3degrees at 1m
– Letters arranged as triplets
– Contrast decreases – log contrast sens from 0.00
to 2.25
– Luminance of white areas 60-120cd/m
52. • Vistech chart – sine wave gratings at 3m –
identify orientation of grating
53. Neural mechanisms
• Campbell and Green – different visual
channels handle different spatial frequencies
• Fovea – high acuity & high freq.
• Peripheral retina – low freq.
54. • Factors affecting contrast sensitivity –
– Refractive errors – high freq.
– Age – decreases with age 10% per decade from
20’s onwards
– Lens – low freq.
– Ocular and systemic diseases
55. TESTS FOR POTENTIAL VISION
• To check whether significant cause of visual
impairment is cataract or associated retinal
pathology
56. INTERFEROMETRY
• Estimation of VA through mild to moderate
media opacification by projection a resolution
target on macula
• Set of interference fringes of light and dark
bands produced on retina by waves from 2
coherent light beams each < 0.1mm in diam.
• Depends only on ability of retina to conduct
signals from photoreceptors to nervous
system
57.
58. Types
• LASER –
– 2 point light sources from Helium-Neon gas laser
(632.8nm)
– Focussed red light penetrates through opaque
media
• White light –
– Polychromatic white light from incandescent bulb
– Contrast of gratings may be reduced by chromatic
abberations
59. OPTICS
• 2 periodic waves go in-phase and out-of-phase
• Maxima – points on retina where both are in-
phase – bright white bars
• Minima – points on the retina where both are
out-of-phase – black bars
• Spacing (fringe pitch) – function of separation of
pinpoint beam areas (grating angle)
• Increased separation – finer fringe – greater
macular resolution
60. • Space is adjusted till patient cannot identify
orientation – last perceived grating value
converted to Snellen’s potential
• Thiry-three maxima per degree of visual angle
corresponds to Snellen’s equivalent of 6/6
61.
62. Technique
• Explain patient – orientation of band patterns,
ignore scotomas
• Pupil dilated, interferometer mounted on slit-
lamp
• Retroillumination, beam passed through area
of maximum transparency of media
• Pupil diam. 1.5mm + steps of 0.1mm
• Patient indicates direction of fringe
63. Interpretation
• Normal – alternate dark and light stripes
• Media opacity – shooting stars, moving
worms, jumble but can identify stripes
• Very dense opacity – no pattern
• False postive – tilted retinal receptors (Stiles-
Crawford phenomenon), healthy receptors in
CME, parafoveal stimulation
• False negative – poor pupillary dilatation, very
dense cataract, VH
64. Potential Acuity Meter
• Guyton & Minkowski
• Small device on slit-lamp
• Projects image of Snellen’s chart through
0.15mm diam aperture
• Slide scale from +13D to -10D
• 20/20 to 20/400
• Pupil dilated, best refractive correction, beam
focussed, reads charts
65.
66. • Factors affecting accuracy of PAM & LI
– Severity of cataract
– Type of cataract
– Preoperative visual acuity < 20/200
67. Comparison
• Moderate cataracts – both useful
• Severe cataracts – PAM underestimates
• Retinal disorders – LI overestimates
• PSCs – both underestimate
