2. Vision – complex process, consists of
visual acuity, peripheral vision, visual
field, contrast sensitivity, dark
adaptation.
We distinguish central and peripheral
vision. Central vision includes VA and
color vision, peripheral vision – VF and
dark adaptation.
4. Visual Acuity (VA)- is ability of eye to
discriminate two points with minimal
distance between them.
Two objects are so close that two
adjacent cones are stimulated,
between them is located one
unstimulated cone.
5. Visual Angle…
The angle subtended at the nodal
point of the eye by the physical
dimensions of an object in the
visual field.
6. 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
7. 3 types of Cone:
1. Green- Chlorolabe
2. Red- Erythrolabe
3. Blue- Cyanolabe
Diameter of each cone : 0.002µm
8. 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)
11. 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.
12. 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).
13. 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.)
14. Minimum Discriminable
Ability to determine whether or not two
parallel and straight lines are aligned in
the frontal plane.
15. 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
17. Visual acuity (VA) is acuteness or
clearness of vision, especially form
vision, which is dependent on the
sharpness of the retinal focus within
the eye, the sensitivity of the nervous
elements, and the interpretative
faculty of the brain.
18. VA – is angular measurement relating testing
distance to the minimal object size resolvable
at the distance.
19. 1862 Hermann Snellen published his
famous letter chart with special
targets - optotypes.
Snellen defined “standard vision” as
the ability to recognize one of his
optotypes when it subtended 5
minutes of arc, thus the optotype can
only be recognized if the person
viewing it can discriminate a spatial
pattern separated by a visual angle of
20. 1923 Soviet ophthalmologists Sergei Golovin and
D. A. Sivtsev developed the table for testing VA,
that was used in the USSR. It consisted of two
parts, ten rows each, both parts with visual acuity
values incrementing by 0.1 from row to row. The
left part contained Cyrillic letters Ш, Б, М, Н, К, Ы, И
in a definite order, with width of each character
equal to its height, and the size of a first row
character being 70mm, in the second row it was
35mm, and in the last row - 7mm. Right part of the
table contained Landolt C symbols. The distance
was set to be 5 meters. This table is called
Golovin-Sivtsev Table
21. Factors determined VA
Light travels from the fixation object to the fovea
through an imaginary path called the visual axis.
The eye's tissues and structures affect the quality
of the image are: tear film, cornea, anterior
chamber, pupil, lens, vitreous, and finally the
retina.
The posterior part of the retina, called the retinal
pigment epithelium (RPE) is responsible for,
among many other things, absorbing light that
crosses the retina so it cannot bounce to other
parts of the retina.
22. Size of the pupil
Normal function of ON
The visual cortex is the part of the cerebral
cortex in the posterior (occipital) part of the
brain responsible for processing visual
stimuli
24. If VA less than O,1 we use
Snellens formula to account VA
VA=d/D
d- distance from patient can read
first line
D- distance from patient must read
first line of chart
26. Name Abbreviation Definition
Counting
Fingers
CF Ability to count fingers
at a given distance.
Hand
Motion
HM Ability to distinguish a
hand if it is moving or
not in front of the
patient's face.
Light
Perception
LP Ability to distinguish if
the eye can perceive
any light.
No Light
Perception
NLP Inability to see any
light. Total blindness.
27. Place the chart at 20 feet (or 6 meters).
If the patient uses glasses, using them.
Place the occluder in front of the eye that is not
being evaluated.
Start first with the big optotypes and proceed to
the smaller ones.
If the measurement is reduced (below 20/20)
then the test using a pinhole should be done
Change the occluder to the other eye and
proceed again from the 4th step.
After both eyes have been evaluated in distant
visual acuity, proceed to evaluate near visual
acuity placing a modifid snellen chart for near
vision at 15.7 inches (or 40 centimeters). Then
repeat the test from the 2nd step.
28. Factors affecting visual acuity
Overall
Illumination of test object.
Contrast of letters.
Pupil size
Cognitive ability
Ocular health
Refractive Status
29. Physical factor
Those which influence the light
characteristics of the distribution
& hence influence the nature of
retinal image.
30. 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
31. Observer related
Retinal locus of stimulation
Pupil size
Accommodation
Effect of eye movement
Optical elements of eye
Development of aspects
32. Distance Acuity chart
Snellen’s distance acuity chart
Bailey-Lovie charts
Landolt ring (or C) chart
Feinbloom Distance Test Chart
Chronister Pocket Acuity Chart
33. 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
36. 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.
39. …
The number used to indicate the “size”
of the letter is the distance at which the
letter height subtends 5 min of arc.
A “6 meter letter” is one that subtends
5 min of arc at 6 m.
A “12 meter letter” is one that subtends
5 min of arc at 12 m.
A “30 meter letter’ is one that subtends
5 min of arc at 30 m.
43. 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
44. Visual field- VF
The term visual field is sometimes
used as a synonym to field of view
Field of view is everything that (at a
given time) causes light to fall onto the
retina. This input is processed by the
visual system, which computes the
visual field as the output.
VF – is a space which we can see
without movement eyes and head
45. Normal limits
The normal human visual field
extends to approximately 60 degrees
nasally (toward the nose, or inward)
in each eye, to 90 degrees temporally
(away from the nose, or outwards),
and approximately 60 degrees above
and 75 below the horizontal
meridian.
46. Measuring the visual field
Confrontation test
Perimetry
Spheroperimetry
Kinetic Perimetry
Static perimetry
Automated perimetry
47. Lesions along the visual pathway
a lesion may arise at different points along the
visual pathway
gives rise to different types of visual field defects
known as hemianopia (half blindness)
perimetry is a test which can detect visual field
defects
48. Left Right
left eye blindness
bitemporal hemianopia
right homonymous hemianop
right homonymous hemianop
normal visual fields
49. Abnormalities of VF
Scotoma – physiologic and
pathologic Blind spot (Mariotte
spot), angioscotomas –
physiologic scotoma
Pathologic- central, periphery,
relative, absolute, positive,
negative
Constriction of VF
